Objective-C enumerateObjectsUsingBlock与快速枚举? [英] Objective-C enumerateObjectsUsingBlock vs fast enumeration?

问题描述

以下两种方法的优缺点是什么?

What are the advantages and disadvantages of the following two approaches:

使用块进行枚举

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;
}

推荐答案

This blog post covers the major differences. In summary:

• 快速枚举在OS X 10.5+上可用，块在10.6+上可用
• 对于简单枚举，快速枚举比基于块的枚举要快
• 使用基于块的枚举比使用快速枚举更容易(并且性能更高)
• 在NSDictionary上进行枚举时，您可以使用基于块的枚举数次获得键和值，而在快速枚举中，必须使用键在单独的消息发送中检索值.

• Fast enumeration is available on OS X 10.5+, blocks are available on 10.6+
• For simple enumeration, fast enumeration is a bit faster than block-based enumeration
• It's easier (and more performant) to do concurrent or reverse enumeration with block-based enumeration than with fast enumeration
• When enumerating over an NSDictionary you can get key and value in one hit with a block-based enumerator, whereas with fast enumeration you have to use the key to retrieve the value in a separate message send.

关于最后一点(NSDictionary枚举)，请进行以下比较:

Regarding the last point (NSDictionary enumeration), compare this:

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

对此:

[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，如果您尝试在基于块的枚举中对集合进行突变，则会得到CoreFoundation的__NSFastEnumerationMutationHandler引发的异常，这表明存在一些通用代码.

In addition, both methods protect mutable collections from mutation inside the enumeration loop. Interestingly, if you try to mutate the collection inside a block-based enumeration, you get an exception thrown by CoreFoundation's __NSFastEnumerationMutationHandler, suggesting that there's some common code under the hood.

 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"

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