带有两个视图的 SwiftUI 卡片翻转

Question

我正在尝试在两个 SwiftUI 视图之间创建卡片翻转效果。单击原始视图时，它会像翻转卡片一样在 Y 轴上 3D 旋转，并且第二个视图应该在 90 度后开始可见。

使用.rotation3DEffect() 我可以轻松地旋转视图，问题是使用animation() 我不知道如何在角度达到 90 度后触发视图更改...

@State var flipped = false

 var body: some View {

    return VStack{
        Group() {
            if !self.flipped {
                MyView(color: "Blue")
            } else {
                MyView(color: "Red")
            }
        }
        .animation(.default)
        .rotation3DEffect(self.flipped ? Angle(degrees: 90): Angle(degrees: 0), axis: (x: CGFloat(0), y: CGFloat(10), z: CGFloat(0)))
        .onTapGesture {
            self.flipped.toggle()
        }

    }

如何在两个视图之间实现这样的旋转？

Answer 1

简单的解决方案 通过将您的两个视图放在 ZStack 中，然后在 flipped 状态更改时显示/隐藏它们，可以使您采用的方法起作用。第二个视图的旋转需要偏移。但是这个解决方案依赖于两个视图之间的交叉淡入淡出。对于某些用例来说可能没问题。但是有一个更好的解决方案 - 虽然它有点复杂（见下文）。

以下是一种让你的方法奏效的方法：

struct SimpleFlipper : View {
      @State var flipped = false

      var body: some View {

            let flipDegrees = flipped ? 180.0 : 0

            return VStack{
                  Spacer()

                  ZStack() {
                        Text("Front").placedOnCard(Color.yellow).flipRotate(flipDegrees).opacity(flipped ? 0.0 : 1.0)
                        Text("Back").placedOnCard(Color.blue).flipRotate(-180 + flipDegrees).opacity(flipped ? 1.0 : 0.0)
                  }
                  .animation(.easeInOut(duration: 0.8))
                  .onTapGesture { self.flipped.toggle() }
                  Spacer()
            }
      }
}

extension View {

      func flipRotate(_ degrees : Double) -> some View {
            return rotation3DEffect(Angle(degrees: degrees), axis: (x: 1.0, y: 0.0, z: 0.0))
      }

      func placedOnCard(_ color: Color) -> some View {
            return padding(5).frame(width: 250, height: 150, alignment: .center).background(color)
      }
}

更好的解决方案 SwiftUI 有一些有用的动画工具 - 例如 GeometryEffect - 可以生成这种效果的非常流畅的版本。 SwiftUI 实验室有一些关于这个主题的优秀博客文章。具体见：https://swiftui-lab.com/swiftui-animations-part2/

我已经简化并改编了该帖子中的一个示例，以提供卡片翻转功能。

struct FlippingView: View {

      @State private var flipped = false
      @State private var animate3d = false

      var body: some View {

            return VStack {
                  Spacer()

                  ZStack() {
                        FrontCard().opacity(flipped ? 0.0 : 1.0)
                        BackCard().opacity(flipped ? 1.0 : 0.0)
                  }
                  .modifier(FlipEffect(flipped: $flipped, angle: animate3d ? 180 : 0, axis: (x: 1, y: 0)))
                  .onTapGesture {
                        withAnimation(Animation.linear(duration: 0.8)) {
                              self.animate3d.toggle()
                        }
                  }
                  Spacer()
            }
      }
}

struct FlipEffect: GeometryEffect {

      var animatableData: Double {
            get { angle }
            set { angle = newValue }
      }

      @Binding var flipped: Bool
      var angle: Double
      let axis: (x: CGFloat, y: CGFloat)

      func effectValue(size: CGSize) -> ProjectionTransform {

            DispatchQueue.main.async {
                  self.flipped = self.angle >= 90 && self.angle < 270
            }

            let tweakedAngle = flipped ? -180 + angle : angle
            let a = CGFloat(Angle(degrees: tweakedAngle).radians)

            var transform3d = CATransform3DIdentity;
            transform3d.m34 = -1/max(size.width, size.height)

            transform3d = CATransform3DRotate(transform3d, a, axis.x, axis.y, 0)
            transform3d = CATransform3DTranslate(transform3d, -size.width/2.0, -size.height/2.0, 0)

            let affineTransform = ProjectionTransform(CGAffineTransform(translationX: size.width/2.0, y: size.height / 2.0))

            return ProjectionTransform(transform3d).concatenating(affineTransform)
      }
}

struct FrontCard : View {
      var body: some View {
            Text("One thing is for sure – a sheep is not a creature of the air.").padding(5).frame(width: 250, height: 150, alignment: .center).background(Color.yellow)
      }
}

struct BackCard : View {
      var body: some View {
            Text("If you know you have an unpleasant nature and dislike people, this is no obstacle to work.").padding(5).frame(width: 250, height: 150).background(Color.green)
      }
}

更新

OP 询问有关在视图之外管理翻转状态的问题。这可以通过使用绑定来完成。下面是一个实现和演示的片段。 OP 还询问有无动画的翻转。这是一个改变翻转状态（这里是showBack var）是否在动画块内完成的问题。 （该片段不包含FlipEffect 结构，这与上面的代码相同。）

struct ContentView : View {

      @State var showBack = false

      let sample1 = "If you know you have an unpleasant nature and dislike people, this is no obstacle to work."
      let sample2 = "One thing is for sure – a sheep is not a creature of the air."

      var body : some View {

            let front = CardFace(text: sample1, background: Color.yellow)
            let back = CardFace(text: sample2, background: Color.green)
            let resetBackButton = Button(action: { self.showBack = true }) { Text("Back")}.disabled(showBack == true)
            let resetFrontButton = Button(action: { self.showBack = false }) { Text("Front")}.disabled(showBack == false)
            let animatedToggle = Button(action: {
                  withAnimation(Animation.linear(duration: 0.8)) {
                        self.showBack.toggle()
                  }
            }) { Text("Toggle")}


            return
                  VStack() {
                        HStack() {
                              resetFrontButton
                              Spacer()
                              animatedToggle
                              Spacer()
                              resetBackButton
                        }.padding()
                        Spacer()
                        FlipView(front: front, back: back, showBack: $showBack)
                        Spacer()
            }
      }
}


struct FlipView<SomeTypeOfViewA : View, SomeTypeOfViewB : View> : View {

      var front : SomeTypeOfViewA
      var back : SomeTypeOfViewB

      @State private var flipped = false
      @Binding var showBack : Bool

      var body: some View {

            return VStack {
                  Spacer()

                  ZStack() {
                        front.opacity(flipped ? 0.0 : 1.0)
                        back.opacity(flipped ? 1.0 : 0.0)
                  }
                  .modifier(FlipEffect(flipped: $flipped, angle: showBack ? 180 : 0, axis: (x: 1, y: 0)))
                  .onTapGesture {
                        withAnimation(Animation.linear(duration: 0.8)) {
                              self.showBack.toggle()
                        }
                  }
                  Spacer()
            }
      }
}

struct CardFace<SomeTypeOfView : View> : View {
      var text : String
      var background: SomeTypeOfView

      var body: some View {
            Text(text)
                  .multilineTextAlignment(.center)
                  .padding(5).frame(width: 250, height: 150).background(background)
      }
}