// 音频播放器（AudioPlayer）类即为上下文。它还会维护指向状态类实例的引用，
// 该状态类则用于表示音频播放器当前的状态。
class AudioPlayer is
    field state: State
    field UI, volume, playlist, currentSong

    constructor AudioPlayer() is
        this.state = new ReadyState(this)

        // 上下文将处理用户输入的工作委派给状态对象。由于每个状态都以不同
        // 的方式处理输入，其结果自然将依赖于当前所处的状态。
        UI = new UserInterface()
        UI.lockButton.onClick(this.clickLock)
        UI.playButton.onClick(this.clickPlay)
        UI.nextButton.onClick(this.clickNext)
        UI.prevButton.onClick(this.clickPrevious)

    // 其他对象必须能切换音频播放器当前所处的状态。
    method changeState(state: State) is
        this.state = state

    // UI 方法将执行工作委派给当前状态。
    method clickLock() is
        state.clickLock()
    method clickPlay() is
        state.clickPlay()
    method clickNext() is
        state.clickNext()
    method clickPrevious() is
        state.clickPrevious()

    // 状态可调用上下文的一些服务方法。
    method startPlayback() is
        // ...
    method stopPlayback() is
        // ...
    method nextSong() is
        // ...
    method previousSong() is
        // ...
    method fastForward(time) is
        // ...
    method rewind(time) is
        // ...


// 所有具体状态类都必须实现状态基类声明的方法，并提供反向引用指向与状态相
// 关的上下文对象。状态可使用反向引用将上下文转换为另一个状态。
abstract class State is
    protected field player: AudioPlayer

    // 上下文将自身传递给状态构造函数。这可帮助状态在需要时获取一些有用的
    // 上下文数据。
    constructor State(player) is
        this.player = player

    abstract method clickLock()
    abstract method clickPlay()
    abstract method clickNext()
    abstract method clickPrevious()


// 具体状态会实现与上下文状态相关的多种行为。
class LockedState extends State is

    // 当你解锁一个锁定的播放器时，它可能处于两种状态之一。
    method clickLock() is
        if (player.playing)
            player.changeState(new PlayingState(player))
        else
            player.changeState(new ReadyState(player))

    method clickPlay() is
        // 已锁定，什么也不做。

    method clickNext() is
        // 已锁定，什么也不做。

    method clickPrevious() is
        // 已锁定，什么也不做。


// 它们还可在上下文中触发状态转换。
class ReadyState extends State is
    method clickLock() is
        player.changeState(new LockedState(player))

    method clickPlay() is
        player.startPlayback()
        player.changeState(new PlayingState(player))

    method clickNext() is
        player.nextSong()

    method clickPrevious() is
        player.previousSong()


class PlayingState extends State is
    method clickLock() is
        player.changeState(new LockedState(player))

    method clickPlay() is
        player.stopPlayback()
        player.changeState(new ReadyState(player))

    method clickNext() is
        if (event.doubleclick)
            player.nextSong()
        else
            player.fastForward(5)

    method clickPrevious() is
        if (event.doubleclick)
            player.previous()
        else
            player.rewind(5)