Multi-Threading question in Objective-C 2.0

Question

I have my main application delegate which contains a method that returns an object. This application delegate runs on the main thread.

I also have a NSOperation that get

Answer 1

Do you absolutely need to perform this invocation on the NSOperation thread, and not simply provide the required object as part of creating the custom operation?

If so, I would recommend not using locks unless performance is critical. If the iPhone supported it, you could use Grand Central Dispatch to get the object onto your thread:

__block id newObject = nil;
dispatch_sync(dispatch_get_main_queue(), ^{
    newObject = [[[[UIApplication sharedApplication] delegate] myMethod] retain];
});

For the iPhone, I would be tempted to create a helper method:

- (void)createNewObject:(NSValue *)returnPtr {
    id newObject = [[[[UIApplication sharedApplication] delegate] myMethod] retain];
    *(id *)[returnPtr pointerValue] = newObject;
}

And invoke it like so from your NSOperation thread:

id newObject = nil;
[self performSelectorOnMainThread:@selector(createNewObject:)
                       withObject:[NSValue valueWithPointer:&newObject]
                    waitUntilDone:YES];

Actually performing the execution on the main thread has fewer implicit risks.