How do you trigger a block after a delay, like -performSelector:withObject:afterDelay:?

Question

Is there a way to call a block with a primitive parameter after a delay, like using performSelector:withObject:afterDelay: but with an argument like int/double/float?

Answer 1

I think you're looking for dispatch_after(). It requires your block to accept no parameters, but you can just let the block capture those variables from your local scope instead.

int parameter1 = 12;
float parameter2 = 144.1;

// Delay execution of my block for 10 seconds.
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 10 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
    NSLog(@"parameter1: %d parameter2: %f", parameter1, parameter2);
});

More: https://developer.apple.com/documentation/dispatch/1452876-dispatch_after

Answer 2

You can use dispatch_after to call a block later. In Xcode, start typing dispatch_after and hit Enter to autocomplete to the following:

Here's an example with two floats as "arguments." You don't have to rely on any type of macro, and the intent of the code is quite clear:

Swift 3, Swift 4

let time1 = 8.23
let time2 = 3.42

// Delay 2 seconds
DispatchQueue.main.asyncAfter(deadline: .now() + 2.0) {
    print("Sum of times: \(time1 + time2)")
}

Swift 2

let time1 = 8.23
let time2 = 3.42

// Delay 2 seconds
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(2.0 * Double(NSEC_PER_SEC))), dispatch_get_main_queue()) { () -> Void in
        println("Sum of times: \(time1 + time2)")
}

Objective C

CGFloat time1 = 3.49;
CGFloat time2 = 8.13;

// Delay 2 seconds
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    CGFloat newTime = time1 + time2;
    NSLog(@"New time: %f", newTime);
});

Answer 3

How about using Xcode built-in code snippet library?

Update for Swift:

Many up votes inspired me to update this answer.

The build-in Xcode code snippet library has dispatch_after for only objective-c language. People can also create their own Custom Code Snippet for Swift.

Write this in Xcode.

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(<#delayInSeconds#> * Double(NSEC_PER_SEC))), dispatch_get_main_queue(), {
        <#code to be executed after a specified delay#>
    })

Drag this code and drop it in the code snippet library area.

Bottom of the code snippet list, there will be a new entity named My Code Snippet. Edit this for a title. For suggestion as you type in the Xcode fill in the Completion Shortcut.

For more info see CreatingaCustomCodeSnippet.

Update Swift 3

Drag this code and drop it in the code snippet library area.

DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(<#delayInSeconds#>)) {
    <#code to be executed after a specified delay#>
}

Answer 4

Expanding on Jaime Cham's answer I created a NSObject+Blocks category as below. I felt these methods better matched the existing performSelector: NSObject methods

NSObject+Blocks.h

#import <Foundation/Foundation.h>

@interface NSObject (Blocks)

- (void)performBlock:(void (^)())block afterDelay:(NSTimeInterval)delay;

@end

NSObject+Blocks.m

#import "NSObject+Blocks.h"

@implementation NSObject (Blocks)

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

- (void)performBlock:(void (^)())block afterDelay:(NSTimeInterval)delay
{
    void (^block_)() = [block copy]; // autorelease this if you're not using ARC
    [self performSelector:@selector(performBlock:) withObject:block_ afterDelay:delay];
}

@end

and use like so:

[anyObject performBlock:^{
    [anotherObject doYourThings:stuff];
} afterDelay:0.15];

Answer 5

Perhaps simpler than going thru GCD, in a class somewhere (e.g. "Util"), or a Category on Object:

+ (void)runBlock:(void (^)())block
{
    block();
}
+ (void)runAfterDelay:(CGFloat)delay block:(void (^)())block 
{
    void (^block_)() = [[block copy] autorelease];
    [self performSelector:@selector(runBlock:) withObject:block_ afterDelay:delay];
}

So to use:

[Util runAfterDelay:2 block:^{
    NSLog(@"two seconds later!");
}];

Answer 6

For Swift I've created a global function, nothing special, using the dispatch_after method. I like this more as it's readable and easy to use:

func performBlock(block:() -> Void, afterDelay delay:NSTimeInterval){
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(delay * Double(NSEC_PER_SEC))), dispatch_get_main_queue(), block)
}

Which you can use as followed:

performBlock({ () -> Void in
    // Perform actions
}, afterDelay: 0.3)

