Check your thread

When you start using Grand Central Dispatch or NSOperation, you’ll want to perform some actions on the main thread and some intentionally off the main thread.

This is a simple and obvious technique, but it took me a while to adopt it: You can do by asserting with NSAssert or NSCAssert for [NSThread isMainThread], just as you would assert any other condition.

Like this:

- (void)updateUI {
    NSAssert([NSThread isMainThread], @"Only update UI from main thread");
    // update UI

- (void)backgroundWork {
    NSAssert(![NSThread isMainThread], @"Only do work off main thread");
    // do work

With assertions like this in place, you’ll sort out what executes where in short order.

I wish I had done this to my code right away; it’s much simpler to get it right than to go back and fix it all later.