Matthew Nieuwenhuys Matthew Nieuwenhuys - 3 months ago 33
Objective-C Question

(NSTimer) creating a Timer Countdown

I am trying to create a simple Countdown Timer so that when a player enters my game the timer begins from 60 down to 0. It seems simple but I am getting confused at how I write this.

So far I have created a method within my GameController.m that looks like this:

-(int)countDownTimer:(NSTimer *)timer {
[NSTimer scheduledTimerWithTimeInterval:-1
invocation:NULL
repeats:YES];
reduceCountdown = -1;
int countdown = [[timer userInfo] reduceCountdown];
if (countdown <= 0) {
[timer invalidate];
}
return time;
}


At the start of the game I initialise the integer Time at 60. The label is then being set within ViewController. But at the moment when I compile the code it just shows the label at 60 and doesn't decrement at all.

Any help would be greatly appreciated - I am new to Objective-C.




EDIT

With some assistance from I have now separated the code into 2 separate methods. The code now looks like this:

-(void)countDown:(NSTimer *)timer {
if (--time == 0) {
[timer invalidate];
NSLog(@"It's working!!!");
}
}

-(void)countDownTimer:(NSTimer *)timer {
NSLog(@"Hello");
[NSTimer scheduledTimerWithTimeInterval:1
target:self
selector:@selector(countDown:)
userInfo:nil
repeats:YES];
}


HOWEVER, the code is still not running properly and when I call the method [game countDownTimer] from my View Controller it breaks saying: "unrecognized selector sent to instance". Can anybody explain what is wrong here?

Answer

Several things are wrong with your code:

  • You are passing a wrong parameter for the time interval - negative numbers are interpreted as the 0.1 ms
  • You are calling the wrong overload - you are expected to pass an invocation object, yet you are passing a NULL
  • You put the code that you want executed on timer together with timer initialization - the code that needs to be executed on timer should go into a separate method.

You should call the overload that takes a selector, and pass 1 for the interval, rather than -1.

Declare NSTimer *timer and int remainingCounts, then add

timer = [NSTimer scheduledTimerWithTimeInterval:1
                                         target:self
                                       selector:@selector(countDown)
                                       userInfo:nil
                                        repeats:YES];
remainingCounts = 60;

to the place where you want to start the countdown. Then add the countDown method itself:

-(void)countDown {
    if (--remainingCounts == 0) {
        [timer invalidate];
    }
}
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