AnimationContext
Contextual values that a custom animation can use to manage state and access a view’s environment.
Declaration
struct AnimationContext<Value> where Value : VectorArithmeticOverview
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 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 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 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 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 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
...
}
}