Animations in SwiftUI

26 Jun 2019

SwiftUI brings declarative and straightforward approach in building User Interfaces. We have List and Form components and Bindings. All of these things make SwiftUI so easy to use and very powerful. But today we are going to talk about another feature of SwiftUI, and it is animations.

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Animation

You can smoothly animate any change in SwiftUI by wrapping it into withAnimation block. By default, SwiftUI uses fade in and fade out for animating changes. Let’s take a look at a small example.

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

    var body: some View {
        VStack {
            Button(action: {
                withAnimation {
                    self.isButtonVisible.toggle()
                }
            }) {
                Text("Press me")
            }

            if isButtonVisible {
                Button(action: {}) {
                    Text("Hidden Button")
                }
            }
        }
    }
}

In the current example, we wrap the State change with withAnimation block, and it produces nice fade in animation. You can modify animation by passing timing and spring values. Another option can be attaching animation modifier to the animating view.

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

    var body: some View {
        VStack {
            Button(action: {
                self.isButtonVisible.toggle()
            }) {
                Text("Press me")
            }

            if isButtonVisible {
                Button(action: {}) {
                    Text("Hidden Button")
                }.animation(.easeInOut, value: isButtonVisible)
            }
        }
    }
}

In the code sample above, we achieve the same animation by simply adding animation modifier. We use easeInOut animation, but you can pass custom animation properties. The second parameter is the equatable value that SwiftUI observes to understand changes and animate them.

Sometimes we have a situation where multiple views depend on some state, and we want to animate all depending view changes together. For this case, we have animatable bindings.

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

    var body: some View {
        VStack {
            Toggle(isOn: $isButtonVisible.animation()) {
                Text("Show/Hide button")
            }

            if isButtonVisible {
                Button(action: {}) {
                    Text("Hidden Button")
                }
            }
        }
    }
}

As you can see, we can easily convert our binding into animatable binding by calling animation method on it. This method wraps every change of binding value into an animation block. You can pass animation settings as parameters to this method. More about bindings you can read in my previous post.

Transitions

As I said before, SwiftUI uses fade in and fade out transition by default, but we can apply any other transition we want. Let’s replace fading with moving.

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

    var body: some View {
        VStack {
            Toggle(isOn: $isButtonVisible.animation()) {
                Text("Show/Hide button")
            }

            if isButtonVisible {
                Button(action: {}) {
                    Text("Hidden Button")
                }.transition(.move(edge: .trailing))
            }
        }
    }
}

In the example above, we attach transition modifier to the view. SwiftUI has a bunch of ready to use transitions like move, slide, scale, offset, opacity, etc. We can combine them into a single transition. Let’s take a look at the example.

extension AnyTransition {
    static func moveAndScale(edge: Edge) -> AnyTransition {
        AnyTransition.move(edge: edge).combined(with: .scale())
    }
}

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

    var body: some View {
        VStack {
            Toggle(isOn: $isButtonVisible.animation()) {
                Text("Show/Hide button")
            }

            if isButtonVisible {
                Button(action: {}) {
                    Text("Hidden Button")
                }.transition(.moveAndScale(edge: .trailing))
            }
        }
    }
}

We created a moveAndScale transition, which is basically a combination of move and scale transitions. SwiftUI applies the current transition symmetrically according to timing or spring values which you pass into the animation method.

SwiftUI provides a way of building asymmetric transitions also. Assume that you need a move transition on insertion and a fade transition on removal. For those cases, we have an asymmetric method on AnyTransition struct, which we can use to build asymmetric transitions.

extension AnyTransition {
    static func moveOrFade(edge: Edge) -> AnyTransition {
        AnyTransition.asymmetric(
            insertion: .move(edge: edge),
            removal: .opacity
        )
    }
}

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

    var body: some View {
        VStack {
            Toggle(isOn: $isButtonVisible.animation()) {
                Text("Show/Hide button")
            }

            if isButtonVisible {
                Button(action: {}) {
                    Text("Hidden Button")
                }.transition(.moveOrFade(edge: .trailing))
            }
        }
    }
}

As you can see, we use asymmetric method to pass two transitions, the first one for insertion and another one for removal. We can pass here combined transition which we created.

Conclusion

Today we discussed multiple ways of animating changes in SwiftUI. It totally depends on you and on use-case which way you have to choose. By spending more and more time with SwiftUI, I understand that it is already a compelling framework. Feel free to follow me on Twitter and ask your questions related to this post. Thanks for reading and see you next week!