swift实现颜色渐变以及转换动画

本文是通过结合使用CAGradientLayer、CABasicAnimation以及CAAnimationDelegate来达到颜色渐变以及转换的动画，下面是今天要达成的效果图：

首先创建一个CAGradientLayer和几个自己喜欢的颜色，让VC持有。

let colorOne = #colorLiteral(red: 0.2392156869, green: 0.6745098233, blue: 0.9686274529, alpha: 1).cgColor
let colorTwo = #colorLiteral(red: 0.8078431487, green: 0.02745098062, blue: 0.3333333433, alpha: 1).cgColor
let colorThree = #colorLiteral(red: 0.9607843161, green: 0.7058823705, blue: 0.200000003, alpha: 1).cgColor
let gradient = CAGradientLayer()

接下来为gradient赋值，将其frame等同于视图的大小，然后颜色先设置为colorOne和colorTwo，起始点和结束点分别为CGPoint(x:0, y:0)和CGPoint(x:1, y:1)，并设置让其在后台线程异步绘制，最后添加到view的layer的sublayer中。

gradient.frame = self.view.bounds
gradient.colors = [colorOne,colorTwo]
gradient.startPoint = CGPoint(x:0, y:0)
gradient.endPoint = CGPoint(x:1, y:1)
gradient.drawsAsynchronously = true
self.view.layer.insertSublayer(gradient, at: 0)

现在运行后会得到下面的结果：

颜色渐变是做到了，那么如何做到颜色渐变的转换呢?这里还是需要用到CABasicAnimation.
在gradient创建完之后，添加并调用一个方法animateGradient，在里面添加一个keyPath为colors的CABasicAnimation，设置动画时长为3s，设置结束值等一系列属性。

func animateGradient() {
  let gradientChangeAnimation = CABasicAnimation(keyPath: "colors")
        gradientChangeAnimation.duration = 3.0
        gradientChangeAnimation.toValue =  [colorTwo,colorThree]
        gradientChangeAnimation.fillMode = CAMediaTimingFillMode.forwards
        gradientChangeAnimation.isRemovedOnCompletion = false
        gradient.add(gradientChangeAnimation, forKey: "gradientChangeAnimation")
      }

这里就完成了转换动画。但是这里有个问题就是这里只转换了一次，无法转换多次颜色。那么这里就需要设置好toValue，让每次的toValue都不一样。
创建一个currentGradient和gradientSet让VC持有。

var currentGradient: Int = 0
var gradientSet = [[CGColor]]()

在animateGradient中每次调用的时候，都对currentGradient的值进行判断和处理。

if currentGradient < gradientSet.count - 1 {
            currentGradient += 1
        } else {
            currentGradient = 0
        }

并修改gradientChangeAnimation的toValue：

let gradientChangeAnimation = CABasicAnimation(keyPath: "colors")
gradientChangeAnimation.duration = 3.0
gradientChangeAnimation.toValue = gradientSet[currentGradient]
gradientChangeAnimation.fillMode = CAMediaTimingFillMode.forwards
gradientChangeAnimation.isRemovedOnCompletion = false
gradientChangeAnimation.repeatCount = Float.infinity
gradient.add(gradientChangeAnimation, forKey: "gradientChangeAnimation")

这里运行后发现还是不行，还是只有一种颜色的转换，这是因为这里只调用了一次animateGradient()。那么如何在合适的时机，也就是动画结束的时候再调用一次animateGradient呢？这里就需要用到CAAnimationDelegate。
在CAAnimationDelegate的animationDidStop方法中重新调用animateGradient。注意这里的gradient.colors 也要改变，否则就会一直是[colorOne, colorTwo]到其他颜色的变换。

func animationDidStop(_ anim: CAAnimation, finished flag: Bool) {
        
        // if our gradient animation ended animating, restart the animation by changing the color set
        if flag {
            gradient.colors = gradientSet[currentGradient]
            animateGradient()
        }
    }

完整代码：

import UIKit

class ViewController: UIViewController, CAAnimationDelegate {
    let colorOne = #colorLiteral(red: 0.2392156869, green: 0.6745098233, blue: 0.9686274529, alpha: 1).cgColor
    let colorTwo = #colorLiteral(red: 0.8078431487, green: 0.02745098062, blue: 0.3333333433, alpha: 1).cgColor
    let colorThree = #colorLiteral(red: 0.9607843161, green: 0.7058823705, blue: 0.200000003, alpha: 1).cgColor
    let gradient = CAGradientLayer()
    
    var currentGradient: Int = 0
    var gradientSet = [[CGColor]]()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        NotificationCenter.default.addObserver(self, selector: #selector(handleEnterForeground), name: UIApplication.willEnterForegroundNotification, object: nil)
        
    }
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        
        createGradientView()
    }
    
    @objc private func handleEnterForeground() {
        animateGradient()
    }
    
    func animateGradient() {
        // cycle through all the colors, feel free to add more to the set
        if currentGradient < gradientSet.count - 1 {
            currentGradient += 1
        } else {
            currentGradient = 0
        }
        
        // animate over 3 seconds
        let gradientChangeAnimation = CABasicAnimation(keyPath: "colors")
        gradientChangeAnimation.duration = 3.0
        gradientChangeAnimation.toValue = gradientSet[currentGradient]
        gradientChangeAnimation.fillMode = CAMediaTimingFillMode.forwards
        gradientChangeAnimation.isRemovedOnCompletion = false
        //gradientChangeAnimation.repeatCount = Float.infinity
        gradientChangeAnimation.delegate = self
        gradient.add(gradientChangeAnimation, forKey: "gradientChangeAnimation")
    }
    
    func createGradientView() {
        
        // overlap the colors and make it 3 sets of colors
        gradientSet.append([colorOne, colorTwo])
        gradientSet.append([colorTwo, colorThree])
        gradientSet.append([colorThree, colorOne])
        
        // set the gradient size to be the entire screen
        gradient.frame = self.view.bounds
        gradient.colors = gradientSet[currentGradient]
        gradient.startPoint = CGPoint(x:0, y:0)
        gradient.endPoint = CGPoint(x:1, y:1)
        gradient.drawsAsynchronously = true
        
        self.view.layer.insertSublayer(gradient, at: 0)
        
        animateGradient()
    }
    func animationDidStop(_ anim: CAAnimation, finished flag: Bool) {
        
        // if our gradient animation ended animating, restart the animation by changing the color set
        if flag {
            gradient.colors = gradientSet[currentGradient]
            animateGradient()
        }
    }
    
}

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