Answer 7

Here are my 2 cents = 5 methods ;)

I like encapsulate these details and have AppCode tell me how to finish my sentences.

void dispatch_after_delay(float delayInSeconds, dispatch_queue_t queue, dispatch_block_t block) {
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
    dispatch_after(popTime, queue, block);
}

void dispatch_after_delay_on_main_queue(float delayInSeconds, dispatch_block_t block) {
    dispatch_queue_t queue = dispatch_get_main_queue();
    dispatch_after_delay(delayInSeconds, queue, block);
}

void dispatch_async_on_high_priority_queue(dispatch_block_t block) {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), block);
}

void dispatch_async_on_background_queue(dispatch_block_t block) {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), block);
}

void dispatch_async_on_main_queue(dispatch_block_t block) {
    dispatch_async(dispatch_get_main_queue(), block);
}

Answer 8

Xcode 10.2 and Swift 5 and above

DispatchQueue.main.asyncAfter(deadline: .now() + 2, execute: {
   // code to execute                 
})

ObjC version

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 2 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
    //code to execute
});

Answer 9

PerformSelector:WithObject always takes an object, so in order to pass arguments like int/double/float etc..... You can use something like this.

//NSNumber is an object..

[self performSelector:@selector(setUserAlphaNumber:)
     withObject: [NSNumber numberWithFloat: 1.0f]       
     afterDelay:1.5];



-(void) setUserAlphaNumber: (NSNumber*) number{

     [txtUsername setAlpha: [number floatValue] ];

}

Same way you can use [NSNumber numberWithInt:] etc.... and in the receiving method you can convert the number into your format as [number int] or [number double].

Answer 10

The dispatch_after function dispatches a block object to a dispatch queue after a given period of time. Use below code to perform some UI related taks after 2.0 seconds.

            let delay = 2.0
            let delayInNanoSeconds = dispatch_time(DISPATCH_TIME_NOW, Int64(delay * Double(NSEC_PER_SEC)))
            let mainQueue = dispatch_get_main_queue()

            dispatch_after(delayInNanoSeconds, mainQueue, {

                print("Some UI related task after delay")
            })

In swift 3.0 :

            let dispatchTime: DispatchTime = DispatchTime.now() + Double(Int64(2.0 * Double(NSEC_PER_SEC))) / Double(NSEC_PER_SEC)
            DispatchQueue.main.asyncAfter(deadline: dispatchTime, execute: {

          })

Answer 11

Press Cmd + Shift + L to show the Xcode built-in code snippet library:

Look for dispatch after, then, just drag and drop to your code.

Answer 12

Here's the Swift 3 way to queue work after a delay.

DispatchQueue.main.asyncAfter(
  DispatchTime.now() + DispatchTimeInterval.seconds(2)) {
    // do work
}

Answer 13

Here's a handy helper to prevent making the annoying GCD call over and over again:

public func delay(bySeconds seconds: Double, dispatchLevel: DispatchLevel = .main, closure: @escaping () -> Void) {
    let dispatchTime = DispatchTime.now() + seconds
    dispatchLevel.dispatchQueue.asyncAfter(deadline: dispatchTime, execute: closure)
}

public enum DispatchLevel {
    case main, userInteractive, userInitiated, utility, background
    var dispatchQueue: DispatchQueue {
        switch self {
        case .main:                 return DispatchQueue.main
        case .userInteractive:      return DispatchQueue.global(qos: .userInteractive)
        case .userInitiated:        return DispatchQueue.global(qos: .userInitiated)
        case .utility:              return DispatchQueue.global(qos: .utility)
        case .background:           return DispatchQueue.global(qos: .background)
        }
    }
}

Now you simply delay your code on the Main thread like this:

delay(bySeconds: 1.5) { 
    // delayed code
}

If you want to delay your code to different thread:

delay(bySeconds: 1.5, dispatchLevel: .background) { 
    // delayed code that will run on background thread
}

---

If you prefer a Framework that also has some more handy features then checkout HandySwift. You can add it to your project via Carthage then use it exactly like in the examples above:

import HandySwift    

delay(bySeconds: 1.5) { 
    // delayed code
}

Answer 14

There's a nice one in the BlocksKit framework.

