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正确使用Block避免Cycle Retain和Crash

2014-06-24 11:43 381 查看



Block简介

Block作为C语言的扩展，并不是高新技术，和其他语言的闭包或lambda表达式是一回事。需要注意的是由于Objective-C在iOS中不支持GC机制，使用Block必须自己管理内存，而内存管理正是使用Block坑最多的地方，错误的内存管理 要么导致return cycle内存泄漏要么内存被提前释放导致crash。 Block的使用很像函数指针，不过与函数最大的不同是：Block可以访问函数以外、词法作用域以内的外部变量的值。换句话说，Block不仅 实现函数的功能，还能携带函数的执行环境。

可以这样理解，Block其实包含两个部分内容

Block执行的代码，这是在编译的时候已经生成好的；

一个包含
Block执行时需要的所有外部变量值
的数据结构。
Block将使用到的、作用域附近到的
变量的值
建立一份快照拷贝到栈上。

Block与函数另一个不同是，Block类似ObjC的对象，可以使用自动释放池管理内存（但Block并不完全等同于ObjC对象，后面将详细说明）。


Block基本语法

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">10</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">11</span>
// 声明一个Block变量
long (^sum) (int, int) = nil;
// sum是个Block变量，该Block类型有两个int型参数，返回类型是long。

// 定义Block并赋给变量sum
sum = ^ long (int a, int b) {
return a + b;
};

// 调用Block：
long s = sum(1, 2);

定义一个实例函数，该函数返回Block：

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span>
- (long (^)(int, int)) sumBlock {
int base = 100;
return [[ ^ long (int a, int b) {
return base + a + b;
} copy] autorelease];
}

// 调用Block
[self sumBlock](1,2);

是不是感觉很怪？为了看的舒服，我们把Block类型typedef一下

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span>
typedef long (^BlkSum)(int, int);

- (BlkSum) sumBlock {
int base = 100;
BlkSum blk = ^ long (int a, int b) {
return base + a + b;
}
return [[blk copy] autorelease];
}


Block在内存中的位置

根据Block在内存中的位置分为三种类型NSGlobalBlock，NSStackBlock, NSMallocBlock。

NSGlobalBlock：类似函数，位于text段；

NSStackBlock：位于栈内存，函数返回后Block将无效；

NSMallocBlock：位于堆内存。

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">10</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">11</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">12</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">13</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">14</span>
BlkSum blk1 = ^ long (int a, int b) {
return a + b;
};
NSLog(@"blk1 = %@", blk1);// blk1 = <__NSGlobalBlock__: 0x47d0>

int base = 100;
BlkSum blk2 = ^ long (int a, int b) {
return base + a + b;
};
NSLog(@"blk2 = %@", blk2); // blk2 = <__NSStackBlock__: 0xbfffddf8>

BlkSum blk3 = [[blk2 copy] autorelease];
NSLog(@"blk3 = %@", blk3); // blk3 = <__NSMallocBlock__: 0x902fda0>

为什么blk1类型是NSGlobalBlock，而blk2类型是NSStackBlock？blk1和blk2的区别在于，blk1没有使用Block以外的任何外部变量，Block不需要建立局部变量值的快照，这使blk1与函数没有任何区别，从blk1所在内存地址0x47d0猜测编译器把blk1放到了text代码段。blk2与blk1唯一不同是的使用了局部变量base，在定义（注意是定义，不是运行）blk2时，局部变量base当前值被copy到栈上，作为
常量
供Block使用。执行下面代码，结果是203，而不是204。

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span>
int base = 100;
base += 100;
BlkSum sum = ^ long (int a, int b) {
return base + a + b;
};
base++;
printf("%ld",sum(1,2));

在Block内变量base是只读的，如果想在Block内改变base的值，在定义base时要用 
__block
修饰：
__block
int base = 100;
。

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span>
__block int base = 100;
base += 100;
BlkSum sum = ^ long (int a, int b) {
base += 10;
return base + a + b;
};
base++;
printf("%ld\n",sum(1,2));
printf("%d\n",base);

