Allows an object to change its behavior when its internal state changes.

The object appears to change its class.

abstract class PlayerState {
  void pressPlay(MusicPlayer player);
}

class StoppedState implements PlayerState {
  @override
  void pressPlay(MusicPlayer player) {
    print('Starting playback');
    player.state = PlayingState();
  }
}

class PlayingState implements PlayerState {
  @override
  void pressPlay(MusicPlayer player) {
    print('Pausing playback');
    player.state = StoppedState();
  }
}

class MusicPlayer {
  PlayerState state = StoppedState();

  void pressPlay() => state.pressPlay(this);
}

// Usage
final player = MusicPlayer();
player.pressPlay(); // Starting playback
player.pressPlay(); // Pausing playback
player.pressPlay(); // Starting playback