iOS之《Effective Objective-C 2.0》读书笔记(42)
2017-09-08 11:18
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第42条:多用GCD,少用performSelector系列方法
1.直接调用某方法
两种方式://方法一: -(id)performSelectoer:(SEL)selector //方法二: [object selectorName];
方法一的缺点:内存管理容易有缺失–>selector是在running time才决定的,这就是它的强大之处。这就等于在动态绑定之上再次使用动态绑定,因而可以实现出下面这种功能
SEL selector; if ( / some condition / ) { selector = @selector(newObject);//newObject返回一个new object } else if ( / some other condition / ) { selector = @selector(copy); // copy根据当前object copy出一个新的object } else { selector = @selector(someProperty));// someProperty可以认为是对象的某个property } id ret = [object performSelector:selector];
2.发消息时顺便传递参数
此方法使用的局限性:(1)由于参数类型是id,所以传入的参数必须是对象才行。如果选择子所接受的参数是整数或浮点数,那就不能采用这些方法了
(2)选择子最多只能接受两个参数,在参数不止两个的情况下,则没有对应的performSelector方法能够执行这种选择子
-(id)performSelector:(SEL)selector withObject:(id)object; -(id)performSelector:(SEL)selector withObject:(id)objectA withObject:(id)objectB;
3.提供的线程功能
优先使用GCD//方法一: -(void)performSelector:(SEL)selector onThread:(NSThread *)thread waitUntilDone:(BOOL)wait; -(void)performSelectorOnMainThread:(SEL)selector withObject:(id)argument waitUnitlDone:(BOOL)wait; //方法二: dispatch_async(dispatch_get_main_queue(), ^{ [self doSomething]; });
4.延迟执行:
优先使用GCD//方法一: -(void)performSelector:(SEL)selector withObject:(id)argument afterDelay:(NSTimeInterval)delay; //方法二: dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5.0 * NSEC_PER_SEC)); dispatch_after(time, dispatch_get_main_queue(), ^{ [self doSomething]; });
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