输出将是214,211。Block中使用
__block
修饰的变量时，将取变量此刻运行时的值，而不是定义时的快照。这个例子中，执行
sum(1,2)
时，base将取
base++
之后的值，也就是201，再执行Block
base+=10;
base+a+b
，运行结果是214。执行完Block时，base已经变成211了。


Block的copy、retain、release操作

不同于NSObjec的copy、retain、release操作：

Block_copy与copy等效，Block_release与release等效；

对Block不管是retain、copy、release都不会改变引用计数retainCount，retainCount始终是1；

NSGlobalBlock：retain、copy、release操作都无效；

NSStackBlock：retain、release操作无效，必须注意的是，NSStackBlock在函数返回后，Block内存将被回收。即使retain也没用。容易犯的错误是[
[mutableAarry
addObject:stackBlock]
，在函数出栈后，从mutableAarry中取到的stackBlock已经被回收，变成了野指针。正确的做法是先将stackBlock copy到堆上，然后加入数组：
[mutableAarry
addObject:[[stackBlock copy] autorelease]]
。支持copy，copy之后生成新的NSMallocBlock类型对象。

NSMallocBlock支持retain、release，虽然retainCount始终是1，但内存管理器中仍然会增加、减少计数。copy之后不会生成新的对象，只是增加了一次引用，类似retain；

尽量不要对Block使用retain操作。


Block对不同类型的变量的存取

基本类型

局部自动变量，在Block中只读。Block定义时copy变量的值，在Block中作为常量使用，所以即使变量的值在Block外改变，也不影响他在Block中的值。

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span>
int base = 100;
BlkSum sum = ^ long (int a, int b) {
// base++; 编译错误，只读
return base + a + b;
};
base = 0;
printf("%ld\n",sum(1,2)); // 这里输出是103，而不是3

static变量、全局变量。如果把上个例子的base改成全局的、或static。Block就可以对他进行读写了。因为全局变量或静态变量在内存中的地址是固定的，Block在读取该变量值的时候是直接从其所在内存读出，获取到的是最新值，而不是在定义时copy的常量。

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span>
static int base = 100;
BlkSum sum = ^ long (int a, int b) {
base++;
return base + a + b;
};
base = 0;
printf("%d\n", base);
printf("%ld\n",sum(1,2)); // 这里输出是3，而不是103
printf("%d\n", base);

输出结果是
0
4 1
，表明Block外部对base的更新会影响Block中的base的取值，同样Block对base的更新也会影响Block外部的base值。

Block变量，被
__block
修饰的变量称作Block变量。
基本类型的Block变量等效于全局变量、或静态变量。

Block被另一个Block使用时，另一个Block被copy到堆上时，被使用的Block也会被copy。但作为参数的Block是不会发生copy的。

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">10</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">11</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">12</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">13</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">14</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">15</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">16</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">17</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">18</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">19</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">20</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">21</span>
void foo() {
int base = 100;
BlkSum blk = ^ long (int a, int b) {
return  base + a + b;
};
NSLog(@"%@", blk); // <__NSStackBlock__: 0xbfffdb40>
bar(blk);
}

void bar(BlkSum sum_blk) {
NSLog(@"%@",sum_blk); // 与上面一样，说明作为参数传递时，并不会发生copy

void (^blk) (BlkSum) = ^ (BlkSum sum) {
NSLog(@"%@",sum);     // 无论blk在堆上还是栈上，作为参数的Block不会发生copy。
NSLog(@"%@",sum_blk); // 当blk copy到堆上时，sum_blk也被copy了一分到堆上上。
};
blk(sum_blk); // blk在栈上

blk = [[blk copy] autorelease];
blk(sum_blk); // blk在堆上
}

ObjC对象，不同于基本类型，Block会引起对象的引用计数变化。

先看下面代码

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">10</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">11</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">12</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">13</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">14</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">15</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">16</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">17</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">18</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">19</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">20</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">21</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">22</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">23</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">24</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">25</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">26</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">27</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">28</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">29</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">30</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">31</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">32</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">33</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">34</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">35</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">36</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">37</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">38</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">39</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">40</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">41</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">42</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">43</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">44</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">45</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">46</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">47</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">48</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">49</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">50</span>
@interface MyClass : NSObject {
NSObject* _instanceObj;
}
@end

