state

Declaration

var state: AnimationState<Value>

Discussion

An instance of CustomAnimation uses this property to read and write state values as the animation runs.

An alternative to using the state property in a custom animation is to create an AnimationStateKey type and extend AnimationContext with a custom property that returns the state as a custom type. For example, the following code creates a state key named PausableState. It’s convenient to store state values in the key type, so the PausableState structure includes properties for the stored state values paused and pauseTime.

private struct PausableState<Value: VectorArithmetic>: AnimationStateKey {
    var paused = false
    var pauseTime: TimeInterval = 0.0

    static var defaultValue: Self { .init() }
}

To provide access to the pausable state, the following code extends AnimationContext to include the pausableState property. This property returns an instance of the custom PausableState structure stored in state, and it can also store a new PausableState instance in state.

extension AnimationContext {
    fileprivate var pausableState: PausableState<Value> {
        get { state[PausableState<Value>.self] }
        set { state[PausableState<Value>.self] = newValue }
    }
}

Now a custom animation can use the pausableState property instead of the state property as a convenient way to read and write state values as the animation runs.

struct PausableAnimation: CustomAnimation {
    func animate<V>(value: V, time: TimeInterval, context: inout AnimationContext<V>) -> V? where V : VectorArithmetic {
        let pausableState = context.pausableState
        var pauseTime = pausableState.pauseTime
        ...
    }
}