为什么 MagnificationGesture 在 SwiftUI 中只更新 state 一次？

Question

我希望下面的视图响应放大手势，但它只在第一次起作用。 视图可以按预期放大，但视图释放后，后续触摸不会触发手势。

有没有办法让手势连续工作？

struct Card: View {
    @State var magScale: CGFloat = 1

    var magnification: some Gesture {
        MagnificationGesture().onChanged { gesture in magScale = gesture }
    }

    var body: some View {
        Rectangle()
            .frame(width: 200, height: 200, alignment: .center)
            .scaleEffect(self.magScale)
            .gesture(magnification)
    }
}

该视图的使用方式如下：

struct ContentView: View {
    var body: some View {
        ZStack {
            Color.blue
            Card()
        }
    }
}

Answer 1

这是一个解决方案。 使用Xcode 12测试（对于以下版本，可能需要调整一些语法）

struct Card: View {
    @State var magScale: CGFloat = 1
    @State var progressingScale: CGFloat = 1

    var magnification: some Gesture {
        MagnificationGesture()
            .onChanged { progressingScale = $0 }
            .onEnded {
                magScale *= $0
                progressingScale = 1
            }
    }

    var body: some View {
        Rectangle()
            .frame(width: 200, height: 200, alignment: .center)
            .scaleEffect(self.magScale * progressingScale)
            .gesture(magnification)
    }
}

Answer 2

下面的代码在设备上运行良好。 除了添加点击和拖动的定义外，没有重大变化。

struct ContentView: View {
    var body: some View {

        ZStack {
            Color.blue
            Card()
        }
    }
}

struct Card: View {
    @State var magScale: CGFloat = 1
    @State var tapped: Bool = false
    @State var isDragging = false

    var magnification: some Gesture {
        MagnificationGesture().onChanged { gesture in
            self.magScale = gesture
        }
    }

    var tap: some Gesture {
        TapGesture(count: 1).onEnded { _ in
        self.tapped = !self.tapped
        }
    }

    var drag: some Gesture {
        DragGesture()
            .onChanged {_ in
                self.isDragging = true
            }
            .onEnded {_ in
                self.isDragging = false
            }
    }


    var body: some View {
        Rectangle()
            .foregroundColor(Color.green)
            .frame(width: 200, height: 200, alignment: .center)
            .scaleEffect(self.magScale)             
          .gesture(tap.simultaneously(with:magnification).simultaneously(with: drag))
    }
}