状态模式允许一个对象在其内部状态改变时改变它的行为。用电梯来举例，电梯可以认为具有开门、关门、运行、停止四种状态，这四种状态之间的切换具有多种限制，比如在开门状态下不电梯不能运行，只能转为关门状态；在运行状态下，电梯只能转为停止状态...
设想一下，如果要常规的if-else或者switch-case描述电梯的这几种状态间的切换，将生成非常复杂的、逻辑相互交织的代码，可读性差且不易维护。
而如果用状态模式来实现，会是怎样的呢？
首先创建LiftState，代表抽象的电梯状态，包含了电梯的四个动作(方法)，通过这些方法可以切换到对应的状态。public abstract class LiftState{ protected Context context; public void SetContext(Context context) { this.context = context; } public abstract void Open(); public abstract void Close(); public abstract void Run(); public abstract void Stop();}
Context是上下文类，它的作用是串联各个状态的过渡，在LiftSate抽象类中把Context类角色聚合进来，并传递到子类，这样4个具体的实现类中自己根据环境来决定如何进行状态的过渡。public class Context{ public readonly static OpenningState openningState = new OpenningState(); public readonly static ClosingState closingState = new ClosingState(); public readonly static RunningState runningState = new RunningState(); public readonly static StoppingState stoppingState = new StoppingState(); private LiftState liftState; public LiftState LiftState { get { return liftState; } set { liftState = value; liftState.SetContext(this); } } public void Open() { this.liftState.Open(); } public void Close() { this.liftState.Close(); } public void Run() { this.liftState.Run(); } public void Stop() { this.liftState.Stop(); }}
接下来是四个具体的状态类，负责状态之间的切换和控制，以OpenningState为例，只能切换到Closing状态，其它切换状态的方法都是空实现。public class OpenningState : LiftState{ public override void Close() { base.context.LiftState = Context.closingState; base.context.LiftState.Close(); } public override void Open() { Console.WriteLine("Openning"); } public override void Run() { // } public override void Stop() { // }}public class ClosingState : LiftState{ public override void Close() { Console.WriteLine("Closing"); } public override void Open() { base.context.LiftState = Context.openningState; base.context.LiftState.Open(); } public override void Run() { base.context.LiftState = Context.runningState; base.context.LiftState.Run(); } public override void Stop() { base.context.LiftState = Context.stoppingState; base.context.LiftState.Stop(); }}public class RunningState : LiftState{ public override void Close() {// } public override void Open() {// } public override void Run() { Console.WriteLine("Running"); } public override void Stop() { base.context.LiftState = Context.stoppingState; base.context.LiftState.Stop(); }}public class StoppingState : LiftState{ public override void Close() { // } public override void Open() { base.context.LiftState = Context.openningState; base.context.LiftState.Open(); } public override void Run() { base.context.LiftState = Context.runningState; base.context.LiftState.Run(); } public .........
