[ios] Make a simple fade in animation in Swift?

I am trying to make a simple animation in Swift. It is a fade in.

I attempted:

self.myFirstLabel.alpha = 0
self.myFirstButton.alpha = 0
self.mySecondButton.alpha = 0

Then, I have:

self.view.addSubview(myFirstLabel)
self.view.addSubview(myFirstButton)
self.view.addSubview(mySecondButton)

And then:

UIView.animateWithDuration(1.5, animations: {
 self.myFirstLabel.alpha = 1.0
 self.myFirstButton.alpha = 1.0
 self.mySecondButton.alpha = 1.0
})

I have all of this in my viewDidLoad function.

How do I make this work?

If you want repeatable fade animation you can do that by using CABasicAnimation like below :

First create handy UIView extension :

extension UIView {

    enum AnimationKeyPath: String {
        case opacity = "opacity"
    }

    func flash(animation: AnimationKeyPath ,withDuration duration: TimeInterval = 0.5, repeatCount: Float = 5){
        let flash = CABasicAnimation(keyPath: animation.rawValue)
        flash.duration = duration
        flash.fromValue = 1 // alpha
        flash.toValue = 0 // alpha
        flash.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)
        flash.autoreverses = true
        flash.repeatCount = repeatCount

        layer.add(flash, forKey: nil)
    }
}

How to use it:

    // You can use it with all kind of UIViews e.g. UIButton, UILabel, UIImage, UIImageView, ...
    imageView.flash(animation: .opacity, withDuration: 1, repeatCount: 5)
    titleLabel.flash(animation: .opacity, withDuration: 1, repeatCount: 5)

Swift 5

Other answers are correct, but in my case I need to handle other properties also (alpha, animate, completion). Because of this, I modified a bit to expose these parameters as below:

extension UIView {
    /// Helper function to update view's alpha with animation
    /// - Parameter alpha: View's alpha
    /// - Parameter animate: Indicate alpha changing with animation or not
    /// - Parameter duration: Indicate time for animation
    /// - Parameter completion: Completion block after alpha changing is finished
    func set(alpha: CGFloat, animate: Bool, duration: TimeInterval = 0.3, completion: ((Bool) -> Void)? = nil) {
        let animation = { (view: UIView) in
            view.alpha = alpha
        }
    
        if animate {
            UIView.animate(withDuration: duration, animations: {
                animation(self)
            }, completion: { finished in
                completion?(finished)
            })
        } else {
            layer.removeAllAnimations()
            animation(self)
            completion?(true)
        }
    }
}

0x7ffffff's answer is ok and definitely exhaustive.

As a plus, I suggest you to make an UIView extension, in this way:

public extension UIView {

  /**
  Fade in a view with a duration

  - parameter duration: custom animation duration
  */
  func fadeIn(duration duration: NSTimeInterval = 1.0) {
    UIView.animateWithDuration(duration, animations: {
        self.alpha = 1.0
    })
  }

  /**
  Fade out a view with a duration

  - parameter duration: custom animation duration
  */
  func fadeOut(duration duration: NSTimeInterval = 1.0) {
    UIView.animateWithDuration(duration, animations: {
        self.alpha = 0.0
    })
  }

}

Swift-3

/// Fade in a view with a duration
/// 
/// Parameter duration: custom animation duration
func fadeIn(withDuration duration: TimeInterval = 1.0) {
    UIView.animate(withDuration: duration, animations: {
        self.alpha = 1.0
    })
}

/// Fade out a view with a duration
///
/// - Parameter duration: custom animation duration
func fadeOut(withDuration duration: TimeInterval = 1.0) {
    UIView.animate(withDuration: duration, animations: {
        self.alpha = 0.0
    })
}

In this way you can do this wherever in your code:

let newImage = UIImage(named: "")
newImage.alpha = 0 // or newImage.fadeOut(duration: 0.0)
self.view.addSubview(newImage)
... 
newImage.fadeIn()

Code reuse is important!

import UIKit

/*
 Here is simple subclass for CAAnimation which create a fadeIn animation
 */

class FadeInAdnimation: CABasicAnimation {
    override init() {
        super.init()
        keyPath = "opacity"
        duration = 2.0
        fromValue = 0
        toValue = 1
        fillMode = CAMediaTimingFillMode.forwards
        isRemovedOnCompletion = false
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
}

/*
 Example of usage
 */

class ViewController: UIViewController {

    weak var label: UILabel!

    override func loadView() {
        let view = UIView()
        view.backgroundColor = .white

        let label = UILabel()
        label.alpha = 0
        label.frame = CGRect(x: 150, y: 200, width: 200, height: 20)
        label.text = "Hello World!"
        label.textColor = .black
        view.addSubview(label)
        self.label = label

        let button = UIButton(type: .custom)
        button.frame = CGRect(x: 0, y: 250, width: 300, height: 100)
        button.setTitle("Press to Start FadeIn", for: UIControl.State())
        button.backgroundColor = .red
        button.addTarget(self, action: #selector(startFadeIn), for: .touchUpInside)
        view.addSubview(button)

        self.view = view
    }

    /*
     Animation in action
     */
    @objc private func startFadeIn() {
        label.layer.add(FadeInAdnimation(), forKey: "fadeIn")
    }

}

Swift only solution

Similar to Luca's anwer, I use a UIView extension. Compared to his solution I use DispatchQueue.main.async to make sure animations are done on the main thread, alpha parameter for fading to a specific value and optional duration parameters for cleaner code.

extension UIView {
  func fadeTo(_ alpha: CGFloat, duration: TimeInterval = 0.3) {
    DispatchQueue.main.async {
      UIView.animate(withDuration: duration) {
        self.alpha = alpha
      }
    }
  }

  func fadeIn(_ duration: TimeInterval = 0.3) {
    fadeTo(1.0, duration: duration)
  }

  func fadeOut(_ duration: TimeInterval = 0.3) {
    fadeTo(0.0, duration: duration)
  }
}

How to use it:

// fadeIn() - always animates to alpha = 1.0
yourView.fadeIn()     // uses default duration of 0.3
yourView.fadeIn(1.0)  // uses custom duration (1.0 in this example)

// fadeOut() - always animates to alpha = 0.0
yourView.fadeOut()    // uses default duration of 0.3
yourView.fadeOut(1.0) // uses custom duration (1.0 in this example)

// fadeTo() - used if you want a custom alpha value
yourView.fadeTo(0.5)  // uses default duration of 0.3
yourView.fadeTo(0.5, duration: 1.0)