@implementation MyClass

NSObject* __globalObj = nil;

- (id) init {
if (self = [super init]) {
_instanceObj = [[NSObject alloc] init];
}
return self;
}

- (void) test {
static NSObject* __staticObj = nil;
__globalObj = [[NSObject alloc] init];
__staticObj = [[NSObject alloc] init];

NSObject* localObj = [[NSObject alloc] init];
__block NSObject* blockObj = [[NSObject alloc] init];

typedef void (^MyBlock)(void) ;
MyBlock aBlock = ^{
NSLog(@"%@", __globalObj);
NSLog(@"%@", __staticObj);
NSLog(@"%@", _instanceObj);
NSLog(@"%@", localObj);
NSLog(@"%@", blockObj);
};
aBlock = [[aBlock copy] autorelease];
aBlock();

NSLog(@"%d", [__globalObj retainCount]);
NSLog(@"%d", [__staticObj retainCount]);
NSLog(@"%d", [_instanceObj retainCount]);
NSLog(@"%d", [localObj retainCount]);
NSLog(@"%d", [blockObj retainCount]);
}
@end

int main(int argc, char *argv[]) {
@autoreleasepool {
MyClass* obj = [[[MyClass alloc] init] autorelease];
[obj test];
return 0;
}
}

执行结果为
1
1 1 2 1
。

__globalObj
和
__staticObj
在内存中的位置是确定的，所以Block
copy时不会retain对象。

_instanceObj
在Block
copy时也没有直接retain 
_instanceObj
对象本身，但会retain
self。所以在Block中可以直接读写
_instanceObj
变量。

localObj
在Block
copy时，系统自动retain对象，增加其引用计数。

blockObj
在Block
copy时也不会retain。

非ObjC对象，如GCD队列dispatch_queue_t。Block copy时并不会自动增加他的引用计数，这点要非常小心。


Block中使用的ObjC对象的行为

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
@property (nonatomic, copy) void(^myBlock)(void);

MyClass* obj = [[[MyClass alloc] init] autorelease];
self.myBlock = ^ {
[obj doSomething];
};

对象obj在Block被copy到堆上的时候自动retain了一次。因为Block不知道obj什么时候被释放，为了不在Block使用obj前被释放，Block retain了obj一次，在Block被释放的时候，obj被release一次。


retain cycle

retain cycle问题的根源在于Block和obj可能会互相强引用，互相retain对方，这样就导致了retain cycle，最后这个Block和obj就变成了孤岛，谁也释放不了谁。比如：

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
ASIHTTPRequest *request = [ASIHTTPRequest requestWithURL:url];
[request setCompletionBlock:^{
NSString* string = [request responseString];
}];

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
+-----------+           +-----------+
| request   |           |   Block   |
---> |           | --------> |           |
| retain 2  | <-------- | retain 1  |
|           |           |           |
+-----------+           +-----------+

解决这个问题的办法是使用弱引用打断retain cycle：

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
__block ASIHTTPRequest *request = [ASIHTTPRequest requestWithURL:url];
[request setCompletionBlock:^{
NSString* string = [request responseString];
}];

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
+-----------+           +-----------+
| request   |           |   Block   |
---->|           | --------> |           |
| retain 1  | < - - - - | retain 1  |
|           |   weak    |           |
+-----------+           +-----------+

request
被持有者释放后。request
的retainCount变成0,request被dealloc，request释放持有的Block，导致Block的retainCount变成0，也被销毁。这样这两个对象内存都被回收。

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
+-----------+           +-----------+
| request   |           |   Block   |
--X->|           | ----X---> |           |
| retain 0  | < - - - - | retain 0  |
|           |   weak    |           |
+-----------+           +-----------+

