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

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        //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: 搜一句補充的話

//forin 迴圈中的便利內容不能被改變, 是因為如果改變其便利的內容會少一個, 而系統是不會允許這個發生的所以就會crash...但是當改變最後一個的內容時, 就不會crash, 是因為此時遍曆已經結束, 結束之後對內容做修改是允許的 。

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