How to Wait in Objective-C and Swift

Question

I want to change my UILabel\'s text after 2 seconds.

I tried setting my UILabel\'s text to \"A text\", and use sleep(2) a

Answer 1

I know I am late to this party. But I found people haven't mention thread sleep. If you are using GCD to call that function. You can use :

[NSThread sleepForTimeInterval:2.0f];   

to delay the thread for 2 seconds.

[self changeText: @"A text"];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        //Here your non-main thread.
        [NSThread sleepForTimeInterval:2.0f];   
        dispatch_async(dispatch_get_main_queue(), ^{
            //Here you returns to main thread.
            [self changeText: @"Another text"];
        });
    });

Edit 2 (Feb 2015):

I think the NSTimer is a great solution. My solution just giving another option to achieve the goal of NSTimer.

Please read: How do I use NSTimer?

[NSTimer scheduledTimerWithTimeInterval:2.0
                                 target:self
                               selector:@selector(doSomethingWhenTimeIsUp:)
                               userInfo:nil
                                repeats:NO];

In the class, you need this method:

- (void) doSomethingWhenTimeIsUp:(NSTimer*)t {

        // YES! Do something here!!

}

Edit 3 (May 2016):

In Swift 2.0, you can use this way:

 NSTimer.scheduledTimerWithTimeInterval(2.0, 
                                         target: self, 
                                       selector: "doSomethingWhenTimeIsUp:", 
                                       userInfo: nil, 
                                        repeats: false)

It creates an NSTimer's entity and adds the timer automatically to the NSRunLoop associated with the NSThread in which the timer is created.

Edit 4 (Jun 2016):

In Swift 2.2, the way to invoke select is:

#selector(doSomethingWhenTimeIsUp(_:))

So, it is something like:

  NSTimer.scheduledTimerWithTimeInterval(2.0,
                                      target: self,
                                      selector: #selector(doSomethingWhenTimeIsUp()),
                                      userInfo: nil,
                                      repeats: false)

Edit 5 (Oct 2016):

In Swift 3, the way to invoke select is:

#selector(doSomethingWhenTimeIsUp)

So, it is something like:

        Timer.scheduledTimer(timeInterval: 2.0,
                             target: self,
                             selector:#selector(doSomethingWhenTimeIsUp),
                             userInfo: nil,
                             repeats: false)

Then, the func should looks like this:

        @objc private func doSomethingWhenTimeIsUp(){
        // Do something when time is up
        }

Edit 6 (May 2018):

In Swift 4, we can do as below way.

    let delaySeconds = 2.0
    DispatchQueue.main.asyncAfter(deadline: .now() + delaySeconds) {
        doSomethingWhenTimeIsUp()
    }  

Then, the func should looks like this:

    private func doSomethingWhenTimeIsUp(){
        // Do something when time is up
        }

Answer 2

Grand Central Dispatch has a helper function dispatch_after() for performing operations after a delay that can be quite helpful. You can specify the amount of time to wait before execution, and the dispatch_queue_t instance to run on. You can use dispatch_get_main_queue() to execute on the main (UI) thread.

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){
    // do something
});

In Swift 3, the above example can be written as:

DispatchQueue.main.asyncAfter(deadline: .now() + 2.0) {
    // do something
}