KVO 两种方式支持属性改变通知发送

Two techniques Ensuring the Changing Notification Emitted

官方文档

There are two techniques for ensuring the change notifications are emitted.

Automatic Change Notification

NSObject provides a basic implementation of automatic key-value change notification. Automatic key-value change notification informs observers of changes made using key-value compliant accessors, as well as the key-value coding methods. Automatic notification is also supported by the collection proxy objects returned by, for example, mutableArrayValueForKey:

NSObject 提供了当属性值发生改变时自动发送通知的机制。

那什么时候自动发送通知呢？

• 使用遵守KVC的属性访问器
• 使用 KVC 方法，如 setValue:forKey: or setValue:forKeyPath:
• mutableArrayValueForKey: 返回的代理集合对象也支持自动发送通知。

Examples of method calls that cause KVO change notifications to be emitted

// Call the accessor method.
[account setName:@"Savings"];
 
// Use setValue:forKey:.
[account setValue:@"Savings" forKey:@"name"];
 
// Use a key path, where 'account' is a kvc-compliant property of 'document'.
[document setValue:@"Savings" forKeyPath:@"account.name"];
 
// Use mutableArrayValueForKey: to retrieve a relationship proxy object.
Transaction *newTransaction = <#Create a new transaction for the account#>;
NSMutableArray *transactions = [account mutableArrayValueForKey:@"transactions"];
[transactions addObject:newTransaction];

若不用 mutableArrayValueForKey: 返回的代理对象，addObject: 不会自动发送改变通知。

Manual Change Notification

背景：

In some cases, you may want control of the notification process, for example, to minimize triggering notifications that are unnecessary for application specific reasons, or to group a number of changes into a single notification. Manual change notification provides means to do this.

有的时候你想要控制通知的过程。

有的时候你想屏蔽一些没必要的通知，例如：当且仅当属性值不同时，才发送通知；观察者只注册一个 key，当一组 keys 发生变化时，通知注册了 key 的观察者，免去了需观察多个 key。

实现

• 重写 NSObject 的 +automaticallyNotifiesObserversForKey:，对于不启用自动通知的 keys，返回 NO，对于不处理的 keys，调用 super。

  + (BOOL)automaticallyNotifiesObserversForKey:(NSString *)theKey {
  	BOOL automatic = NO;
      if ([theKey isEqualToString:@"balance"]) {
      	automatic = NO;
  	}
  	else {
      	automatic = [super automaticallyNotifiesObserversForKey:theKey];
  	}
  	return automatic;
  }
  

• 手动调用 -willChangeValueForKey, -didChangeValueForKey: 触发通知

  - (void)setBalance:(double)theBalance {
      [self willChangeValueForKey:@"balance"];
  	_balance = theBalance;
  	[self didChangeValueForKey:@"balance"];
  }
  

• to-many relationships 不仅仅要指定 key，还要指定改变的类型，改变元素的 indexes

  - (void)removeTransactionsAtIndexes:(NSIndexSet *)indexes {
      [self willChange:NSKeyValueChangeRemoval
      valuesAtIndexes:indexes forKey:@"transactions"];
  
  	// Remove the transaction objects at the specified indexes.
  
  	[self didChange:NSKeyValueChangeRemoval
      valuesAtIndexes:indexes forKey:@"transactions"];
  }
  

Manual Change Notification 优化点

You can minimize sending unnecessary notifications by first checking if the value has changed.

- (void)setBalance:(double)theBalance {
    if (theBalance != _balance) {
        [self willChangeValueForKey:@"balance"];
        _balance = theBalance;
        [self didChangeValueForKey:@"balance"];
    }
}

group a number of changes into a single notification

 (void)setBalance:(double)theBalance {
    [self willChangeValueForKey:@"balance"];
    [self willChangeValueForKey:@"itemChanged"];
    _balance = theBalance;
    _itemChanged = _itemChanged+1;
    [self didChangeValueForKey:@"itemChanged"];
    [self didChangeValueForKey:@"balance"];
}