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How to start an animation immediately after a view appears

Paul Hudson    @twostraws   

Updated for Xcode 12.0

If you want a SwiftUI view to start animating as soon as it appears, you should use the onAppear() modifier to attach an animation. I’ll show you the basic code first, then show you two extensions I use to make this process easier.

First, the simple version – this creates a circle that scales up and down forever:

struct ContentView: View {
    @State var scale: CGFloat = 1

    var body: some View {
        Circle()
            .scaleEffect(scale)
            .onAppear {
                let baseAnimation = Animation.easeInOut(duration: 1)
                let repeated = baseAnimation.repeatForever(autoreverses: true)

                return withAnimation(repeated) {
                    self.scale = 0.5
                }
            }
    }
}

If you intend to add initial animations frequently, it’s a smart idea to add some extensions to the View protocol to make it easier. The two extensions below add animate() and animateForever() modifiers that let you customize the animation you want, and wrap up the whole behavior neatly:

extension View {
    func animate(using animation: Animation = Animation.easeInOut(duration: 1), _ action: @escaping () -> Void) -> some View {
        return onAppear {
            withAnimation(animation) {
                action()
            }
        }
    }
}

extension View {
    func animateForever(using animation: Animation = Animation.easeInOut(duration: 1), autoreverses: Bool = false, _ action: @escaping () -> Void) -> some View {
        let repeated = animation.repeatForever(autoreverses: autoreverses)

        return onAppear {
            withAnimation(repeated) {
                action()
            }
        }
    }
}

With that in place, animating our circle is much easier:

Circle()
    .scaleEffect(scale)
    .animateForever(autoreverses: true) { self.scale = 0.5 }
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