How to Wait in Objective-C and Swift

You can use

[self performSelector:@selector(changeText:) withObject:text afterDelay:2.0];

or if you want to display it periodically, check the NSTimer class.

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
}

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
}