Bug-iOS: Collection <__NSArrayM: 0x> was mutated while being enumerated.

//init a NSArray
NSMutableArray *arr2 = [NSMutableArray array];
for (int i=0; i<10; ++i) {
NSMutableDictionary *d = [NSMutableDictionary dictionaryWithObjectsAndKeys:[NSString stringWithFormat:@"%d", i], @"a", nil];
[arr2 addObject:d];
}

//Wrong
for (NSMutableDictionary *s in arr2) {
if ([s[@"a"] isEqualToString:@"3"]) {
[arr2 removeObject:s]; //while here is safe for s[@"a"] = @"eeee";
}
}

//Safe 1.
[arr2 enumerateObjectsUsingBlock:^(NSMutableDictionary *obj, NSUInteger idx, BOOL *stop) {
if ([obj[@"a"] isEqualToString:@"3"]) {
*stop = YES;
[arr2 removeObject:obj]; // or obj[@"a"] = @"4ewewewe";
}
}];

//Safe 2.
NSArray *tmp = [NSArray arrayWithArray:arr2];
for (NSMutableDictionary *s in tmp) {
if ([s[@"a"] isEqualToString:@"3"]) {
s[@"a"] = @"eeee";
[arr2 removeObject:s];
}
}

PS: 搜一句补充的话

//forin 循环中的便利内容不能被改变, 是因为如果改变其便利的内容会少一个, 而系统是不会允许这个发生的所以就会crash...但是当改变最后一个的内容时, 就不会crash, 是因为此时遍历已经结束, 结束之后对内容做修改是允许的 。