onChange(of:initial:_:)

Declaration

nonisolated func onChange<V>(of value: V, initial: Bool = false, _ action: @escaping (V, V) -> Void) -> some View where V : Equatable

Parameters

• value:

  The value to check against when determining whether to run the closure.

• initial:

  Whether the action should be run when this view initially appears.

• action:

  A closure to run when the value changes.

  oldValue

  The old value that failed the comparison check (or the initial value when requested).

  newValue

  The new value that failed the comparison check.

Return Value

A view that fires an action when the specified value changes.

Discussion

You can use onChange to trigger a side effect as the result of a value changing, such as an Environment key or a Binding.

The system may call the action closure on the main actor, so avoid long-running tasks in the closure. If you need to perform such tasks, detach an asynchronous background task.

When the value changes, the new version of the closure will be called, so any captured values will have their values from the time that the observed value has its new value. The old and new observed values are passed into the closure. In the following code example, PlayerView passes both the old and new values to the model.

struct PlayerView: View {
    var episode: Episode
    @State private var playState: PlayState = .paused

    var body: some View {
        VStack {
            Text(episode.title)
            Text(episode.showTitle)
            PlayButton(playState: $playState)
        }
        .onChange(of: playState) { oldState, newState in
            model.playStateDidChange(from: oldState, to: newState)
        }
    }
}