参考：http://stackoverflow.com/questions/1049001/get-notification-when-nsoperationqueue-finishes-all-tasks
多线程编程中，操作队列NSOperationQueue我们经常会用到的，简化了多线程的操作。至于用法就不多介绍了。这里要说的是队列执行完毕的状态检查。
我们很多时候需要在队列完成之后再进行操作，而何时队列完成，NSOperationQueue并没有内置的didFinishedSelector来供使用，因此需要自己去检查其状态。
因为NSOperationQueue兼容 key-value coding (KVC) and key-value observing (KVO)机制，因此我们可以观察NSOperationQueue的属性。NSOperationQueue可供监控观察的属性有：
-
operations- read-only property -
operationCount- read-only property -
maxConcurrentOperationCount- readable and writable property -
suspended- readable and writable property -
name- readable and writable property
实现如下：
1.初始_parseQueue
- - (NSOperationQueue *)parseQueue
- {
- if (nil == _parseQueue)
- {
- _parseQueue = [[NSOperationQueue alloc] init];
- [_parseQueue setSuspended:YES];
- //[_parseQueue setMaxConcurrentOperationCount:1];
- [_parseQueue addObserver:self
- forKeyPath:@"operations"
- options:0
- context:nil];
- }
- return _parseQueue;
- }
2.加入operation
- NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self
- selector:@selector(myTask)
- object:nil];
- [self.parseQueue addOperation:operation];
- [operation release];<strong>
- rong>
3.观察值的改变：
- //KVO,观察parseQueue是否执行完
- - (void)observeValueForKeyPath:(NSString *)keyPath
- ofObject:(id)object
- change:(NSDictionary *)change
- context:(void *)context
- {
- if (object == self.parseQueue && [keyPath isEqualToString:@"operations"])
- {
- if (0 == self.parseQueue.operations.count)
- {
- DLog(@"parse finished");
- //other operation
- [_parseQueue setSuspended:YES];
- }
- }
- else
- {
- [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
- }
- }
选择对NSOperationQueue的operations进行观察，而不选operationCount是因为operationCount需要iOS 4.0以上