# subscript(_:) Accesses the state for a custom key. ``` subscript(key: K.Type) -> K.Value where K : AnimationStateKey { get set } ``` ## Overview Create a custom animation state value by defining a key that conforms to the [`AnimationStateKey`](/documentation/SwiftUI/AnimationStateKey) protocol and provide the [`defaultValue`](/documentation/SwiftUI/AnimationStateKey/defaultValue) for the key. Also include properties to read and write state values that your [`CustomAnimation`](/documentation/SwiftUI/CustomAnimation) uses. For example, the following code defines a key named `PausableState` that has two state values, `paused` and `pauseTime`: ``` private struct PausableState: AnimationStateKey { var paused = false var pauseTime: TimeInterval = 0.0 static var defaultValue: Self { .init() } } ``` Use that key with the subscript operator of the [`AnimationState`](/documentation/SwiftUI/AnimationState) structure to get and set a value for the key. For more convenient access to the key value, extend [`AnimationContext`](/documentation/SwiftUI/AnimationContext) with a computed property that gets and sets the key’s value. ``` extension AnimationContext { fileprivate var pausableState: PausableState { get { state[PausableState.self] } set { state[PausableState.self] = newValue } } } ``` To access the state values in a [`CustomAnimation`](/documentation/SwiftUI/CustomAnimation), call the custom computed property, then read and write the state values that the custom [`AnimationStateKey`](/documentation/SwiftUI/AnimationStateKey) provides. ``` struct PausableAnimation: CustomAnimation { let base: Animation func animate(value: V, time: TimeInterval, context: inout AnimationContext) -> 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 } } ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)