备忘录模式

备忘录模式UML.png

class Originator{
    private int stateA = 0;
    private int stateB = 0;
    private int stateC = 0;
    
    public Memoto createMemoto(){
        Memoto memoto = new Memoto();
        memoto.setStateA(stateA);
        memoto.setStateB(stateB);
        memoto.setStateC(stateC);
    }
    
    public void restore(Memoto memoto){
        this.stateA = memoto.getStateA();
        this.stateB = memoto.getStateB();
        this.stateC = memoto.getStateC();
    }
    
    public void change(){
        stateA = 1;
        stateB = 2;
        stateC = 3;
    }
    
    public void showData(){
        System.out.println(stateA+"=="+stateB+"=="+stateC);
    }
    
}

原始类

class Memoto{
    private int stateA;
    private int stateB;
    private int stateC;
    
    public int getStateA(){
        return this.stateA;
    }
    public int getStateB(){
        return this.stateB;
    }
    public int getStateC(){
        return this.stateC;
    }
    public void setStateA(int a){
        this.stateA = a;
    }
    public void setStateB(int b){
        this.stateB = b;
    }
    public void setStateC(int c){
        this.stateC = c;
    }
}   

数据bean类

class Caretaker{
    Memoto memoto;
    public Memoto restoreMemoto(){
        return this.memoto;//恢复数据
    }
    
    public void storeMemoto(Memoto memoto){
        this.memoto = memoto;
    }
}

备忘录对象，管理Memoto对象

class Cliten{
    public static void main(String args[]){
        Originator originator = new Originator();
        Caretaker caretaker = new Caretaker();
        originator.showData();//原始数据为 0 0 0 
        originator.change();//改变之后为 1 2 3
        caretaker.storeMemoto(originator.createMemoto());//存储状态
        //模拟下次登录之后 重新开始
        Originator originatorNew = new Originator();    
        originatorNew.restore(caretaker.restoreMemoto());
        originatorNew.showData();//数据为 1 2 3
    }
}

客户端模拟备忘录模式。当需要保存状态的时候将状态存入Caretaker即可。Caretaker这里只是保存最后一次变化状态，可以扩展需求做的更丰富一些。

个人总结

例子比较简单，仅仅只是备忘录模式的原型。一句话概括就是备忘录类会记住应该记住的状态，而存储与恢复状态在原始类中调用。
备忘录模式在Android源码里面例子，当界面退居后台或者由于内存不足被干翻，在这之前会调用Activity中onSaveInstanceState方法保存状态。当界面再次出现在界面上时在onRestoreInstanceState方法中获取之前保存的状态。