与上面情况类似的陷阱：

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
self.myBlock = ^ {
[self doSomething];
};

这里self和myBlock循环引用，解决办法同上：

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
__block MyClass* weakSelf = self;
self.myBlock = ^ {
[weakSelf doSomething];
};

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
@property (nonatomic, retain) NSString* someVar;

self.myBlock = ^ {
NSLog(@"%@", _someVer);
};

这里在Block中虽然没直接使用self，但使用了成员变量。在Block中使用成员变量，retain的不是这个变量，而会retain self。解决办法也和上面一样。

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
@property (nonatomic, retain) NSString* someVar;

__block MyClass* weakSelf = self;
self.myBlock = ^ {
NSLog(@"%@", self.someVer);
};

或者

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
NSString* str = _someVer;
self.myBlock = ^ {
NSLog(@"%@", str);
};

retain cycle不只发生在两个对象之间，也可能发生在多个对象之间，这样问题更复杂，更难发现

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
ClassA* objA = [[[ClassA alloc] init] autorelease];
objA.myBlock = ^{
[self doSomething];
};
self.objA = objA;

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span>
+-----------+           +-----------+           +-----------+
|   self    |           |   objA    |           |   Block   |
|           | --------> |           | --------> |           |
| retain 1  |           | retain 1  |           | retain 1  |
|           |           |           |           |           |
+-----------+           +-----------+           +-----------+
^                                                |
|                                                |
+------------------------------------------------+

解决办法同样是用
__block
打破循环引用

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span>
ClassA* objA = [[[ClassA alloc] init] autorelease];

MyClass* weakSelf = self;
objA.myBlock = ^{
[weakSelf doSomething];
};
self.objA = objA;

注意：MRC中
__block
是不会引起retain；但在ARC中
__block
则会引起retain。ARC中应该使用
__weak
或
__unsafe_unretained
弱引用。
__weak
只能在iOS5以后使用。


Block使用对象被提前释放

看下面例子，有这种情况，如果不只是
request
持有了Block，另一个对象也持有了Block。

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
+-----------+           +-----------+
| request   |           |   Block   |   objA
---->|           | --------> |           |<--------
| retain 1  | < - - - - | retain 2  |
|           |   weak    |           |
+-----------+           +-----------+

这时如果request 被持有者释放。

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
+-----------+           +-----------+
| request   |           |   Block   |   objA
--X->|           | --------> |           |<--------
| retain 0  | < - - - - | retain 1  |
|           |   weak    |           |
+-----------+           +-----------+

这时request已被完全释放，但Block仍被objA持有，没有释放，如果这时触发了Block，在Block中将访问已经销毁的request，这将导致程序crash。为了避免这种情况，开发者必须要注意对象和Block的生命周期。

另一个常见错误使用是，开发者担心retain cycle错误的使用
__block
。比如

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
__block kkProducView* weakSelf = self;
dispatch_async(dispatch_get_main_queue(), ^{
weakSelf.xx = xx;
});

将Block作为参数传给dispatch_async时，系统会将Block拷贝到堆上，如果Block中使用了实例变量，还将retain self，因为dispatch_async并不知道self会在什么时候被释放，为了确保系统调度执行Block中的任务时self没有被意外释放掉，dispatch_async必须自己retain一次self，任务完成后再release self。但这里使用
__block
，使dispatch_async没有增加self的引用计数，这使得在系统在调度执行Block之前，self可能已被销毁，但系统并不知道这个情况，导致Block被调度执行时self已经被释放导致crash。

<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">10</span>
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">11</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">12</span>
// MyClass.m
- (void) test {
__block MyClass* weakSelf = self;
double delayInSeconds = 10.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
NSLog(@"%@", weakSelf);
});

// other.m
MyClass* obj = [[[MyClass alloc] init] autorelease];
[obj test];

这里用dispatch_after模拟了一个异步任务，10秒后执行Block。但执行Block的时候
MyClass*
obj
已经被释放了，导致crash。解决办法是不要使用
__block
。
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