创建NSMutableDictionary
使用构造函数是否明显更有效
[NSMutableDictionary dictionaryWithObjectsAndKeys:@"object1", @"key1", @2, @"key2", nil];
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而不是采取文字的可变副本
[@{@"key1":@"object1", @"key2":@2} mutableCopy];
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