Call back to delegate using the main thread rather than secondary thread?

Question

I've defined a class called 'AsyncNetworkOperation', a subclass of NSOperation used to perform database queries in my app. My AsyncNetworkOperation class has a protocol, to be used by objects that initiate the AsyncNetworkOperation:

@protocol AsyncNetworkOperationDelegate
@optional - (void)operationAboutToFinish;
@required - (void)operationFinishedWithData:(id)retrievedData;
@required - (void)operationFinishedWithError:(NSError*)error;
@end

This all works fine, except in the case where the delegate I'm calling back to wants to show a UIAlertView, start an NSTimer, or do something else that secondary threads either aren't supposed to do, or require extra effort to do.

So here's my question: when I callback to the delegate, should I do so via -[NSObject performSelectorOnMainThread:]? It would seem that in doing so, I relieve my delegate from the burden of having to know whether it's being called back via a secondary or primary thread, thereby allowing it to implement functionality in the most straightforward way possible, with no limitations.

Just wondering about the standard way to approach this problem.

Thanks.

Answer 1

I would recommend having the AsyncNetworkOperation call the delegate object in the same thread that it was invoked on. So if the async operation was first invoked on the main thread, then it should call the delegate on the main thread!

You can imagine future use where the delegate doesn't need to update the UI and calling it from a secondary thread would be good.