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How to animate changes in binding values

Paul Hudson    @twostraws   

Fully updated for Xcode 11.2

SwiftUI’s two-way bindings let us adjust the state of our program, and we can respond to that by adjusting our view hierarchy. For example, we might make some text appear or disappear, or adjust the opacity of a view.

Rather than having a state change happen immediately, we can animate changes caused by a binding being modified by adding animation() to our binding. For example, this view has a toggle that shows or hides a text view depending on the stage of the toggle:

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

    var body: some View {
        VStack {
            Toggle(isOn: $showingWelcome) {
                Text("Toggle label")
            }

            if showingWelcome {
                Text("Hello World")
            }
        }
    }
}

Without animation, the text view will just appear or disappear immediately, which causes a visual jump. If we modify the toggle so it was bound to $showingWelcome.animation() then the text view will slide in smoothly:

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

    var body: some View {
        VStack {
            Toggle(isOn: $showingWelcome.animation()) {
                Text("Toggle label")
            }

            if showingWelcome {
                Text("Hello World")
            }
        }
    }
}

If you want more control over the animation, you can pass parameters to animation() that affect how the transition happens. For example, this will bring the text in with a simple spring animation:

Toggle(isOn: $showingWelcome.animation(.spring())) {
    Text("Toggle label")
}

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