iPhone 4 is there an absolutely certain way to have a long term NSTimer fire

Question

I keep having troubles with my NSTimers and background selectors. It is driving me nuts and takes a very long time to try out each tweak. To preserve my sanity, and the sani

Answer 1

For as long as you depend on using scheduledTimerWithTimeInterval:... you cannot achieve what you want:
The timer will always be tied to the run-loop of the calling thread.

If there is no run-loop associated with that thread by the time of that message's invocation, there surely is one when the method returns as -[NSRunLoop currentRunLoop] creates a run-loop if necessary.

What you can do, if you don't like the other APIs for creation of a timer, is providing a category on NSTimer, which takes care of all the scheduling and so forth and that you can reuse in other projects.

Here is an example of what such a category might look like:

#pragma mark - setting up a timer:
+ (NSTimer *)yourPrefix_mainLoopScheduledTimerWithTimeInterval:(NSTimeInterval)interval target:(id)target selector:(SEL)selector userInfo:(id)userInfo repeats:(BOOL)shouldRepeat
{
    NSTimer *timer = [self yourPrefix_timerWithTimeInterval:interval target:target selector:selector userInfo:userInfo repeats:shouldRepeat];

    void (^scheduler)() = ^{
        [[NSRunLoop currentRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
    };

    if ([NSThread isMainThread]) {
        scheduler();
    } else {
        // you should really be able to rely on the fact, that the timer is ready to roll, when this method returns
        dispatch_sync(dispatch_get_main_queue(), scheduler);
    }

    return timer;
}

// this is just a convenience for the times where you actually want an _unscheduled_ timer
+ (NSTimer *)yourPrefix_timerWithTimeInterval:(NSTimeInterval)interval target:(id)target selector:(SEL)selector userInfo:(id)userInfo repeats:(BOOL)shouldRepeat
{
    NSDate *fireDate = [NSDate dateWithTimeIntervalSinceNow:interval];

    NSTimer *timer = [[self alloc] initWithFireDate:fireDate interval:interval target:target selector:selector userInfo:userInfo repeats:shouldRepeat];

    return [timer autorelease];
}

#pragma mark - tearing it down:
- (void)yourPrefix_invalidateMainLoopTimer
{
    [self yourPrefix_invalidateMainLoopTimerAsynchronous:NO];
}

- (void)yourPrefix_invalidateMainLoopTimerAsynchronous:(BOOL)returnsImmediately
{
    void (^invalidator)() = ^{
        [self invalidate];
    };

    dispatch_queue_t mainQueue = dispatch_get_main_queue();
    if (returnsImmediately) {
        dispatch_async(mainQueue, invalidator);
        return;
    }

    if (![NSThread isMainThread]) {
        dispatch_sync(mainQueue, invalidator);
        return;
    }

    invalidator();
}

Note the thread checks before using dispatch_sync because...

dispatch_sync

Discussion

[…] Calling this function and targeting the current queue results in deadlock.

(from The GCD Reference — emphasis mine)