NSTimer Troubles

Question

I am trying to run the code below but it keeps locking up my simulator after the "Tick" is written to the console. It never outputs "Tock" so my guess is that it has to do with the line "NSTimeInterval elapsedTime = [startTime timeIntervalSinceNow];" The IBactions are activated by buttons. timer and startTime are defined in the .h as NSTimer and NSDate respectively.

Can anyone tell me what I am doing wrong?

code:

- (IBAction)startStopwatch:(id)sender
{
    startTime = [NSDate date];
    NSLog(@"%@", startTime);
    timer = [NSTimer scheduledTimerWithTimeInterval:1 //0.02
                                             target:self
                                           selector:@selector(tick:)
                                           userInfo:nil
                                            repeats:YES];
}

- (IBAction)stopStopwatch:(id)sender
{
    [timer invalidate];
    timer = nil;
}

- (void)tick:(NSTimer *)theTimer
{
    NSLog(@"Tick!");
    NSTimeInterval elapsedTime = [startTime timeIntervalSinceNow];
    NSLog(@"Tock!");
    NSLog(@"Delta: %d", elapsedTime);
}

I have the following in the .h:

@interface MainViewController : UIViewController <FlipsideViewControllerDelegate> {

    NSTimer *timer;
    NSDate *startTime;
}


- (IBAction)startStopwatch:(id)sender;
- (IBAction)stopStopwatch:(id)sender;
- (void)tick:(NSTimer *)theTimer;

@property(nonatomic, retain) NSTimer *timer;
@property(nonatomic, retain) NSDate *startTime;

@end

Answer 1

Where you have:

startTime = [NSDate date];

You need:

startTime = [[NSDate date] retain];

Anything that is created with out an alloc, new, init will be auto-released (rule of thumb). So what is happening is you are creating the NSDate, assigning it to startTime, it's getting auto-released (destroyed), then you are trying to call timeIntervalSinceNow on an object that was fully released so it blows up.

Adding the retain increased the retain count so it still sticks around after the auto-release. Don't forget to manually release it when you're done with it though!

Answer 2

To take advantage of the @property you need to do: self.startTime = [NSDate date]