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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