Objective-C enumerateObjectsUsingBlock vs 快速枚举？

Question

以下两种方式的优缺点是什么:

enumerate using block

NSArray *myArray = [NSArray new];
[myArray enumerateObjectsUsingBlock:^(id anObject, NSUInteger idx, BOOL *stop) {
    if (anObject == someOtherObject) {
        [anObject doSomething:idx];
        *stop = YES;
    }
}];

fast enumeration

NSArray *myArray = [NSArray new];
int idx = 0
for (id anObject in myArray) {
    if (anObject == someOtherObject) {
        [anObject doSomething:idx];
        break;
    }
    ++idx;
}

Answer 1

This blog post涵盖主要差异。总结:

• 快速枚举在 OS X 10.5+ 上可用， block 在 10.6+ 上可用
• 对于简单枚举，快速枚举比分块枚举快一点
• 使用基于 block 的枚举比使用快速枚举更容易(并且更高效)进行并发或反向枚举
• 在 NSDictionary 上枚举时，您可以使用基于 block 的枚举器一次性获取键和值，而对于快速枚举，您必须使用键在单独的消息发送中检索值.

关于最后一点(NSDictionary枚举)，对比一下:

for (id key in dictionary)
{
    id obj = [dictionary objectForKey: key];
    // do something with key and obj
}

为此:

[dictionary enumerateKeysAndObjectsUsingBlock: ^(id key, id obj, BOOL *stop) {
    // do something with key and obj
}];

此外，这两种方法都可以防止可变集合在枚举循环内发生突变。有趣的是，如果您尝试在基于 block 的枚举中改变集合，您会收到 CoreFoundation 的 __NSFastEnumerationMutationHandler 抛出的异常，这表明幕后有一些通用代码。

 NSMutableArray *myArray = [NSMutableArray arrayWithObjects:@"a", @"b", nil];
 [myArray enumerateObjectsUsingBlock:^(id anObject, NSUInteger idx, BOOL *stop) {
     // Attempt to mutate the array during enumeration
     [myArray addObject:@"c"];
 }];

输出:

2011-12-14 22:37:53.716 Untitled[5809:707] *** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <__NSArrayM: 0x109614190> was mutated while being enumerated.'
*** First throw call stack:
(
    0   CoreFoundation                      0x00007fff8cca7286 __exceptionPreprocess + 198
    1   libobjc.A.dylib                     0x00007fff8319ad5e objc_exception_throw + 43
    2   CoreFoundation                      0x00007fff8cd311dc __NSFastEnumerationMutationHandler + 172
    3   CoreFoundation                      0x00007fff8cc9efb4 __NSArrayEnumerate + 612
    4   Untitled                            0x00000001094efcea main + 250
    5   Untitled                            0x00000001094efbe4 start + 52
    6   ???                                 0x0000000000000001 0x0 + 1
)
terminate called throwing an exceptionRun Command: line 1:  5809 Abort trap: 6           ./"$2"