如果希望 SwiftUI 视图一出现就立即开始动画处理，则应使用 onAppear() 修饰符附加动画。 首先，我将向您展示基本代码，然后再展示两个用来简化此过程的扩展程序。
首先，是简单的版本–这会创建一个永远缩放的圆圈：
struct ContentView: View {
@State var scale: CGFloat = 1
var body: some View {
Circle()
.frame(width: 200, height: 200)
.scaleEffect(scale)
.onAppear {
let baseAnimation = Animation.easeInOut(duration: 1)
let repeated = baseAnimation.repeatForever(autoreverses: true)
withAnimation(repeated) {
scale = 0.5
}
}
}
}
如果您打算频繁添加初始动画，则明智的做法是在 View 协议中添加一些扩展，以使其变得更容易。
为了说明这一点，下面的两个扩展添加了 animate() 和 animateForever() 修饰符，使您可以自定义所需的动画并整齐地包装整个行为：
// Create an immediate animation.
extension View {
func animate(using animation: Animation = Animation.easeInOut(duration: 1), _ action: @escaping () -> Void) -> some View {
onAppear {
withAnimation(animation) {
action()
}
}
}
}
// Create an immediate, looping animation
extension View {
func animateForever(using animation: Animation = Animation.easeInOut(duration: 1), autoreverses: Bool = false, _ action: @escaping () -> Void) -> some View {
let repeated = animation.repeatForever(autoreverses: autoreverses)
return onAppear {
withAnimation(repeated) {
action()
}
}
}
}
// Try out our extensions with the scaling circle.
struct ContentView: View {
@State var scale: CGFloat = 1
var body: some View {
Circle()
.frame(width: 200, height: 200)
.scaleEffect(scale)
.animateForever(autoreverses: true) { scale = 0.5 }
}
}