BlocksKit

(and the class)

BBlocksKit.m

Answer 15

In swift 3, We can simply use DispatchQueue.main.asyncAfter function to trigger any function or action after the delay of 'n' seconds. Here in code we have set delay after 1 second. You call any function inside the body of this function which will trigger after the delay of 1 second.

let when = DispatchTime.now() + 1
DispatchQueue.main.asyncAfter(deadline: when) {

    // Trigger the function/action after the delay of 1Sec

}

Answer 16

Updated in April 2021 with Xcode 12.4. This still works, except now the + button presents more icons, including Views library and Modifiers library (see below), and Views may be the default. So be sure to select the Snippets library, as in the image below.

---

(verified on Xcode 11.3.1 in June 2020)

Xcode provides a code snippet to do this. 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, while you are editing some code (rather than, say, in the Project Navigator, where other libraries like Capabilities would be presented)
2. Make sure the Snippets library (see screenshot, the icon with { }) is selected, out of the icons available.
3. search for after
4. It will return only 1 search result, which is the desired snippet (see screenshot). Double click it and you're good to go.

Answer 17

You can either wrap the argument in your own class, or wrap the method call in a method that doesn't need to be passed in the primitive type. Then call that method after your delay, and within that method perform the selector you wish to perform.

Answer 18

Here is how you can trigger a block after a delay in Swift:

runThisAfterDelay(seconds: 2) { () -> () in
    print("Prints this 2 seconds later in main queue")
}

/// EZSwiftExtensions
func runThisAfterDelay(seconds seconds: Double, after: () -> ()) {
    let time = dispatch_time(DISPATCH_TIME_NOW, Int64(seconds * Double(NSEC_PER_SEC)))
    dispatch_after(time, dispatch_get_main_queue(), after)
}

Its included as a standard function in my repo.

Answer 19

Swift 3 & Xcode 8.3.2

This code will help you, i add an explanation too

// Create custom class, this will make your life easier
class CustomDelay {

    static let cd = CustomDelay()

    // This is your custom delay function
    func runAfterDelay(_ delay:Double, closure:@escaping ()->()) {
        let when = DispatchTime.now() + delay
        DispatchQueue.main.asyncAfter(deadline: when, execute: closure)
    }
}


// here how to use it (Example 1)
class YourViewController: UIViewController {

    // example delay time 2 second
    let delayTime = 2.0

    override func viewDidLoad() {
        super.viewDidLoad()

        CustomDelay.cd.runAfterDelay(delayTime) {
            // This func will run after 2 second
            // Update your UI here, u don't need to worry to bring this to the main thread because your CustomDelay already make this to main thread automatically :)
            self.runFunc()
        }
    }

    // example function 1
    func runFunc() {
        // do your method 1 here
    }
}

// here how to use it (Example 2)
class YourSecondViewController: UIViewController {

    // let say you want to user run function shoot after 3 second they tap a button

    // Create a button (This is programatically, you can create with storyboard too)
    let shootButton: UIButton = {
        let button = UIButton(type: .system)
        button.frame = CGRect(x: 15, y: 15, width: 40, height: 40) // Customize where do you want to put your button inside your ui
        button.setTitle("Shoot", for: .normal)
        button.translatesAutoresizingMaskIntoConstraints = false
        return button
    }()

    override func viewDidLoad() {
        super.viewDidLoad()

        // create an action selector when user tap shoot button
        shootButton.addTarget(self, action: #selector(shoot), for: .touchUpInside)   
    }

    // example shoot function
    func shoot() {
        // example delay time 3 second then shoot
        let delayTime = 3.0

        // delay a shoot after 3 second
        CustomDelay.cd.runAfterDelay(delayTime) {
            // your shoot method here
            // Update your UI here, u don't need to worry to bring this to the main thread because your CustomDelay already make this to main thread automatically :)
        }
    }   
}

Answer 20

I believe the author is not asking how to wait for a fractional time (delay), but instead how to pass a scalar as argument of the selector (withObject:) and the fastest way in modern objective C is:

[obj performSelector:...  withObject:@(0.123123123) afterDelay:10]

your selector have to change its parameter to NSNumber, and retrieve the value using a selector like floatValue or doubleValue