Why does my NSTimer double decrement my timer? - ios

Question

I have been staring at my code for hours now so I thought I might try coming here for some fresh eyes. I needed to create a timer so I used the code below to do that. The first line is where I create the timer and the second part is my decrementTime method. This is in Objective C for an IOS app. This is my first time posting on StackOverflow (I usually find the answer I am looking for), so please let me know of any unwritten rules that I am not following.
_timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:#selector(decrementTime) userInfo:nil repeats:YES];
- (void)decrementTime{
self.timeLeft--;
}
I'll add here where I invalidate the first timer
-(IBAction)infoClick:(id)sender{
[_timer invalidate];
}
Then here is info message, where I create another timer
- (void)hideInfoMessage {
_secondTimer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:#selector(decrementTime) userInfo:nil repeats:YES];
}
clarification on my code: the order of events starts with my first code block (creating the timer). Then my second code block is called (invalidating the timer). then finally my third code block is called(making a new timer).
I know it is double incrementing because when I run the code I can visually see the timer double incrementing.

Answer 1

It's pretty easy to invoke the code that creates a timer twice. When you do that you actually have 2 timers running concurrently. Each one will decrement your value, so it will get decremented twice per second.
If you create a timer in your viewWillAppear method, for example, then you need to invalidate it in your viewWillDisappear method so you're sure you only have one running.
The same approach applies to other situations where you create a timer. You need to make sure you balance every call that creates a timer with a call that invalidates that timer.
If you use one of the scheduledTimer... methods, you can save a weak pointer to the timer. The run loop will retain it as long as it's running. When you invalidate it, the system run loop will release it and it will be deallocated. When that happens your weak pointer gets zeroed, so you don't even have to test it to see if it's valid/nil in your viewWillAppear method.
EDIT:
You need to instrument your code. In your infoClick method, is the variable _timer nil? What is it's address?
BTW, the target of an NSTimer is supposed to be a method that takes a single parameter, the timer itself. You should change your decrementTime method to look like this:
- (void) decrementTime: (NSTimer *) timer
{
NSLog(#"In method decrementTime, timer = %X", (unsigned long) timer)
self.timeLeft--;
}
Then look at your log and see if your decrementTime method is being called from 2 different timers (I would bet money that it is.)
You might also want to log the address of the timers you get back from your calls to scheduledTimerWithTimeInterval...