Proper usage of NSInvocationOperation and NSOperationQueue

Question

I have set up an operation queue and an invocation operation. Do I need to signal that the invocation is commpleted? If not how will the operation queue knows the invocation is finished and move on to the next one? The operation queue has been set to execute one operation at a time.

Answer 1

No, there is no need to signal that the invocation has been completed. An NSOperationQueue knows that an operation is finished when its isFinished property is set to YES. This happens by default when the operation's -main method returns.

NSInvocationOperation's -main method, for all intents and purposes, just invokes its NSInvocation and returns, so its isFinished flag should be set to YES immediately after the invocation completes.

Answer 2

Boon,

It seems like what you really want here is to subclass NSOperation yourself and call your asynchronous inside of it. When the async code completes and you get your callback, you would then notify the queue via KVO that isExecuting and isFinished are updated. This is explained much more in detail over at Dave Dribin's blog:

http://www.dribin.org/dave/blog/archives/2009/05/05/concurrent%5Foperations/

Answer 3

It's automatic for NSInvocationOperation. You're already good to go.

If you need to tell other parts of your app that the operation has completed, you can use a notification. Be sure the notification goes to the right thread. On the iPhone, I send them to the main thread because I often change the UI in response to a notification, and all UI stuff must happen on the main thread.

[self performSelectorOnMainThread:@selector(postOpDoneNote) withObject:nil waitUntilDone:NO];

-(void) postOpDoneNote
{
    [[NSNotificationCenter defaultCenter] postNotificationName:@"someOpDone" object:self];
}