animation - 简单的过渡动画在 SwiftUI 中不起作用

Question

我想了解 SwiftUI 中的过渡动画，但它对我不起作用。在下面的代码中，一秒后的转换是立即的(无淡入淡出)。

struct TestApp: App {
    
    @State private var isReady: Bool = false
    
    var body: some Scene {
        WindowGroup {
            if isReady {
                Color.red
            } else {
                Color.blue
                    .transition(.opacity)
                    .onAppear {
                        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
                            withAnimation {
                                isReady = true
                            }
                        }
                    }
            }
        }
    }
}

Answer 1

过渡和动画是不同的东西，有一个要显示过渡的 View ，还有一个为该过渡设置动画的 View 。此外，应用程序不会更新 View ，状态应该在 View 内。

所以这是一个正确的变体:

struct TestApp: App {
    var body: some Scene {
        WindowGroup {
          ContentView()  // root view of the scene !!
        }
    }
}

struct ContentView: View {
    // state of the view refreshes the view
    @State private var isReady: Bool = false

    var body: some View {
       ZStack {           // animating container !!
            if isReady {
                Color.red
//                    .transition(.opacity)  // probably you wanted this as well
            } else {
                Color.blue
                    .transition(.opacity)  // << transition here !!
            }
        }
        .onAppear {   // << responsible for animation !!
            DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
                withAnimation {
                    isReady = true
                }
            }
        }
    }
}