购物车中的观察者模式的应用

观察者模式其实就是   比如 给一个adapter设置了一个观察者  当adapter调用notifyDatasetchaged（）方法时 会执行观察者中的回调方法

 1 package com.xqx.adapterobserver;
 2
 3 import android.app.Activity;
 4 import android.database.DataSetObserver;
 5 import android.os.Bundle;
 6 import android.view.View;
 7 import android.widget.*;
 8 import org.w3c.dom.Text;
 9
10 import java.util.ArrayList;
11 import java.util.List;
12
13 public class MainActivity extends Activity {
14     /**
15      * Called when the activity is first created.
16      */
17
18     private ArrayAdapter<Integer> adapter ;
19     private List<Integer> list;
20     private ListView listView;
21     private TextView text;
22
23     //创建观察者
24     private DataSetObserver sumObserver = new DataSetObserver() {
25         /**
26          * 当Adapter的notifyDataSetChanged方法执行时被调用
27          */
28         @Override
29         public void onChanged() {
30             super.onChanged();
31             //执行相应的操作
32             int sum = 0;
33             for (int i = 0; i < list.size(); i++) {
34                 sum+=list.get(i);
35             }
36             text.setText("总金额:"+sum);
37         }
38
39         /**
40          * 当Adapter 调用 notifyDataSetInvalidate方法执行时被调用
41          */
42         @Override
43         public void onInvalidated() {
44             super.onInvalidated();
45             //执行相应的操作
46         }
47     };
48     @Override
49     public void onCreate(Bundle savedInstanceState) {
50         super.onCreate(savedInstanceState);
51         setContentView(R.layout.main);
52
53         list = new ArrayList<Integer>();
54         for (int i = 0; i < 30; i++) {
55             list.add(0);
56         }
57         adapter = new ArrayAdapter<Integer>(this,android.R.layout.simple_list_item_1,list);
58         //注册观察者
59         adapter.registerDataSetObserver(sumObserver);
60         text = (TextView) findViewById(R.id.text);
61         listView = (ListView) findViewById(R.id.listView);
62         listView.setAdapter(adapter);
63
64         listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
65             @Override
66             public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
67                 list.set(i,2);
68                 //将列表项的0变为2 更新适配器，
69                 adapter.notifyDataSetChanged();
70                 //执行该方法后DataSetObserver观察者观察到
71             }
72         });
73
74     }
75
76     @Override
77     protected void onDestroy() {
78         super.onDestroy();
79         //注销观察者
80         adapter.unregisterDataSetObserver(sumObserver);
81     }
82 }
83
84 MainActivity.java

activity