制作CALayer动画的安全方法

问题描述

当我寻找CALayer动画时，我发现了类似的解决方案：

let basicAnimation = CABasicAnimation(keyPath: "opacity")
basicAnimation.fromValue = 0
basicAnimation.tovalue = 1
basicAnimation.duration = 0.3
add(basicAnimation,forKey: "opacity")

但是fromValue和tovalue是Any的类型，并且作为键，我们可以使用任何字符串，这是不安全的。使用最新的Swift功能有更好的方法吗？

解决方法

我想出了使用非常简单的解决方案：

layer.animate(.init(
    keyPath: \.opacity,value: "1",// this will produce an error
    duration: 0.3)
)
layer.animate(.init(
    keyPath: \.opacity,value: 1,// correct
    duration: 0.3)
)
layer.animate(.init(
    keyPath: \.backgroundColor,value: UIColor.red,// this will produce an error
    duration: 0.3,timingFunction: .init(name: .easeOut),beginFromCurrentState: true)
)
layer.animate(.init(
    keyPath: \.backgroundColor,value: UIColor.red.cgColor,// correct
    duration: 0.3,beginFromCurrentState: true)
)

解决方案代码为：

import QuartzCore

extension CALayer {
    struct Animation<Value> {
        let keyPath: ReferenceWritableKeyPath<CALayer,Value>
        let value: Value
        let duration: TimeInterval
        let timingFunction: CAMediaTimingFunction? = nil
        let beginFromCurrentState = false
    }
    
    @discardableResult func animate<Value>(
        _ animation: Animation<Value>,completionHandler: (() -> Void)? = nil)
    -> CABasicAnimation?
    {
        CATransaction.begin()
        CATransaction.setCompletionBlock(completionHandler)
        defer {
            // update actual value with the final one
            self[keyPath: animation.keyPath] = animation.value
            CATransaction.commit()
        }
        guard animation.duration > 0 else { return nil }
        let fromValueLayer: CALayer
        if animation.beginFromCurrentState,let presentation = presentation() {
            fromValueLayer = presentation
        } else {
            fromValueLayer = self
        }
        let basicAnimation = CABasicAnimation(
            keyPath: NSExpression(forKeyPath: animation.keyPath).keyPath
        )
        basicAnimation.timingFunction = animation.timingFunction
        basicAnimation.fromValue = fromValueLayer[keyPath: animation.keyPath]
        basicAnimation.toValue = animation.value
        basicAnimation.duration = animation.duration
        
        add(basicAnimation,forKey: basicAnimation.keyPath)
        return basicAnimation
    }
}

优点：

• 在CALayer上可用的keyPath自动补全
• 值类型取决于keyPath，因此您将无法设置错误的值
• 清除代码

缺点：

• 我们仍然可以选择不可设置动画的keyPath