ios - 如何为 CATextLayer 的字符串设置动画？

Question

我目前正在使用 AVMutableComposition 在视频中叠加一些文本，我想根据某个时间间隔重复更改字符串。我正在尝试使用核心动画来实现这一点；但是，字符串属性似乎不可设置动画。有没有其他方法可以达到目的？谢谢。

代码(不工作):

func getSubtitlesAnimation(withFrames frames: [String], duration: CFTimeInterval)->CAKeyframeAnimation {
    let animation = CAKeyframeAnimation(keyPath:"string")
    animation.calculationMode = kCAAnimationDiscrete
    animation.duration = duration
    animation.values = frames
    animation.keyTimes = [0,0.5,1]
    animation.repeatCount = Float(frames.count)
    animation.isRemovedOnCompletion = false
    animation.fillMode = kCAFillModeForwards
    animation.beginTime = AVCoreAnimationBeginTimeAtZero
    return animation
}

Answer 1

字符串不是 CATextLayer 上的动画路径。这是 list of keyPaths你可以在任何 CALayer 上使用。

关于将 Core Animation 与 AVFoundation 一起使用的其他一些重要事项。

• 所有的动画都必须有 removedCompletion 的 false。
• 您只能将一个关键路径动画应用于一个图层一次。这意味着如果你想添加淡入和淡出的不透明度，它需要组合成一个关键帧动画。在核心动画中，您可以使用 beginTime 并应用两个不同的不透明度动画，但根据我使用 AVFoundation 和核心动画的经验，这不起作用。因此，如果您想在 CALayer 的同一键路径上使用两种不同的(前不透明度)动画，您将必须计算出您希望发生什么的键时间和值。
• 您的离散曲线仍然有效，但您必须计算出关键时间和值。
• 因为字符串不能用作动画属性，所以下一个最好的办法是使用多个 CATextLayer 并将它们一个接一个地淡入淡出。这是一个例子。将 CACurrentMediaTime() 替换为 AVCoreAnimationBeginTimeAtZero 以便与 AVFoundation 一起使用。这只是一个示例，因此您可以想象您想要什么。

Example1Using-Discrete

 import UIKit

class ViewController: UIViewController {

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

        //center the frame
        let textFrame = CGRect(x: (self.view.bounds.width - 200)/2, y: (self.view.bounds.height - 100)/2, width: 200, height: 50)

        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now()) {
            //animation spacing could be a negative value of half the animation to appear to fade between strings
            self.animateText(subtitles: ["Hello","Good Morning","Good Afternoon","Good Evening","Goodnight","Goodbye"], duration: 2, animationSpacing: 0, frame: textFrame, targetLayer: self.view.layer)
        }
    }

    func animateText(subtitles:[String],duration:Double,animationSpacing:Double,frame:CGRect,targetLayer:CALayer){
        var currentTime : Double = 0
        for x in 0..<subtitles.count{
            let string = subtitles[x]
            let textLayer = CATextLayer()
            textLayer.frame = frame
            textLayer.string = string
            textLayer.font = UIFont.systemFont(ofSize: 20)
            textLayer.foregroundColor = UIColor.black.cgColor
            textLayer.fontSize = 20.0
            textLayer.alignmentMode = kCAAlignmentCenter
            let anim = getSubtitlesAnimation(duration: duration, startTime: currentTime)
            targetLayer.addSublayer(textLayer)
            textLayer.add(anim, forKey: "opacityLayer\(x)")
            currentTime += duration + animationSpacing
        }
    }
    func getSubtitlesAnimation(duration: CFTimeInterval,startTime:Double)->CAKeyframeAnimation {
        let animation = CAKeyframeAnimation(keyPath:"opacity")
        animation.duration = duration
        animation.calculationMode = kCAAnimationDiscrete
        //have to fade in and out with a single animation because AVFoundation
        //won't allow you to animate the same propery on the same layer with
        //two different animations
        animation.values = [0,1,1,0,0]
        animation.keyTimes = [0,0.001,0.99,0.999,1]
        animation.isRemovedOnCompletion = false
        animation.fillMode = kCAFillModeBoth
        //Replace with AVCoreAnimationBeginTimeAtZero for AVFoundation
        animation.beginTime = CACurrentMediaTime() + startTime
        return animation
    }
}

示例 2 - 使用长淡入淡出的 Gif 附件

import UIKit

class ViewController: UIViewController {

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

        //center the frame
        let textFrame = CGRect(x: (self.view.bounds.width - 200)/2, y: (self.view.bounds.height - 100)/2, width: 200, height: 50)

        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now()) {
            //animation spacing could be a negative value of half the animation to appear to fade between strings
            self.animateText(subtitles: ["Hello","Good Morning","Good Afternoon","Good Evening","Goodnight","Goodbye"], duration: 4, animationSpacing: -2, frame: textFrame, targetLayer: self.view.layer)
        }
    }

    func animateText(subtitles:[String],duration:Double,animationSpacing:Double,frame:CGRect,targetLayer:CALayer){
        var currentTime : Double = 0
        for x in 0..<subtitles.count{
            let string = subtitles[x]
            let textLayer = CATextLayer()
            textLayer.frame = frame
            textLayer.string = string
            textLayer.font = UIFont.systemFont(ofSize: 20)
            textLayer.foregroundColor = UIColor.black.cgColor
            textLayer.fontSize = 20.0
            textLayer.alignmentMode = kCAAlignmentCenter
            let anim = getSubtitlesAnimation(duration: duration, startTime: currentTime)
            targetLayer.addSublayer(textLayer)
            textLayer.add(anim, forKey: "opacityLayer\(x)")
            currentTime += duration + animationSpacing
        }
    }
    func getSubtitlesAnimation(duration: CFTimeInterval,startTime:Double)->CAKeyframeAnimation {
        let animation = CAKeyframeAnimation(keyPath:"opacity")
        animation.duration = duration
        //have to fade in and out with a single animation because AVFoundation
        //won't allow you to animate the same propery on the same layer with
        //two different animations
        animation.values = [0,0.5,1,0.5,0]
        animation.keyTimes = [0,0.25,0.5,0.75,1]
        animation.isRemovedOnCompletion = false
        animation.fillMode = kCAFillModeBoth
        //Replace with AVCoreAnimationBeginTimeAtZero for AVFoundation
        animation.beginTime = CACurrentMediaTime() + startTime
        return animation
    }
}

结果:持续时间为 2 秒进出 2 秒。可以是即时的。