Sequential fade in Swift

I'm using the excellent answer here to implement a fade in for a text label.

However, I want to introduce a delay so I can sequentially fade in several text labels.

So far (taken from the answer), i'm using :

extension UIView {

    func fadeIn(duration: TimeInterval = 1.0, delay: TimeInterval = 0.0, completion: @escaping ((Bool) -> Void) = {(finished: Bool) -> Void in}) {
        UIView.animate(withDuration: duration, delay: delay, options: UIViewAnimationOptions.curveEaseIn, animations: {
            self.alpha = 1.0
        }, completion: completion)  }
}

and then implementing with :

override func viewDidLoad() {

        self.line1Outlet.alpha = 0
        self.line1Outlet.fadeIn(completion: {
            (finished: Bool) -> Void in
        })

    }

I was thinking the best solution would be to implement the delay as a parameter in the extension so I could easily add a different delay to each label. (e.g.

override func viewDidLoad() {

        self.line1Outlet.alpha = 0
        //add a parameter here for the delay (here line 1 gets '1second' then line 2 could come in after 2seconds etc)
        self.line1Outlet.delay = 1second
        self.line1Outlet.fadeIn(completion: {
            (finished: Bool) -> Void in
        })

    }

I've tried adding self.delay into the extension (underneath self.alpha) but that doesn't work and I'm not sure how to refactor that extension to allow what I'm after.

The answer to this would then be a reusable method of implementing sequential fades that hopefully would be useful to lots of other people!

In the extension you created, first add self.alpha = 0.0 at the top in fadeIn function, i.e.

extension UIView {
    func fadeIn(duration: TimeInterval = 1.0, delay: TimeInterval = 0.0, completion: ((Bool)->())? = nil) {
        self.alpha = 0.0
        UIView.animate(withDuration: duration, delay: delay, options: .curveEaseIn, animations: {
            self.alpha = 1.0
        }, completion: completion)
    }
}

Now lets assume you've 3 labels in your view, i.e.

@IBOutlet weak var label1: UILabel!
@IBOutlet weak var label2: UILabel!
@IBOutlet weak var label3: UILabel!

Add animation to the labels in sequence like,

self.label1.fadeIn(delay: 0.1) { _ in
    self.label2.fadeIn(delay: 0.2, completion: { _ in
        self.label3.fadeIn(delay: 0.3, completion: { _ in
            print("Done All")
        })
    })
}

Since the duration parameter in fadeIn method is having a default value, we can avoid that.

The way you're calling fadeIn is one way of calling it. Since the method contains multiple default params, it can be called in other ways as well.

Read more about default parameters here.