1，定义

状态模式（State）当一个状态的内在状态改变的时候允许改变其行为，这个对象看起来像是改变了其类

2，UML图

状态模式

3，基本代码

namespace 状态模式
{
    class Program
    {
        static void Main(string[] args)
        {
            Context c = new Context(new ConcreteStateA());

            c.Request();
            c.Request();
            c.Request();
            c.Request();

            Console.Read();
        }
    }

    class Context
    {
        private State state;

        public Context(State state)
        {
            this.state = state;
        }

        public State State
        {
            get
            {
                return state;
            }
            set
            {
                state = value;
                Console.WriteLine("当前状态:" + state.GetType().Name);
            }
        }

        public void Request()
        {
            state.Handle(this);
        }
    }

    abstract class State
    {
        public abstract void Handle(Context context);
    }

    class ConcreteStateA : State
    {
        public override void Handle(Context context)
        {
            context.State = new ConcreteStateB();
        }
    }

    class ConcreteStateB : State
    {
        public override void Handle(Context context)
        {
            context.State = new ConcreteStateA();
        }
    }

}

4，简单应用

namespace 状态模式
{
    class Program
    {
        static void Main(string[] args)
        {
            //紧急项目
            Work emergencyProjects = new Work();
            emergencyProjects.Hour = 9;
            emergencyProjects.WriteProgram();
            emergencyProjects.Hour = 10;
            emergencyProjects.WriteProgram();
            emergencyProjects.Hour = 12;
            emergencyProjects.WriteProgram();
            emergencyProjects.Hour = 13;
            emergencyProjects.WriteProgram();
            emergencyProjects.Hour = 14;
            emergencyProjects.WriteProgram();
            emergencyProjects.Hour = 17;

            //emergencyProjects.WorkFinished = true;
            emergencyProjects.TaskFinished = false;

            emergencyProjects.WriteProgram();
            emergencyProjects.Hour = 19;
            emergencyProjects.WriteProgram();
            emergencyProjects.Hour = 22;
            emergencyProjects.WriteProgram();


            Console.Read();
        }
    }


    //抽象状态
    public abstract class State
    {
        public abstract void WriteProgram(Work w);
    }

    //上午工作状态
    public class ForenoonState : State
    {
        public override void WriteProgram(Work w)
        {
            if (w.Hour < 12)
            {
                Console.WriteLine("当前时间：{0}点 上午工作，精神百倍", w.Hour);
            }
            else
            {
                w.SetState(new NoonState());
                w.WriteProgram();
            }
        }
    }

    //中午工作状态
    public class NoonState : State
    {
        public override void WriteProgram(Work w)
        {
            if (w.Hour < 13)
            {
                Console.WriteLine("当前时间：{0}点 饿了，午饭；犯困，午休。", w.Hour);
            }
            else
            {
                w.SetState(new AfternoonState());
                w.WriteProgram();
            }
        }
    }

    //下午工作状态
    public class AfternoonState : State
    {
        public override void WriteProgram(Work w)
        {
            if (w.Hour < 17)
            {
                Console.WriteLine("当前时间：{0}点 下午状态还不错，继续努力", w.Hour);
            }
            else
            {
                w.SetState(new EveningState());
                w.WriteProgram();
            }
        }
    }

    //晚间工作状态
    public class EveningState : State
    {
        public override void WriteProgram(Work w)
        {
            if (w.TaskFinished)
            {
                w.SetState(new RestState());
                w.WriteProgram();
            }
            else
            {
                if (w.Hour < 21)
                {
                    Console.WriteLine("当前时间：{0}点 加班哦，疲累之极", w.Hour);
                }
                else
                {
                    w.SetState(new SleepingState());
                    w.WriteProgram();
                }
            }
        }
    }

    //睡眠状态
    public class SleepingState : State
    {
        public override void WriteProgram(Work w)
        {
            Console.WriteLine("当前时间：{0}点 不行了，睡着了。", w.Hour);
        }
    }

    //下班休息状态
    public class RestState : State
    {
        public override void WriteProgram(Work w)
        {
            Console.WriteLine("当前时间：{0}点 下班回家了", w.Hour);
        }
    }

    //工作
    public class Work
    {
        private State current;
        public Work()
        {
            current = new ForenoonState();
        }

        private double hour;
        public double Hour
        {
            get { return hour; }
            set { hour = value; }
        }

        private bool finish = false;
        public bool TaskFinished
        {
            get { return finish; }
            set { finish = value; }
        }


        public void SetState(State s)
        {
            current = s;
        }

        public void WriteProgram()
        {
            current.WriteProgram(this);
        }
    }

}

5，思考

1，状态模式解决的是当控制一个对象状态转换的条件表达式过于复杂时候的情况，把状态的判断逻辑转移到表示不同的状态的一系列类当中，可以简化判断的逻辑
2，好处，将与特定状态的行为局部化，并且将不同的状态的行为分开来，将特定的状态的相关行为都放在一个对象中，消除了条件判断语句，状态模式通过把状态转换的逻辑分布到state的子类当中，来减少相互间的依赖
3，使用，当一个对象的行为取决于他的状态，并且它必须在运行的时候根据它的状态改变它的行为的时候，可以用状态模式
4，状态模式主要解决的是当控制一个对象的转换条件复杂的时候，把状态的判断逻辑转移到表示不同状态的一系列类当中，可以简化复杂的判断逻辑