Adding delay between execution of two following lines

Question

I need to add a delay between the execution of two lines in a(same) function. Is there any favorable option to do this?

Note: I don't need two different functions to do this, and the delay must not affect other functions' execution.

eg:

line 1: [executing first operation];

line 2: Delay                        /* I need to introduce delay here */

line 3: [executing second operation];

Answer 1

You can use gcd to do this without having to create another method

// ObjC

NSTimeInterval 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){
  NSLog(@"Do some work");
});

// Swift

DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
    print("Do some work)
}

You should still ask yourself "do I really need to add a delay" as it can often complicate code and cause race conditions

Answer 2

You can use the NSThread method:

[NSThread sleepForTimeInterval: delay];

However, if you do this on the main thread you'll block the app, so only do this on a background thread.

---

or in Swift

NSThread.sleepForTimeInterval(delay)

in Swift 3

Thread.sleep(forTimeInterval: delay)

Answer 3

This line calls the selector secondMethod after 3 seconds:

[self performSelector:@selector(secondMethod) withObject:nil afterDelay:3.0 ];

Use it on your second operation with your desired delay. If you have a lot of code, place it in its own method and call that method with performSelector:. It wont block the UI like sleep

Edit: If you do not want a second method you could add a category to be able to use blocks with performSelector:

@implementation NSObject (PerformBlockAfterDelay)

- (void)performBlock:(void (^)(void))block 
          afterDelay:(NSTimeInterval)delay
{
    block = [block copy];
    [self performSelector:@selector(fireBlockAfterDelay:) 
               withObject:block 
               afterDelay:delay];
}

- (void)fireBlockAfterDelay:(void (^)(void))block
{
    block();
}

@end

Or perhaps even cleaner:

void RunBlockAfterDelay(NSTimeInterval delay, void (^block)(void))
{
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, NSEC_PER_SEC*delay),
      dispatch_get_current_queue(), block);
}

Answer 4

I have a couple of turn-based games where I need the AI to pause before taking its turn (and between steps in its turn). I'm sure there are other, more useful, situations where a delay is the best solution. In Swift:

        let delay = 2.0 * Double(NSEC_PER_SEC) 
        let time = dispatch_time(DISPATCH_TIME_NOW, Int64(delay)) 
        dispatch_after(time, dispatch_get_main_queue()) { self.playerTapped(aiPlayView) }

I just came back here to see if the Objective-C calls were different.(I need to add this to that one, too.)

Answer 5

[checked 27 Nov 2020 and confirmed to be still accurate with Xcode 12.1]

The most convenient way these days: Xcode provides a code snippet to do this where you just have to enter the delay value and the code you wish to run after the delay.

1. click on the + button at the top right of Xcode.
2. search for after
3. It will return only 1 search result, which is the desired snippet (see screenshot). Double click it and you're good to go.

Answer 6

If you're targeting iOS 4.0+, you can do the following:

[executing first operation];
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){
    [executing second operation];
});

Answer 7

Like @Sunkas wrote, performSelector:withObject:afterDelay: is the pendant to the dispatch_after just that it is shorter and you have the normal objective-c syntax. If you need to pass arguments to the block you want to delay, you can just pass them through the parameter withObject and you will receive it in the selector you call:

[self performSelector:@selector(testStringMethod:) 
           withObject:@"Test Test" 
           afterDelay:0.5];

- (void)testStringMethod:(NSString *)string{
    NSLog(@"string  >>> %@", string);
}

If you still want to choose yourself if you execute it on the main thread or on the current thread, there are specific methods which allow you to specify this. Apples Documentation tells this:

If you want the message to be dequeued when the run loop is in a mode other than the default mode, use the performSelector:withObject:afterDelay:inModes: method instead. If you are not sure whether the current thread is the main thread, you can use the performSelectorOnMainThread:withObject:waitUntilDone: or performSelectorOnMainThread:withObject:waitUntilDone:modes: method to guarantee that your selector executes on the main thread. To cancel a queued message, use the cancelPreviousPerformRequestsWithTarget: or cancelPreviousPerformRequestsWithTarget:selector:object: method.