Question

My code is:

-(void) timerRun{...}

-(void) createTimer
{
   NSTimer *timer;
   timer = [NSTimer timerWithTimeInterval:1.0 
                                   target:self 
                                 selector:@selector(timerRun) 
                                 userInfo:nil 
                                  repeats:YES];
}

viewDidLoad
{
   [NSThread detachNewThreadSelector:@selector(createTimmer) 
                            toTarget:self withObject:nil];
   ...

}

When I debug, the method createTimer runs ok, but the method does timerRun not run?

Answer 1

Just creating a timer doesn’t start it running. You need to both create it and schedule it.

You’re actually going to have to do slightly more work than that if you want it to run on a background thread. NSTimers attach to NSRunloops, which are the Cocoa form of an event loop. Each NSThread inherently has a a run loop but you have to tell it to run explicitly.

A run loop with a timer attached can run itself indefinitely but you probably don’t want it to because it won’t be managing autorelease pools for you.

So, in summary, you probably want to (i) create the timer; (ii) attach it to that thread’s run loop; (iii) enter a loop that creates an autorelease pool, runs the run loop for a bit and then drains the autorelease pool.

Code will probably look like:

// create timer
timer = [NSTimer timerWithTimeInterval:1.0 
                                target:self 
                              selector:@selector(timerRun) 
                              userInfo:nil 
                               repeats:YES];

// attach the timer to this thread's run loop
[[NSRunLoop currentRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];

// pump the run loop until someone tells us to stop
while(!someQuitCondition)
{
    // create a autorelease pool
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    // allow the run loop to run for, arbitrarily, 2 seconds
    [[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:2.0]];

    // drain the pool
    [pool drain];
}

// clean up after the timer
[timer invalidate];