Pausing an NSTimer

Question

I have a UIView with a UITableView and a UIImageView in it. The UITableView takes up the top half of the UIView

Answer 1

To get access to the thread from outside the startTimerThread method, you should add a property to your class. I would suggest property over ivar because you can have atomic accessors synthesized automatically. What you do with the timer depends on whether you want to pause or stop it.

Stopping the timer means the next time you start it, it will start with the full 3 seconds remaining. This means if the timer will fire in 1 second when you stop it, it will still take 3 seconds to fire after you start it again. However, stopping is very simple. A stopTimerThread would be defined as follows:

- (void)stopTimerThread {
    NSTimer *theTimer = self.timer;
    [theTimer invalidate];
    self.timer = nil;
}

Since the runloop in your other thread now has no sources, it will automatically exit and the thread will end, so you can use startTimerThread when you want to start it again.

Pausing the timer means that, if it will fire in 1 second when you pause it, it will fire 1 second after you restart it. To do this, you will also need a property containing the time remaining before the timer fires, which will be set in stopTimerThread and used in startTimerThread. The implementation of stopTimerThread is as follows:

- (void)stopTimerThread {
    NSTimer *theTimer = self.timer;
    NSDate *fireDate = [theTimer fireDate];
    [theTimer invalidate];
    self.timerTimeRemaining = - [fireDate timeIntervalSinceNow];
    self.timer = nil;
}

Notice that the time remaining is set to be the negative of the time interval, since the fire date will be before now. You will also have to modify startTimerThread to take the time remaining into account when setting up the timer.

- (void)startTimerThread {
    // set up autorelease pool, get run loop
    NSTimeInterval timeLeft = self.timerTimeRemaining;
    if(timeLeft <= 0.0) timeLeft = 3.0;
    NSDate *fireDate = [NSDate dateWithTimeIntervalSinceNow:timeLeft];
    NSTimer *theTimer = [[NSTimer alloc] initWithFireDate:fireDate interval:3.0 target:self selector:@selector(loadNextPhoto) userInfo:nil repeats:YES];
    [runLoop addTimer:theTimer forMode:NSDefaultRunLoopMode];
    self.timer = theTimer;
    [theTimer release];
    [runLoop run];
    // clean up
}