iphone - 快速枚举中的突变？

Question

实际上我收到了一个奇怪的异常:我迭代了一个 MutableDictionary 并想在其中设置一个新值:

    selectionIndex = [NSMutableDictionary dictionaryWithDictionary:selection];
    NSString *whatever = @"999999";
    id keys;
    NSEnumerator *keyEnum = [selectionIndex keyEnumerator];

    while (keys = [keyEnum nextObject]) 
    {
        [selectionIndex setObject:whatever forKey:keys];
    }

顺便说一句，选择，传递给这个方法的是一个 MutableDictionary。如果我运行这段代码，我会收到以下异常:

2011-12-05 15:28:05.993 lovelini[1333:207] * Terminating app due to uncaught exception 'NSGenericException', reason: '* Collection <__NSCFDictionary: 0x6a33ed0> was mutated while being enumerated.{type = mutable dict, count = 8, entries => 0 : {contents =

好吧，我知道我无法更改 NSDictionary，但据我所知，我没有!那么为什么我会得到这个异常呢？这是快速枚举的限制吗？？？据我所知，无法在快速枚举中添加或删除条目，但我没有添加或删除任何内容？!

Answer 1

您不能在枚举集合时对其进行任何更改。您可以枚举字典的键而不是字典本身:

for (NSString *key in selectionIndex.allKeys) {
    [selectionIndex setObject:whatever forKey:key];
}