【设计模式】【23】状态模式

使用场景

状态模式，英文名State Pattern

该模式根据对象状态来决定对象行为，由State来决定行为，而不是Context自身

这种设计模式的目的在于，将与状态切换相关的工作，交给State自己处理，Context只关心自身的其它业务

根据业务复杂程度，该模式可以有简单和复杂两种实现方式

• 简单模式下，每个状态之间是独立的，每个State对象直接处理当前状态下的工作即可，不需要考虑旧状态
• 复杂模式下，多个状态之间是有关联的，从旧状态切换到新状态，需要做一些额外的切换工作，并且不同状态之间的切换工作可能是完全不同的

代码实现 - 简单模式

	abstract public class State {
	
	    public static final State STATE_A = new StateA();
	    public static final State STATE_B = new StateB();
	
	    Context context;
	
	    abstract public void work();
	}
	
	public class StateA extends State {
	
	    @Override
	    public void work() {
	        System.out.print("Apply State A and Work as Role A");
	    }
	}
	
	public class StateB extends State {
	
	    @Override
	    public void work() {
	        System.out.print("Apply State B and Work as Role B");
	    }
	}

	public class Context {
	
	    State currentState;
	
	    public void applyStateAndWork(State state) {
	        currentState = state;
	        currentState.context = this;
	        currentState.work();
	    }
	}

	public class APP {
	
	    public static void main(String... args) {
	        Context ctx = new Context();
	        ctx.applyStateAndWork(State.STATE_A);
	        ctx.applyStateAndWork(State.STATE_B);
	    }
	}

代码实现 - 复杂模式

	abstract public class State {
	
	    public static final State STATE_A = new StateA();
	    public static final State STATE_B = new StateB();
	
	    Context context;
	
	    public void setContext(Context context) {
	        this.context = context;
	    }
	
	    abstract public void workAsRoleA();
	
	    abstract public void workAsRoleB();
	}
	
	public class StateA extends State {
	
	    @Override
	    public void workAsRoleA() {
	        System.out.print("Apply State A and Work as Role A");
	    }
	
	    @Override
	    public void workAsRoleB() {
	        context.currentState = State.STATE_B;
	        context.workAsRoleB();
	    }
	}
	
	public class StateB extends State {
	
	    @Override
	    public void workAsRoleA() {
	        context.currentState = State.STATE_A;
	        context.workAsRoleA();
	    }
	
	    @Override
	    public void workAsRoleB() {
	        System.out.print("Apply State B and Work as Role B");
	    }
	}

	public class Context {
	
	    State currentState = State.STATE_A;
	
	    public void workAsRoleA() {
	        currentState.setContext(this);
	        currentState.workAsRoleA();
	    }
	
	    public void workAsRoleB() {
	        currentState.setContext(this);
	        currentState.workAsRoleB();
	    }
	}

	public class APP {
	
	    public static void main(String... args) {
	        Context ctx = new Context();
	        ctx.workAsRoleA();
	        ctx.workAsRoleB();
	        ctx.workAsRoleA();
	    }
	}