iOS5 ARC is it safe to schedule NSTimers from back

Question

I'm trying to debug my application.

I've been using some NSTimer instances in my non-arc code like this (from the main thread):

[NSTimer scheduledTimerWithTimeInterval:5 target:musicPlayer selector:@selector(playPause:) userInfo:nil repeats:NO];

This works fine if I assign this code to a button and click a button. The timer fires.

I've also tried:

 if( self.deliveryTimer == nil)
  {                 
self.deliveryTimer = [NSTimer scheduledTimerWithTimeInterval:10 target:self selector:@selector(playPause:) userInfo:nil repeats:NO];
    }

    -(void)playPause:(NSTimer*)timer
    {
           [deliveryTimer invalidate];
            deliveryTimer = nil;
//more code here

    }

I would expect the timer to execute, hit the play/pause method below, then turn to nil, so I can reset the timer later. The reason why I'm checking for nil is because I have 3 different code paths that may set the timer. Each one has an NSLog statement indicating that the timer has been scheduled.

My code runs, and I see that the timers are being scheduled, but they don't seem to fire in the course of normal app execution. I'm investigating why. Short term timers, using the same logic fire fine. It is when I let the app run for a while that I'm running into issues.

Could the NSTimers be reclaimed by ARC?

Does it matter if I set the timer from a performSelectorInBackground? As I was writing up this question, I noticed that some of my timers were created from a code path that is being called through:

[self performSelectorInBackground:@selector(notifyDelegateOfDataPoint:) withObject:data];

could the background selector be the reason why my timers do not fire/get reclaimed earlier?

Any help is appreciated, this bug has been bugging me for over 2 weeks!

Update: after changing the code to use the main thread for NSTimers, the timers fire correctly, causing the music to play:

   [self performSelectorOnMainThread:@selector(deliverReminder:) withObject:nil waitUntilDone:NO];


-(void)deliverReminder:(id)sender{
     [ NSTimer scheduledTimerWithTimeInterval:10 target:reminderDeliverySystem selector:@selector(playAfterDelay:) userInfo:nil repeats:NO];
    [self postMessageWithTitle:nil message:@"Deliver Reminder Called" action:kNoContextAction];
}

-(void)playAfterDelay:(id)sender
{
    int reminderDelay = reminder.delayValue.intValue;

    [playTimers addObject:[NSTimer scheduledTimerWithTimeInterval:reminderDelay target:self selector:@selector(appMusicPlayerPlay:) userInfo:nil repeats:NO]];


}

Here I have a whole bunch of timers, which is because I don't know how to pass a primitive to a target with a selector.

Answer 1

An NSTimer requires a run loop to be running in that background thread for it to keep firing. The main thread already has an active run loop, which is why your timers work fine when executed on it.

If you want to use your timers within a background thread, you can do something like the following:

NSRunLoop* runLoop = [NSRunLoop currentRunLoop];
self.deliveryTimer = [NSTimer scheduledTimerWithTimeInterval:10 target:self selector:@selector(playPause:) userInfo:nil repeats:NO];
[runLoop run];

What was probably happening with your short duration timers firing, but the longer ones not, was that they were firing while the thread was still active, but without a run loop to keep it going, were failing after the thread reached the end of its execution.

I don't believe this is ARC-related, although there may be something there you'll have to watch for, because the NSRunLoop holds on to a timer that is attached to it. Following standard procedure with NSTimers should avoid ARC problems.