ObjC中的消息传递机制小结

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引子

本文是学习ObjC中的消息传递机制的一篇小结小结

KVO

  1. One to Many: 一个对象的value changes 可以被多个对象监听
  2. 但recipient 必须知道 sender 的生命周期，因为必须在sender对象deallcated之前unregister the observer
  3. 在Core Data 中使用要注意 Core Data的Faulting 机制，因为managed对象变成fault时也会引发KVO监听

Notifications

  1. Broadcast: sender 和 recipient 不需要知道彼此的生命周期，低相关性；
  2. 可以通过userInfo传递数据

Delegation

  1. 通过函数参数传递数据

  2. 松耦合，使用广泛

    Delegation is very flexible and straightforward communication pattern if you only need to communicate between two specific objects.

  3. 紧耦合不适用 ，容易造成循环引用？

Blocks

  1. 潜在隐患：

    if the sender needs to retain the block and cannot guarantee that this reference will be a nilled out, the every reference to self from within the block becomes a potential retain cycle.
    如

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    self.myTableView.selectionHandler = ^void(NSIndexPath *selectedIndexPath) {
        // handle selection ...
    };

    而 如下就不会，原因是operation 执行完后会立即释放

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self.queue = [[NSOperationQueue alloc] init];
MyOperation *operation = [[MyOperation alloc] init];
operation.completionBlock = ^{
    [self finishedOperation];
};
[self.queue addOperation:operation];

Target-Action

  1. Target-Action is the typical pattern used to send messages in response to user-interface events.
  2. A limitation of target-action-based communication is that the messages sent cannot carry any custom payloads.
  3. 非常松耦合，target 和 action 不用知道对方的存在，甚至target=nil ,Action 会沿着响应链一直传递直到找到响应者。On iOS, each control can even be associated with multiple target-action pairs.

参考资料

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