Weak Reference to NSTimer Target To Prevent Retain

Question

I'm using an NSTimer like this:

timer = [NSTimer scheduledTimerWithTimeInterval:30.0f target:self selector:@selector(tick) userInfo:nil repeats:YES];

Of course, NSTimer retains the target which creates a retain cycle. Furthermore, self isn't a UIViewController so I don't have anything like viewDidUnload where I can invalidate the timer to break the cycle. So I'm wondering if I could use a weak reference instead:

__weak id weakSelf = self;
timer = [NSTimer scheduledTimerWithTimeInterval:30.0f target:weakSelf selector:@selector(tick) userInfo:nil repeats:YES];

I've heard that the timer must be invalidated (i guess to release it from the run loop). But we could do that in our dealloc, right?

- (void) dealloc {
    [timer invalidate];
}

Is this a viable option? I've seen a lot of ways that people deal with this issue, but I haven't seen this.

Answer 1

If you are using Swift here is an auto-cancelling timer:

https://gist.github.com/evgenyneu/516f7dcdb5f2f73d7923

The timer cancels itself automatically on deinit.

var timer: AutoCancellingTimer? // Strong reference

func startTimer() {
  timer = AutoCancellingTimer(interval: 1, repeats: true) {
    print("Timer fired")
  }
}

Answer 2

It doesn't matter that weakSelf is weak, the timer still retains the object so there's still a retain cycle. Since a timer is retained by the run loop, you can (and I suggest to ) hold a weak pointer to the timer:

NSTimer* __weak timer = [NSTimer scheduledTimerWithTimeInterval:30.0f target: self selector:@selector(tick) userInfo:nil repeats:YES];

About invalidate you're way of doing is correct.

Answer 3

The proposed code:

__weak id weakSelf = self;
timer = [NSTimer scheduledTimerWithTimeInterval:30.0f target:weakSelf selector:@selector(tick) userInfo:nil repeats:YES];

has the effect that (i) a weak reference is made to self; (ii) that weak reference is read in order to provide a pointer to NSTimer. It won't have the effect of creating an NSTimer with a weak reference. The only difference between that code and using a __strong reference is that if self is deallocated in between the two lines given then you'll pass nil to the timer.

The best thing you can do is create a proxy object. Something like:

[...]
@implementation BTWeakTimerTarget
{
    __weak target;
    SEL selector;
}

[...]

- (void)timerDidFire:(NSTimer *)timer
{
    if(target)
    {
        [target performSelector:selector withObject:timer];
    }
    else
    {
        [timer invalidate];
    }
}
@end

Then you'd do something like:

BTWeakTimerTarget *target = [[BTWeakTimerTarget alloc] initWithTarget:self selector:@selector(tick)];
timer = [NSTimer scheduledTimerWithTimeInterval:30.0 target:target selector:@selector(timerDidFire:) ...];

Or even add a class method to BTWeakTimerTarget of the form +scheduledTimerWithTimeInterval:target:selector:... to create a neater form of that code. You'll probably want to expose the real NSTimer so that you can invalidate it, otherwise the rules established will be:

1. the real target isn't retained by the timer;
2. the timer will fire once after the real target has begun (and probably completed) deallocation, but that firing will be ignored and the timer invalidated then.