在iPhone上排队和处理多个请求

在我的应用程序中,我使用nsOperationQueue和nsInvocationOperation对象来执行所有操作。我的应用程序中有多个队列,我正在使用kvo“isfinished”,调用它时,我将在主线程上执行下一个操作。

问题是:每当我一个接一个地执行两个操作时,应用程序崩溃,并说:

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '<myViewController: 0x481b200>: An     -observeValueForKeyPath:ofObject:change:context: message was received but not handled.
Key path: isFinished
Observed object: <NSInvocationOperation: 0x45d9ea0>
Change: {
 kind = 1;
}
Context: 0x0'

我的一般代码如下:

operation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(somemethod) object:nil];

[operation addObserver:self forKeyPath:@"isFinished" options:0 context:nil];

[operationQueue addOperation:operation];
[operation release];    

- (void)observeValueForKeyPath:(NSString *)keyPath 
                        ofObject:(id)object 
                           change:(NSDictionary *)change 
                         context:(void *)context
{
   if([keyPath isEqual:@"isFinished"] && operation == object)
 {


    [operation removeObserver:self forKeyPath:@"isFinished"];
     [self performSelectorOnMainThread:@selector(newerPostLoadingNumberOfUdates) withObject:nil waitUntilDone:YES];

 } 


 else 
 {
     [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
 }
}

我想知道将请求的操作排队并相应地执行它们的最佳实践是什么?

提前完成。