SwiftUI 动态添加子视图，但 animation 不起作用

Question

I would like to create a view in SwiftUI that add a subview dynamically and with animation.

struct ContentView : View {
    @State private var isButtonVisible = false

    var body: some View {
        VStack {
            Toggle(isOn: $isButtonVisible.animation()) {
                Text("add view button")
            }

           if isButtonVisible {
                 AnyView(DetailView())
                      .transition(.move(edge: .trailing))
                      .animation(Animation.linear(duration: 2))
             }else{
                    AnyView(Text("test"))
            }
        }
    }
}

The above code works fine with the animation. however when i move the view selection part into a function, the animation is not working anymore (since i want to add different views dynamically, therefore, I put the logic in a function.)

struct ContentView : View {
    @State private var isButtonVisible = false
    var body: some View {
        VStack {
            Toggle(isOn: $isButtonVisible.animation()) {
                Text("add view button")
            }

            subView().transition(.move(edge: .trailing))
                     .animation(Animation.linear(duration: 2))   
    }

     func subView() -> some View {
         if isButtonVisible {
             return AnyView(DetailView())
         }else{
            return AnyView(Text("test"))
        }
    }
}

it looks totally the same to me, however, i don't understand why they have different result. Could somebody explain me why? and any better solutions? thanks alot!

Answer 1

Here's your code, modified so that it works:

struct ContentView : View {
    @State private var isButtonVisible = false

    var body: some View {
        VStack {
            Toggle(isOn: $isButtonVisible.animation()) {
                Text("add view button")
            }

            subView()
                .transition(.move(edge: .trailing))
                .animation(Animation.linear(duration: 2))
        }
    }

    func subView() -> some View {
        Group {
            if isButtonVisible {
                DetailView()
            } else {
                Text("test")
            }
        }
    }
}

Note two things:

1. Your two examples above are different, which is why you get different results. The first applies a transition and animation to a DetailView, then type-erases it with AnyView. The second type-erases a DetailView with AnyView, then applies a transition and animation.
2. Rather that using AnyView and type-erasure, I prefer to encapsulate the conditional logic inside of a Group view. Then the type you return is Group , which will animate properly.
3. If you wanted different animations on the two possibilities for your subview, you can now apply them directly to DetailView() or Text("test") .

Update

The Group method will only work with if , elseif , and else statements. If you want to use a switch, you will have to wrap each branch in AnyView() . However, this breaks transitions/animations. Using switch and setting custom animations is currently not possible.

Answer 2

I was able to get it to work with a switch statement by wrapping the function that returns an AnyView in a VStack . I also had to give the AnyView an .id so SwiftUI can know when it changes. This is on Xcode 11.3 and iOS 13.3

struct EnumView: View {

    @ObservedObject var viewModel: ViewModel

    var body: some View {
        VStack {
            view(for: viewModel.viewState)
                .id(viewModel.viewState)
                .transition(.opacity)
        }
    }

    func view(for viewState: ViewModel.ViewState) -> AnyView {
        switch viewState {
        case .loading:
            return AnyView(LoadingStateView(viewModel: self.viewModel))
        case .dataLoaded:
            return AnyView(LoadedStateView(viewModel: self.viewModel))
        case let .error(error):
            return AnyView(ErrorView(error: error, viewModel: viewModel))
        }
    }

}

Also for my example in the ViewModel I need to wrap the viewState changes in a withAnimation block

withAnimation {
    self.viewState = .loading
}

Answer 3

In iOS 14 they added the possibility to use if let and switch statements in function builders. Maybe it helps for your issues:

https://www.hackingwithswift.com/articles/221/whats-new-in-swiftui-for-ios-14 (at the article's bottom)