Question

I’ve used both GCD and performSelectorOnMainThread:waitUntilDone in my apps, and tend to think of them as interchangeable–that is, performSelectorOnMainThread:waitUntilDone is an Obj-C wrapper to the GCD C syntax. I’ve been thinking of these two commands as equivalent:

dispatch_sync(dispatch_get_main_queue(), ^{ [self doit:YES]; });


[self performSelectorOnMainThread:@selector(doit:) withObject:YES waitUntilDone:YES];

Am I incorrect? That is, is there a difference of the performSelector* commands versus the GCD ones? I’ve read a lot of documentation on them, but have yet to see a definitive answer.

Answer 1

performSelectorOnMainThread: does not use GCD to send messages to objects on the main thread.

Here’s how the documentation says the method is implemented:

- (void) performSelectorOnMainThread:(SEL) selector withObject:(id) obj waitUntilDone:(BOOL) wait {
  [[NSRunLoop mainRunLoop] performSelector:selector target:self withObject:obj order:1 modes: NSRunLoopCommonModes];
}

And on performSelector:target:withObject:order:modes:, the documentation states:

This method sets up a timer to perform the aSelector message on the current thread’s run loop at the start of the next run loop iteration. The timer is configured to run in the modes specified by the modes parameter. When the timer fires, the thread attempts to dequeue the message from the run loop and perform the selector. It succeeds if the run loop is running and in one of the specified modes; otherwise, the timer waits until the run loop is in one of those modes.