写高质量OC代码52建议总结：48.多用块枚举，少用for循环

以下是for循环针对NSArray，NSSet，NSDictionary的遍历。字典和set都是无序的，无法根据特定的整数下标直接访问其值。遍历set和dictionary都需要而外创建数组储存对象和key。这个中介数组增加了不必要的而外开支。

-(void)demoforArray{
NSArray *anArray = /***/;
for (int i = 0; i < anArray.count; i++) {
id object = anArray[i];
// do something
}
}

-(void)demoforSet{
NSSet *aSet = /***/;
NSArray *objects = [aSet allObjects];
for (int i = 0; i < objects.count; i++) {
id object = object[i];
// do something
}
}

-(void)demoforDic{
NSDictionary *aDictionary = /***/;
NSArray *keys = [aDictionary allKeys];
for (int i = 0; i < keys.count; i++) {
id key = keys[i];
id value = aDictionary[key];
// do something
}
} 以下是通过NSEnumerator遍历，nextObject方法可以返回枚举里的下一个对象，等到所有数据都已经遍历，该方法会返回nil。reverseObjectEnumerator返回数组反向排列。-

-(void)demoNSEnumeratorForArray{
NSArray *anArray = /***/;
NSEnumerator *enumerator = [anArray objectEnumerator];
id object;
while ((object = [enumerator nextObject]) != nil) {
//  do something
}
}

-(void)demoNSEnumeratorForSet{
NSSet *anSet = /***/;
NSEnumerator *enumerator = [anSet objectEnumerator];
id object;
while ((object = [enumerator nextObject]) != nil) {
//  do something
}
}

-(void)demoNSEnumeratorForDic{
NSDictionary *anDic = /***/;
NSEnumerator *enumerator = [anDic keyEnumerator];
id key;
while ((key = [enumerator nextObject]) != nil) {
//  do something
}
}

以下是快速遍历方法
-(void)demoFastForArray{
NSArray *anArray = /***/;
for (id object in anArray) {
// do something
}
}

-(void)demoFastForSet{
NSSet *anSet = /***/;
for (id object in anSet) {
// do something
}
}

-(void)demoFastForDic{
NSDictionary *anDic = /***/;
for (id key in anDic) {
id value = anDic[key];
// do something
}
} 以下是基于块的便利方式

 -(void)enumerateObjectsUsingBlock:(void(^)(id object, NSUInteger idx, BOOL *stop))block;

 前两个参数，分别提供了所针对的对象和下标，第三个参数可以终止便利操作。
-(void)demoBlockForArray{
NSArray *anArray = /***/;
[anArray enumerateObjectsUsingBlock:^(id _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
// do something
if (....) {
*stop = YES;
}
}];
}

-(void)demoBlockForSet{
NSSet *aSet = /***/;
[aSet enumerateObjectsUsingBlock:^(id _Nonnull obj, BOOL * _Nonnull stop) {
// do something
if (...) {
*stop = YES;
}
}];
}

-(void)demoBlockForDic{
NSDictionary *dic = /***/;
[dic enumerateKeysAndObjectsUsingBlock:^(id _Nonnull key, id _Nonnull obj, BOOL * _Nonnull stop) {
// do something
if (shouldStop) {
*stop = YES;
}

4000
}];
}
 反向遍历:NSEnumerationReverse，数组和字典都对应两个方法

 - (void)enumerateObjectsWithOptions:(NSEnumerationOptions)opts usingBlock:(void (NS_NOESCAPE ^)(ObjectType obj, NSUInteger idx, BOOL *stop))block NS_AVAILABLE(10_6, 4_0);

 - (void)enumerateKeysAndObjectsWithOptions:(NSEnumerationOptions)opts usingBlock:(void (NS_NOESCAPE ^)(KeyType key, ObjectType obj, BOOL *stop))block NS_AVAILABLE(10_6, 4_0);

 

 typedef NS_OPTIONS(NSUInteger, NSEnumerationOptions) {

     NSEnumerationConcurrent = (1UL << 0),

     NSEnumerationReverse = (1UL << 1),

 };

 

 总结：

 1.遍历collection有四种方式，最基本的是for循环，其次是NSEnumerator遍历法，快速遍历法，块枚举法。

 2.s块枚举发本身就可以通过GCD来并发执行遍历操作。