AC在应用中大量使用了block，由于Objective-C语言的内存管理是基于引用计数的，为了避免循环引用问题，在block中如果要引用self，需要使用@weakify(self)和@strongify(self)来避免强引用。
一、block的循环引用问题
- - (void)loadView
- {
- [superloadView];
- _observer = [[NSNotificationCenter defaultCenter] addObserverForName:@"testKey"
- object:nil
- queue:nil
- usingBlock:^(NSNotification *note) {
- [self dismissModalViewControllerAnimated:YES];
- }];
- }
- - (void)dealloc
- {
- [[NSNotificationCenter defaultCenter] removeObserver:_observer];
- }
代码分析：
在上面代码中，我们添加向通知中心注册了一个观察者，然后在 dealloc 时解除该注册，一切看起来正常。但这里有两个问题：
这段代码中涉及到的对象包括：notificationcenter, _observer, block, self.
a) 在消息通知 block 中引用到了 self，所以这里 self 对象被 block retain；而 _observer 又对该 block 进行retain，通知中心 notificationcentre 又持有 _observer。因此只要 _observer 对象还没有被解除注册，block 就会一直被持有，从而 self 就不会被释放，那么 dealloc 就不会被调用。而我们却又期望在 dealloc 中通过 removeObserver 来解除注册以消除通知中心 notificationcenter 对 _observer 的 retain。
小结：notificationcenter --> _observer --> block --> self 只有在 self 释放，dealloc 调用的时候，notificationcenter 才会释放 _observer，显然其中存在循环引用。
b) 同时，_observer 是在 self 所在类中定义赋值，因此是被 self retain 的，这样就形成了循环引用。
小结： self --> _observer --> block --> self 显然这也是一个循环引用。
二、Weak-Strong Dance
对于在block中的retain cycle，在2011 WWDC Session #322 (Objective-C Advancements in Depth)有一个解决方案weak-strong dance，很漂亮的名字。其实现如下：
- - (void)dealloc
- {
- [[NSNotificationCenter defaultCenter] removeObserver:_observer];
- }
- - (void)loadView
- {
- [superloadView];
- __weak TestViewController *wself = self;
- _observer = [[NSNotificationCenter defaultCenter] addObserverForName:@"testKey"
- object:nil
- queue:nil
- usingBlock:^(NSNotification *note) {
- TestViewController *sself = wself;
- [sself dismissModalViewControllerAnimated:YES];
- }];
- }
这样的话就可以打破循环引用了。
当然，__weak 和 __strong 只在 ARC 情形下有效；对于非 ARC ，就需要用到 __block 了，效果相同，如下：
- - (void)dealloc
- {
- [[NSNotificationCenter defaultCenter] removeObserver:_observer];
- [_observer release];
- [superdealloc];
- }
- - (void)loadView
- {
- [superloadView];
- __block TestViewController *bself = self;
- _observer = [[NSNotificationCenter defaultCenter] addObserverForName:@"testKey"
- object:nil
- queue:nil
- ngBlock:^(NSNotification *note) {
- [bself retain];
- [bself dismissModalViewControllerAnimated:YES];
- [bself release];
- }];
- }
三、ReactiveCocoa中的Weak-Strong Dance
例如：
- @weakify(self);
- [RACObserve(self,photosArray) subscribeNext:^(id x){
- @strongify(self);
- [self.collectionView reloadData];
- }];
RACObserver is a C macro that takes two parameters: an object and a key path on that object. It returns a signal whose values are sent whenever the key path’s value changes. A completion value is sent when the object, in this case self, is deallocated. --> ? We subscribe to this signal in order to reload our collection view whenever our photosArray property is changed.
译注：RACObserver 是一个宏定义，有两个参数：an object and a key path on that object。当 object key path value 变化时，就会返回一个 signal。
我们对这个 signal 进行订阅，一旦 photoArray 属性发送变化，返回signal，就可以 reload collection view。
The weakify/strongify dance is all too common in Objective-C under ARC.Weakify creates a new, weak variable assigned to self. Strongify then creates a new, strong variable in its scope assigned to the weak self. When strongify does this, it’s using what’s called a “shadow variable” – so named because the new, strong variable is called self, replacing the former strong reference to self.
Basically, the subscribeNext: block is going to capture self in its lexical scope, causing a reference cycle between self and the block. The block is strongly referenced by the return value of subscribeNext:, a RACSubscriber instance. This is then captured by the RACObserver macro, which will be automatically deallocated once its first parameter, self is deallocated. Without the weakify/strongify invocations, self would never be deallocated.
译注：分析一下其中可能存在的 block 循环引用问题。
self --> RACObserver macro --> RACSubscriber instance --> block --> self. 假如不使用 weakify/strongify 那么现实其中的循环引用导致 self 始终无法释放。
最后友情提示：在使用时应该注意block的嵌套层数，不恰当的滥用多层嵌套block可能给程序的可维护性带来灾难。