Creating a generic completion handler in Swift

Question

I have a generic NSObject animation class I created for a new project and I'm attempting to refactor some older projects with the new class. All was well until I found some animations w/ completion blocks, which threw off my plan to remove exorbitant amounts of redundant code from my view controllers. Here's what I've got...

Animator.swift

class Animator: NSObject {
    var control: UIControl? // Can accept everything that's a subclass of UIControl

    override init() {
        super.init()
    }

    // FIXME: figure out how to add a completion block as a parameter on a method call
    func animateControl(control: UIControl) {
        control.transform = CGAffineTransformMakeScale(0.75, 0.75)
        UIView.animateWithDuration(0.5,
                                   delay: 0,
                                   usingSpringWithDamping: 0.3,
                                   initialSpringVelocity: 5.0,
                                   options: UIViewAnimationOptions.AllowUserInteraction,
                                   animations: {
                                    control.transform = CGAffineTransformIdentity
        }) { (value: Bool) -> Void in
            // completion block
            // ** method as a parameter goes here? ** 
        }
    }
}

To use it, rather than typing in all stuff to animate a button, I just call the class from MyViewController.swift

MyViewController.swift

// Property declaration
let animator = Animator()

// Tie it to an IBAction
@IBAction func myButtonAction(sender: UIButton) {
    animator.animateControl(sender, methodIWantToRunAfterAnimateControlFinishes)
}

func methodIWantToRunAfterAnimateControlFinishes() {
    // Do Stuff
}

How would I feed animateControl's initializer a method to run as a completion block? I looked at this (not my website, but it's how I feel), but I'm unable to get it to work.

Update

I had to wrestle with the syntax a little bit, but here's the initializer code that got me over the finish line:

func animateControl(control: UIControl, completion: (() -> Void)?) {
    control.transform = CGAffineTransformMakeScale(0.75, 0.75)
    UIView.animateWithDuration(0.5,
                               delay: 0,
                               usingSpringWithDamping: 0.3,
                               initialSpringVelocity: 5.0,
                               options: UIViewAnimationOptions.AllowUserInteraction,
                               animations: {
                                control.transform = CGAffineTransformIdentity
    }) { _ in
        // completion block
        completion?()
    }
}

This handles completion blocks w/ code and nil completion blocks.

Adrian Sluyters · Accepted Answer · 2016-04-06 16:42:06Z

3

A very simple completion block when used as a parameter looks like this:

onCompletion: (() -> Void)?

The reason why the () -> Void is nullable ? is to be usable in Objective-C. If this isn't needed then you needn't enclose it in brackets nor mark it as nullable ?.

This will let you pass in a function that expects no parameters and returns nothing. You would add this to your signature like so:

func animateControl(control: UIControl, onCompletion: (() -> Void)?)

If you want to add parameters to that, add them into the signature of the completion block:

func animateControl(control: UIControl, onCompletion: ((value: AnyObject) -> Void)?)

The same goes for the return statement:

func animateControl(control: UIControl, onCompletion: ((value: AnyObject) -> AnyObject)?)

Once it's in your function's signature, you can call it by it's name:

func animateControl(control: UIControl, onCompletion: ((value: AnyObject) -> Void)?) {
    //do stuff here
    onCompletion(obj)
}

edited Apr 6, 2016 at 16:42

Adrian Sluyters

2,2491 gold badge17 silver badges21 bronze badges

answered Apr 6, 2016 at 16:16

Maxwell

6,8125 gold badges40 silver badges55 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Adrian Over a year ago

Thank you for getting me squared away on this. Greatly appreciated.

dan · Accepted Answer · 2016-04-06 16:15:27Z

1

You can add a completion handler like this and you should be able to call it the way you have it in your example.

func animateControl(control: UIControl, _ completion: (() -> ())? = nil) {
    control.transform = CGAffineTransformMakeScale(0.75, 0.75)
    UIView.animateWithDuration(0.5,
                               delay: 0,
                               usingSpringWithDamping: 0.3,
                               initialSpringVelocity: 5.0,
                               options: UIViewAnimationOptions.AllowUserInteraction,
                               animations: {
                                control.transform = CGAffineTransformIdentity
    }) { _ in
        completion?()
    }
}

answered Apr 6, 2016 at 16:15

dan

9,8251 gold badge44 silver badges40 bronze badges

1 Comment

Adrian Over a year ago

Thank you for your help on this! I greatly appreciate it.

Collectives™ on Stack Overflow

Creating a generic completion handler in Swift

2 Answers 2

1 Comment

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related