Question

I have been having some crashes in my app. When checking the logs and using atos, it is telling me exactly where I get the crash, which is where I tell my NSRunLoop to run:

/**
 * Create a new thread for the timer
 *
 * @version $Revision: 0.1
 */
- (void)createTimerThread {
    NSThread *timerThread = [[NSThread alloc] initWithTarget:self selector:@selector(startTimerThread) object:nil];
    [timerThread start];
    [timerThread release];
}//end


/**
 * Start the actual timer
 *
 * @version $Revision: 0.1
 */
- (void)startTimerThread {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    NSRunLoop *runLoop = [NSRunLoop currentRunLoop];

    // Start timer
    self.countTimer = [NSTimer scheduledTimerWithTimeInterval:1.0f target:self selector:@selector(updateCounter:) userInfo:nil repeats:YES];

    [runLoop run];// <--- Crash happened here
    [pool release];

}//end


/**
 * Update the counter
 *
 * @version $Revision: 0.1
 */
- (void)updateCounter:(NSTimer *)theTimer {

    // Does tons of timer stuff here

}//end

As you can see, the crash happens on [runLoop run] but I have no idea why. It normally happens the second time that I call the createTimerThread method.

What am I doing wrong here? All I was wanting to do was run a timer in the background so that it wasn’t on the main thread because I needed to update a UILabel.

Should I be using something new like Grand Central Dispatch (GCD)?

Answer 1

You said updateCounter is updating a UILabel and that’s being called from the timer that’s running on the background thread. You can’t do that, you need to update UI views on the main thread.

You can either use performSelectorOnMainThread or GCD (dispatch to the main queue). I compared both using samples on this SO post:

GCD, Threads, Program Flow and UI Updating

This SO article specifically has an example of a bg timer with GCD:

iOS4 Create Background Timer

Look at the post from mrwalker