iOS: Perform action after time while user is interacting / scrolling

Question

I'm trying to get my application to perform an action after a delay, but it will have to be done WHILE the user is interacting with/scrolling on a UIScrollView.

I'm not sure why neither performSelector:withObject:afterDelay or scheduledTimerWithTimeInterval:target:selector:userInfo:repeats: will fire. Is it because they're on a background thread?

Any suggestions or help?

Congruity

What is not working with performSelector: afterDelay:? What is it you are trying to do? Are you SURE they don't fire - i.e you put an NSLog statement in to check? – Grubstake

I have put in a log statement. It fires after the user stops interacting with the screen. I am trying to update a label. – Congruity

I've found this: #2866260, but there isn't any way to incorporate a delay with that. – Congruity

Ok, good. Have you tried performSelectorOnMainThread in your timer callback? – Grubstake

Theres no delay in that, though. – Congruity

Have your timer call a method - which THEN calls performSelectorOnMainThread. The timer is doing the delay. – Grubstake

The timer doesn't work either. – Congruity

Answer 1

Both NSTimer and performSelector:withObject:afterDelay: by default only fire in the normal run loop mode. When scrolling, the run loop is in event tracking mode.

You have to schedule your timed action in all common modes:

NSTimer *timer = [NSTimer timerWithTimeInterval:0.016 target:self selector:@selector(fire:) userInfo:nil repeats:YES];
[[NSRunLoop currentRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];

or

[self performSelector:@selector(fire:) withObject:nil afterDelay:1.0 inModes:[NSArray arrayWithObject:NSRunLoopCommonModes]];

There's also the dedicated NSEventTrackingRunLoopMode.

Agenesis

So, its just like what I have in the comment? – Congruity

@Congruity I'm not sure I understand what you mean. Which comment? – Agenesis

Sorry. I'm being very vague! The comment where I mention a post I was looking at before. The post is: #2866260 – Congruity

Exactly. The answer matches my answer, except for the timer based approach. – Agenesis

Would you be willing to answer another issue I'm having with this? I'm about to edit my post. – Congruity

You should post that as a new question. – Agenesis

Answer 2

Be sure the delayed trigger happens on the NSRunLoopCommonModes. The default is NSDefaultRunLoopMode which won't get messages while e.g. scrolling.

[self performSelector:@selector(fire:) withObject:nil afterDelay:2.0 inModes:@[NSRunLoopCommonModes]];

Alternatively you can use GCD which also behaves the same way (not sure which runloop mode it uses)

double delayInSeconds = 2.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
    <#code to be executed on the main queue after delay#>
});

Nason

Answer 3

For Swift:

performSelector(#selector(fire:), withObject: sender, afterDelay: 1.0, inModes: [NSRunLoopCommonModes])

Rockett