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工厂模式很简单，也很好上手，那么接下来学习策略模式，我们就以商场的收银系统作为需求，使用策略模式完成。达到学习了解策略模式的目的。

策略模式（strategy）:它定义了算法家族，分别封装起来，让它们之间可以互相替换，此模式让算法的变化，不会影响到使用算法的客户。

先看看策略模式的结构图：

通过Context的实例去调用不同具体实例的方法。那么根据策略模式的结构图，商场收银系统的UML

商场的收银算法多种多样，并且可能随时更改，那么在设计该系统时，模式选择上就应该注重系统的可扩展性，可维护性。那么我们就以商场有现金全额收费算法，打折算法，满返现算法。当然上诉分层结构时，在具体策略层可以加个工厂，这样可以提高代码的可复用性。

先从上到下，抽象算法

/** *抽象策略，一个算法 *@author Shuwen *created at 2016/8/25 15:54 */public abstract class CashSuper {    public abstract double CashMoney(double money);    public abstract String getType();}

接着是具体策略

/** * Created by Shuwen on 2016/8/25. */public class CashDebate extends CashSuper {    private double debate = 1;    public CashDebate(double debate){        this.debate = debate;    }    @Override    public double CashMoney(double money) {        return money*debate;    }    @Override    public String getType() {        return "打"+debate+"折";    }}

接着就是业务逻辑，整个银行系统，把它抽出来设计为一个自定义的View，这样可以增强代码的复用性，在其他地方要用，直接就可以用这个控件，而不用再去重复写业务逻辑。

package com.abings.strategymodel;import android.content.Context;import android.util.AttributeSet;import android.view.LayoutInflater;import android.view.View;import android.widget.AdapterView;import android.widget.ArrayAdapter;import android.widget.Button;import android.widget.EditText;import android.widget.LinearLayout;import android.widget.Spinner;import android.widget.TextView;import com.abings.strategymodel.Strategy.CashContext;import com.abings.strategymodel.StrategyFactory.CashFactory;/** * Created by Shuwen on 2016/8/25. */public class BankSysView extends LinearLayout {    private EditText singlemoney;    private EditText number;    private Button sure;    private Button reset;    private TextView history;    private TextView total;    private StringBuilder stringBuilder;    private Context context;    private Spinner spinner;    private ArrayAdapter
   
     adapter;    private CashContext cashContext;    private double totalMoney;    private double allMoney;    private String[]types = {"全额付款","打8折","打5折","满300返50"};    public BankSysView(Context context) {        this(context,null);    }    public BankSysView(Context context, AttributeSet attrs) {        this(context, attrs, 0);    }    public BankSysView(Context context, AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);        this.context =context;        LayoutInflater.from(context).inflate(R.layout.cashmoneysys, this);        initView();        setEvents();    }    private void setEvents() {        spinner.setAdapter(adapter);        spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {            @Override            public void onItemSelected(AdapterView
     parent, View view, int position, long id) {                cashContext = CashFactory.createCashSuper(position);            }            @Override            public void onNothingSelected(AdapterView
     parent) {            }        });        sure.setOnClickListener(new OnClickListener() {            @Override            public void onClick(View v) {                double singleMoney = Double.parseDouble(singlemoney.getText().toString().trim());                double numBer = Double.parseDouble(number.getText().toString().trim());                totalMoney = cashContext.getResult(singleMoney * numBer);                allMoney += totalMoney;                stringBuilder.append("单价:"+singleMoney+"元,"+"数量:"+(int)numBer+","+cashContext.getType()+",合计:"+totalMoney+"元\n");                total.setText("总计：" + allMoney + "元");                history.setText(stringBuilder);            }        });        reset.setOnClickListener(new OnClickListener() {            @Override            public void onClick(View v) {                singlemoney.setText("");                number.setText("");                allMoney = 0;                stringBuilder.delete(0, stringBuilder.length());                history.setText("");                total.setText("总计：");            }        });    }    private void initView() {        singlemoney = (EditText) findViewById(R.id.singlemoney);        number = (EditText) findViewById(R.id.num);        sure = (Button) findViewById(R.id.sure);        reset = (Button) findViewById(R.id.reset);        history = (TextView) findViewById(R.id.history);        total = (TextView) findViewById(R.id.total);        spinner = (Spinner) findViewById(R.id.spinner);        adapter = new ArrayAdapter<>(context,R.layout.text,types);        stringBuilder = new StringBuilder();    }}
   

最后根据需求，写个工厂类

package com.abings.strategymodel.StrategyFactory;import com.abings.strategymodel.Strategy.CashContext;import com.abings.strategymodel.Strategy.CashDebate;import com.abings.strategymodel.Strategy.CashNormal;import com.abings.strategymodel.Strategy.CashReturn;/** * Created by Shuwen on 2016/8/26. */public class CashFactory {    public static CashContext createCashSuper(int position){        CashContext cashContext = null;        switch (position) {            case 0:                cashContext = new CashContext(new CashNormal());                break;            case 1:                cashContext = new CashContext(new CashDebate(0.8));                break;            case 2:                cashContext = new CashContext(new CashDebate(0.5));                break;            case 3:                cashContext = new CashContext(new CashReturn(300, 50));                break;        }        return cashContext;    }}

这就是策略模式，一个抽象策略，多个具体策略，一个Context统一算法的调用，那么以后再添加算法，或是修改算法，只需要在具体算法层做改动，客户端和抽象策略不需要修改，这样代码的可维护性，可扩张性也提高了不少。很不错的设计模式。

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