Question

So I am trying to set up a basic timer but I am failing miserably. Basically all I want is to start a 60 second timer when the user clicks a button, and to update a label with the time remaining(like a countdown). I created my label and button and connected them in IB. Next I created a IBAction for the button. Now when I tried to update the label based on the timer, my app screws up. Here’s my code:

NSTimer *t = [NSTimer scheduledTimerWithTimeInterval: 1
                      target: self
                      selector:@selector(updateLabelDisplay)
                      userInfo: nil repeats:YES];

I also have an updateLabelDisplay function that determines how many times the timer has ran and then subtracted that number from 60 and displays that number in the countdown label. Can anyone tell me what I am doing wrong?

Answer 1

Ok, well for starters, check this out if you haven’t already: Official Apple Docs about Using Timers

Based on your description, you probably want code that looks something like this. I’ve made some assumptions regarding behavior, but you can suit to taste.

This example assumes that you want to hold on to a reference to the timer so that you could pause it or something. If this is not the case, you could modify the handleTimerTick method so that it takes an NSTimer* as an argument and use this for invalidating the timer once it has expired.

@interface MyController : UIViewController
{
  UILabel * theLabel;

  @private
  NSTimer * countdownTimer;
  NSUInteger remainingTicks;
}

@property (nonatomic, retain) IBOutlet UILabel * theLabel;

-(IBAction)doCountdown: (id)sender;

-(void)handleTimerTick;

-(void)updateLabel;

@end

@implementation MyController
@synthesize theLabel;

// { your own lifecycle code here.... }

-(IBAction)doCountdown: (id)sender
{
  if (countdownTimer)
    return;


  remainingTicks = 60;
  [self updateLabel];

  countdownTimer = [NSTimer scheduledTimerWithTimeInterval: 1.0 target: self selector: @selector(handleTimerTick) userInfo: nil repeats: YES];
}

-(void)handleTimerTick
{
  remainingTicks--;
  [self updateLabel];

  if (remainingTicks <= 0) {
    [countdownTimer invalidate];
    countdownTimer = nil;
  }
}

-(void)updateLabel
{
  theLabel.text = [[NSNumber numberWithUnsignedInt: remainingTicks] stringValue];
}


@end