Objective-C计时器不会失效

Question

Here is my situation: I am making a countdown app which works fine but does not appear to stop when I call [stopWatchTimer invalidate]; , and I have no idea why. Here is my code:

- (IBAction)btnStartPressed:(id)sender {
//Start countdown with the time on the Date Picker.

    timeLeft = [pkrTime countDownDuration];

    [self currentCount];

    lblTimer.text = time; //sets the label to the time set above

    pkrTime.hidden = YES;
    btnStart.hidden = YES;
    btnStop.hidden = NO;

    //Fire this timer every second.
    stopWatchTimer = [NSTimer scheduledTimerWithTimeInterval:1.0/1.0
                                                  target:self
                                                selector:@selector(reduceTimeLeft:)
                                                userInfo:nil
                                                 repeats:YES];
}
- (void)reduceTimeLeft:(NSTimer *)timer {
    //Countown timeleft by a second each time this function is called
    timeLeft--;
    // Get the system calendar
    NSCalendar *sysCalendar = [NSCalendar currentCalendar];

    // Create the NSDates
    NSDate *date1 = [[NSDate alloc] init];
    NSDate *date2 = [[NSDate alloc] initWithTimeInterval:timeLeft sinceDate:date1];

    // Get conversion to months, days, hours, minutes
    unsigned int unitFlags = NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit;

    NSDateComponents *conversionInfo = [sysCalendar components:unitFlags fromDate:date1  toDate:date2  options:0];

    int sec = [conversionInfo second];
    int min = [conversionInfo minute];
    int hour = [conversionInfo hour];

    NSString *seconds = [NSString stringWithFormat:@"%d",sec];
    NSString *minutes = [NSString stringWithFormat:@"%d",min];

    if (sec <= 9)
        seconds = [NSString stringWithFormat:@"0%d", sec];
    if (min <= 9)
        minutes = [NSString stringWithFormat:@"0%d", min];

    if ([conversionInfo hour] == 0)
        time = [NSString stringWithFormat:@"%@:%@", minutes, seconds];
    else
        time = [NSString stringWithFormat:@"%d:%@:%@", hour, minutes, seconds];

    lblTimer.text = time; //sets the label to the time set above

    NSLog(@"%d", timeLeft);

    if (timeLeft == 0) {
        [self timerDone];
        [stopWatchTimer invalidate];
        stopWatchTimer = nil;
    }
}

-(void)timerDone {

    pkrTime.hidden = NO;
    btnStart.hidden = NO;
    btnStop.hidden = YES;

    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Timer Done" message:nil delegate:self cancelButtonTitle:nil otherButtonTitles:@"Ok", nil];
    [alert show];

    [self playAlert];
}

Please let me know what the problem is... I cannot find a problem with my code anywhere!

Answer 1

In your btnStartPressed: method, you have nothing preventing a second NSTimer from being allocated and assigned to stopWatchTimer . If you press the button twice, you'll end up with two timers, but only one will ever be invalidated.

Add something like:

 if (stopWatchTimer) return;

To the beginning of btnStartPressed: . If that doesn't fix the problem, then there isn't enough context to know for sure what is going on beyond conjecturing that timeLeft is zero?

---

What Nate said, but here is another explanation.

Imagine if you do this (where stopWatchTimer is a global or instance variable, doesn't matter):

 stopWatchTimer = [NSTimer scheduledTimerWithTimeInterval:....];

Now, do this:

 stopWatchTimer = nil;
 [stopWatchTimer invalidate];

The timer won't invalidate, but it'll still fire. stopWatchTimer is a reference to the object. It isn't the object itself. Thus, when you assign a second timer to stopWatchTimer , you are overwriting the reference to the first timer, but that timer is still going to fire!