Question

I have a problem when creating an NSTimer without repeat.

Imagine I create a new instance

timer = [NSTimer scheduledTimerWithTimeInterval:3.0 target:self selector:@selector(updateSystems) userInfo:nil repeats:NO];

somewhere in my code.

If I press the home button on the Iphone, my app will enter background state, and the timer will still be around, just be paused. So in my applicationDidEnterBackground method I say

if(timer.isValid == YES)
{
    [timer invalidate];
    timer = nil;
}

The problem arises if I press the Home button after the timer has completed. Then the timer has been released, resulting in a crash when trying to access isValid.

Trying

if(timer != nil) 
{
    [timer invalidate];
    timer = nil;
} 

results in the same crash, since Cocoa’s auto-invalidation doesn’t seem to set the timer to nil.

How is it possible to check wether the timer has auto-invalidated or not?

Thank you.

NOTE: The reason I need to do this is because I’m sending a request to a web service, once the request has been successful (or a failure) I start a timer that will send a new request once it gets fired. If I don’t invalidate the timer before it’s finished, several timers will be created (since I’m making a new request on applicationDidBecomeActive) making multiple request when only one is needed.

Answer 1

You need to retain the timer yourself to guarantee that it’s still around after it has fired. scheduledTimerWithTimeInterval returns an autoreleased instance that you don’t own.