AnimationContext

struct · iOS 17.0+ · SDK 27.1

struct AnimationContext<Value> where Value : VectorArithmetic

Contextual values that a custom animation can use to manage state and access a view's environment. The system provides an `AnimationContext` to a ``CustomAnimation`` instance so that the animation can store and retrieve values in an instance of ``AnimationState``. To access these values, use the context's ``AnimationContext/state`` property. For more convenient access to state, create an ``AnimationStateKey`` and extend `AnimationContext` to include a computed property that gets and sets the ``AnimationState`` value. Then use this property instead of ``AnimationContext/state`` to retrieve the state of a custom animation. For example, the following code creates an animation state key named `PausableState`. Then the code extends `AnimationContext` to include the `pausableState` property: private struct PausableState<Value: VectorArithmetic>: AnimationStateKey { var paused = false var pauseTime: TimeInterval = 0.0 static var defaultValue: Self { .init() } } extension AnimationContext { fileprivate var pausableState: PausableState<Value> { get { state[PausableState<Value>.self] } set { state[PausableState<Value>.self] = newValue } } } To access the pausable state, the custom animation `PausableAnimation` uses the `pausableState` property instead of the ``AnimationContext/state`` property: struct PausableAnimation: CustomAnimation { let base: Animation func animate<V>(value: V, time: TimeInterval, context: inout AnimationContext<V>) -> V? where V : VectorArithmetic { let paused = context.environment.animationPaused let pausableState = context.pausableState var pauseTime = pausableState.pauseTime if pausableState.paused != paused { pauseTime = time - pauseTime context.pausableState = PausableState(paused: paused, pauseTime: pauseTime) } let effectiveTime = paused ? pauseTime : time - pauseTime let result = base.animate(value: value, time: effectiveTime, context: &context) return result } } The animation can also retrieve environment values of the view that created the animation. To retrieve a view's environment value, use the context's ``AnimationContext/environment`` property. For instance, the following code creates a custom ``EnvironmentValues`` property named `animationPaused`, and the view `PausableAnimationView` uses the property to store the paused state: extension EnvironmentValues { @Entry var animationPaused: Bool = false } struct PausableAnimationView: View { @State private var paused = false var body: some View { VStack { ... } .environment(\.animationPaused, paused) } } Then the custom animation `PausableAnimation` retrieves the paused state from the view's environment using the ``AnimationContext/environment`` property: struct PausableAnimation: CustomAnimation { func animate<V>(value: V, time: TimeInterval, context: inout AnimationContext<V>) -> V? where V : VectorArithmetic { let paused = context.environment.animationPaused ... } }

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