GCD, Threads, Program Flow and UI Updating

GCD and performSelectorInBackground samples below. But first, let’s look at your code.

You cannot wait where you want to in the code above.
Here’s the code you had. Where you say wait in the comment is incorrect. See where I added NO.

- (void) solvePuzzle:(id)sender{
    solveButton.enabled = NO;
    solveButton.title = @"Working . . . .";

    // I've tried using this as a Background thread, but I can't get the code to waitTilDone before continuing and changing the button state.
    [self performSelectorInBackground:@selector(createTreeFromNode:) withObject:rootNode];

    // NO - do not wait or enable here.
    // Need to wait here until createTreeFromNode is finished.
    solveButton.enabled=YES;

}

A UI message loop is running on the main thread which keeps the UI running. solvePuzzle is getting called on the main thread so you can’t wait – it will block the UI. It also can’t set the button back to enabled – the work hasn’t been done yet.

It is the worker function’s job on the background thread to do the work and then when it’s done to then update the UI. But you cannot update the UI from a background thread. If you’re not using blocks and using performSelectInBackground, then when you’re done, call performSelectorOnMainThread which calls a selector to update your UI.

performSelectorInBackground Sample:

In this snippet, I have a button which invokes the long running work, a status label, and I added a slider to show I can move the slider while the bg work is done.

// on click of button
- (IBAction)doWork:(id)sender
{
    [[self feedbackLabel] setText:@"Working ..."];
    [[self doWorkButton] setEnabled:NO];

    [self performSelectorInBackground:@selector(performLongRunningWork:) withObject:nil];
}

- (void)performLongRunningWork:(id)obj
{
    // simulate 5 seconds of work
    // I added a slider to the form - I can slide it back and forth during the 5 sec.
    sleep(5);
    [self performSelectorOnMainThread:@selector(workDone:) withObject:nil waitUntilDone:YES];
}

- (void)workDone:(id)obj
{
    [[self feedbackLabel] setText:@"Done ..."];
    [[self doWorkButton] setEnabled:YES];
}

GCD Sample:

// on click of button
- (IBAction)doWork:(id)sender
{
    [[self feedbackLabel] setText:@"Working ..."];
    [[self doWorkButton] setEnabled:NO];

    // async queue for bg work
    // main queue for updating ui on main thread
    dispatch_queue_t queue = dispatch_queue_create("com.sample", 0);
    dispatch_queue_t main = dispatch_get_main_queue();

    //  do the long running work in bg async queue
    // within that, call to update UI on main thread.
    dispatch_async(queue, 
                   ^{ 
                       [self performLongRunningWork]; 
                       dispatch_async(main, ^{ [self workDone]; });
                   });    
}

- (void)performLongRunningWork
{
    // simulate 5 seconds of work
    // I added a slider to the form - I can slide it back and forth during the 5 sec.
    sleep(5);
}

- (void)workDone
{
    [[self feedbackLabel] setText:@"Done ..."];
    [[self doWorkButton] setEnabled:YES];
}