一个用Block实例变量语法，当有一个参考的实例变量，常引起retain cycle。

capturing self strongly in this block is likely to lead to a retain cycle

_items = [[NSMutableArray alloc] init];      _block = ^{          [_items addObject:@"Hello!"]; //_block引用了_items，导致retain cycle。      };

写成以下格式

__block ViewController *blockSelf = self;  _block = ^{      [blockSelf->_items addObject:@"Hello!"];  };

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