NSTimer's isValid always returns YES - ios

Question

So I've got a timer that is not repetitive. Each time it fires, the method that being executed decide if to reschedule it or not according to some inner logic of my app.
This method is available from other parts of the app, so the first thing that I'm doing in the method is to check if the timer is still valid (to know if the initiator was the timer or a different entity) so in case it wasn't initiated by the timer I want to invalidate it:
if (self.pollingTimer.isValid) {
[self.pollingTimer invalidate];
self.pollingTimer = nil;
}
I've noticed that if the method is being called due to the timer being fired - I always receive a true value from the isValid property, even though when looking at the NSTimer documentations under the scheduledTimerWithTimeInterval:(NSTimeInterval)seconds target:(id)target selector:(SEL)aSelector userInfo:(id)userInfo repeats:(BOOL)repeats method:
repeats
If YES, the timer will repeatedly reschedule itself until invalidated. If NO, the timer will be invalidated after it fires.
Discussion
After seconds seconds have elapsed, the timer fires,
sending the message aSelector to target.
I'm having hard time to understand when the timer is being automatically invalidated which bring me to my questions:
Any idea why I always get YES from isValid?
What is the exact definition of the timer fires? Is it just sending the message aSelector to target as stated in the documentation? or is it finishing the execution of the method? (which might explain what I'm experiencing)
Thanks in advance.

Answer 1

A timer is not a real-time mechanism; it fires only when one of the run loop modes to which the timer has been added is running and able to check if the timer’s firing time has passed. Therefore, the timer does not immediately invalidate itself, but at the end of the run loop.
As a simple test, you can see:
- (void)viewDidLoad {
[super viewDidLoad];
_timer = [NSTimer scheduledTimerWithTimeInterval:0.5 target:self selector:#selector(timerFired) userInfo:nil repeats:NO];
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
if (self.timer.isValid){
NSLog(#"--> TIMER VALID");
} else {
NSLog(#"TIMER INVALID!");
}
});
}
- (void) timerFired {
if (self.timer.isValid){
NSLog(#"--> TIMER VALID");
} else {
NSLog(#"TIMER INVALID!");
}
}
This will log --> TIMER VALID from the timerFired method and when the block from dispatch_after is called, you will see TIMER INVALID!. So, when you schedule a timer with repeats:NO, it is guaranteed to not reschedule itself but it will not invalidate immediately.
So, to answer your question:
repeats
If YES, the timer will repeatedly reschedule itself until
invalidated. If NO, the timer will be invalidated after it fires (but not immediately)

Answer 2

I made a test like this:
self.timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:#selector(xx) userInfo:nil repeats:NO];
- (void)xx
{
if ([self.timer isValid]) {
NSLog(#"isvalid");
}
dispatch_async(dispatch_get_main_queue(), ^{
if ([self.timer isValid]) {
NSLog(#"isvalid");
}else{
NSLog(#"isInvalid");
}
});
static int i = 0;
NSLog(#"%d",i++);
}
and the result is:
isvalid
0
isInvalid
thus, I guess when timer is fired,the function is execute like this:
void __NSFireTimer(){
id yourObj;
[yourObj performSelector:#selector(yourSelector)];
timer.isvalid = NO;
}
what you believe is:
void __NSFireTimer(){
id yourObj;
timer.isvalid = NO;
[yourObj performSelector:#selector(yourSelector)];
}
So, just accept it.You can put your check valid code in dispatch_asyn() ,like the test code.

Answer 3

This is how I used my timers. First initialise it on the top as
#property (strong, nonatomic) NSTimer *refreshTimer;
then this two methods, to create and invalidate the timer. "Its very important to invalidate the current timer if you want to create another timer with same name" otherwise their will be two timers.
- (void)startTimer {
if (_refreshTimer) {
[self invalidateTimer];
}
_refreshTimer = [NSTimer scheduledTimerWithTimeInterval:15.0
target:self
selector:#selector(determineIfPartOfgroup)
userInfo:nil
repeats:YES];
}
- (void)invalidateTimer {
if (_refreshTimer) {
[_refreshTimer invalidate];
_refreshTimer = nil;
}
}
I hope this will help you.