Question about NSTimer and retain - iphone

Question

This code works well
#property (nonatomic, retain) NSTimer *timer;
self.timer = [[NSTimer timerWithTimeInterval:kAdsAppearTimeInterval target:self selector:#selector(timerFired:) userInfo:nil repeats:NO] retain];
this code get CFRelease . But why? i use retain property
self.timer = [NSTimer timerWithTimeInterval:kAdsAppearTimeInterval target:self selector:#selector(timerFired:) userInfo:nil repeats:NO];

Answer 1

Not a lot to go on... but:
#property (nonatomic, retain) NSTimer *timer;
self.timer = [[NSTimer timerWithTimeInterval:kAdsAppearTimeInterval target:self selector:#selector(timerFired:) userInfo:nil repeats:NO] retain];
That'll end up retaining the timer 3 times and self once.
Timer +1 for -retain
Timer +1 for scheduling it
Timer +1 for the property assignment
self +1 for being the target of the timer
The timer will be released once when fired (because it'll be unscheduled from the run loop). self will be released when the timer is invalidated or released (you shouldn't have to care).
So, you have two retain counts to account for. The call to retain in the code above is noise; don't bother as the property assignment will retain it.
That leaves the property's retain. The most obvious way is to release the timer in -dealloc.
However, unless you need to potentially invalidate the timer before it fires, there is no reason to have an instance variable referring to the timer at all. Even if you do have an iVar, there is no reason to retain the timer either as long as you set self.timer = nil in your timerFired: method (and set it to nil if you invalidate anywhere).

Answer 2

For a non-repeating timer, if you need a reference to the instance variable, I would not recommend a retain property in its declaration to avoid confusion.
setting the instance variable (myTimer)
myTimer = [NSTimer scheduledTimerWithTimeInterval:myTimerInterval
target:self
selector:#selector(myTimerFired:)
userInfo:nil
repeats:NO];
when the timer fires, you can mark the instance variable as nil since its released when the timer is fired
- (void) myTimerFired: (NSTimer *) theTimer{
myTimer = nil;
//etc
}
This way if you have to reference your instance variable (for example to disable the timer when exiting a View controller)
-(void) onBack {
if(myTimer){
[myTimer invalidate];
myTimer = nil;
}
}