将操作添加到队列时，NSOperationQueue（代理/回调/通知）

Question

有没有办法知道何时将某个操作添加到NSOperationQueue实例？ PS：目前，我不想对NSOperationQueue进行子类化并覆盖所需的addOperation API。

Answer 1

你可以定期检查operations属性，看看它是否改变了以前的引用（也许用一种运行的方法？）。如果确实如此，则检查是否添加了操作。

这样你就不需要子类NSOperationQueue或覆盖addOperation。

Answer 2

operationsNSOperationQueue的属性是KVO兼容的。这意味着您可以通过addObserverForKeyPath和observeValueForKeyPath方法观察此属性的内容何时更改。请看下面的代码片段：

@interface BigBrother : NSObject 
//Just a demo method for filling queue with something 
- (void) addSomeOperationsAndWaitTillFinished; 

@end 

@implementation BigBrother 


- (void) observeValueForKeyPath:(NSString *)keyPath 
         ofObject:(id)object 
         change:(NSDictionary<NSString *,id> *)change 
         context:(void *)context 
{ 
    //Triggers each time 'operations' property is changed 
    //(ex. operation is added or finished in this case) 
    //object is your NSOperationQueue 
    NSLog(@"Key path:%@\nObject:%@\nChange%@", keyPath, object, change); 

} 

- (void) addSomeOperationsAndWaitTillFinished 
{ 
    NSOperationQueue* queue = [[NSOperationQueue alloc] init]; 
    //Tell the queue we want to know when something happens to 'operations' property 
    [queue addObserver:self 
      forKeyPath:@"operations" 
       options:NSKeyValueObservingOptionNew 
       context:nil]; 

    for (NSUInteger opNum = 0; opNum < 5; opNum++) 
    { 
     [queue addOperationWithBlock:^{sleep(1);}]; 
    } 

    [queue waitUntilAllOperationsAreFinished]; 
} 

你可以看到它是如何工作的，加入这一行到某处：

[[[BigBrother alloc] init] addSomeOperationsAndWaitTillFinished]; 

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