android——handler机制原理

　　在android版本4.0及之后的版本中多线程有明确的分工，子线程可以写所有耗时的代码（数据库、蓝牙、网络服务），但是绝对不能碰UI,想碰UI跟着主线程走，那么我们如何才能让主线程知道我们要对 UI进行操作呢？这时我们就可以利用用消息机制——handler去通知主线程（因为子线程本身不可以发消息）

　　下面是handler简单的工作原理图（此图为转载）

　　 handler用来发消息和处理消息

下面用个案例来说明：

　　activity_main.xml：

　　　　　　

 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     >

     <TextView
         android:id="@+id/textView1"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:text="@string/hello_world" />

     <Button
         android:id="@+id/button1"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_alignParentRight="true"
         android:layout_alignTop="@+id/textView1"
         android:layout_marginRight="53dp"
         android:text="Button" />

     <ListView
         android:id="@+id/listView1"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         android:layout_below="@+id/button1"
         android:layout_marginTop="84dp" >
     </ListView>

    <!--  <ProgressBar
         android:id="@+id/progressBar1"
         style="?android:attr/progressBarStyleHorizontal"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_alignLeft="@+id/listView1"
         android:layout_alignRight="@+id/button1"
         android:layout_below="@+id/button1"
         android:layout_marginTop="28dp" /> -->

 </RelativeLayout>

listview中的布局item.xml

 <?xml version="1.0" encoding="utf-8"?>
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:orientation="horizontal" >
     <TextView
         android:id="@+id/username"
         android:layout_weight="1"
         android:layout_width="match_parent"
         android:layout_height="60dp"
         android:textSize="45dp"
         />

     <TextView
         android:id="@+id/sex"
         android:layout_width="match_parent"
         android:layout_height="60dp"
         android:layout_weight="1"
         android:textSize="45dp" />

 </LinearLayout>

User.java(用于模拟数据)

 public class User {
     private String username;
     private String sex;
     public String getUsername() {
         return username;
     }
     public void setUsername(String username) {
         this.username = username;
     }
     public String getSex() {
         return sex;
     }
     public void setSex(String sex) {
         this.sex = sex;
     }

 }

mainActivity.java

　

 import java.util.ArrayList;
 import java.util.List;

 import android.app.Activity;
 import android.os.Bundle;
 import android.os.Handler;
 import android.os.Message;
 import android.util.Log;
 import android.view.LayoutInflater;
 import android.view.Menu;
 import android.view.MenuItem;
 import android.view.View;
 import android.view.View.OnClickListener;
 import android.view.ViewGroup;
 import android.widget.BaseAdapter;
 import android.widget.Button;
 import android.widget.ListView;
 import android.widget.TextView;
 import android.widget.Toast;

 public class MainActivity extends Activity {

     private String fromDb_str1 = "";
     private Button btn;
     private TextView tv;
     private ListView lv;
     private BaseAdapter adapter;
     private List<User> userList = new ArrayList<User>();
     private Runnable doInBackground1;
     private Runnable doInBackground2;

     //1.跟着主线程走，可以碰UI
     //2.能够接受子线程发送的消息(Message)
     //        子线程类本身不可以发信息
     private Handler handler;

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);

         Log.i("UI_MainThread","id:"+Thread.currentThread().getId());

         //模拟数据访问产生数据
         for (int i = 0; i < 5; i++) {
             User u = new User();
             u.setUsername("小明"+i);
             u.setSex("女"+i);
             userList.add(u);
         }

         tv =(TextView)findViewById(R.id.textView1);
         btn =(Button)findViewById(R.id.button1);
         btn.setOnClickListener(new OnClickListener() {

             @Override
             public void onClick(View v) {
                 //1.访问数据库或者互联网（但会卡的）
                 //2.更新界面
                 Thread t1 = new Thread(doInBackground1);
                 t1.start();

                 Thread t2 = new Thread(doInBackground2);
                 t2.start();

             }
         });
         adapter = new BaseAdapter(){

             @Override
             public int getCount() {
                 // TODO Auto-generated method stub
                 return userList.size();
             }

             @Override
             public View getView(int position, View convertView, ViewGroup parent) {
                 LayoutInflater inflater = MainActivity.this.getLayoutInflater();
                 View view;
                 if (convertView==null){
                     view = inflater.inflate(R.layout.item, null);
                 }
                 else{
                     view = convertView;
                 }

                 TextView tv_username = (TextView)view.findViewById(R.id.username);
                 TextView tv_sex = (TextView)view.findViewById(R.id.sex);
                 tv_username.setText(userList.get(position).getUsername());
                 tv_sex.setText(userList.get(position).getSex());
                 return view;
             }

             @Override
             public Object getItem(int position) {
                 // TODO Auto-generated method stub
                 return null;
             }

             @Override
             public long getItemId(int position) {
                 // TODO Auto-generated method stub
                 return 0;
             }
         };
         lv = (ListView)findViewById(R.id.listView1);
         lv.setAdapter(adapter);

         handler = new Handler(){

             //1.消息msg来自于子线程
             //2.消息可以多个，采用msg.what识别
             //3.处理消息，一般就会更新UI
             //4.此方法可以参考onPostExecute
             @Override
             public void handleMessage(Message msg) {

                 super.handleMessage(msg);
                 int msgwhat = msg.what;
                 Log.i("handler","已经收到消息，消息what："+msgwhat+",id:"+Thread.currentThread().getId());

                 if (msgwhat==1){
                     //更新helloworld
                     tv.setText("子线程让我更新"+msgwhat);
                 }
                 if (msgwhat==2){
                     //更新ListView
                     adapter.notifyDataSetChanged();
                 }

             }

         };

         //子线程代码1
         doInBackground1 = new Runnable() {

             @Override
             public void run() {
                 Log.i("sub_Thread","子线程1启动,id:"+Thread.currentThread().getId());

                 try {
                     Thread.sleep(3000);
                 } catch (InterruptedException e) {
                     // TODO Auto-generated catch block
                     e.printStackTrace();
                 }

                 //1.访问数据库或者互联网,不在UI进程，所以不卡
                 Message msg = new Message();
                 //对消息一个识别号，便于handler能够识别
                 msg.what = 1;
                 handler.sendMessage(msg);
                 Log.i("sub_Thread","子线程1已经发送消息给handler");
             }
         };

         //子线程代码1
         doInBackground2 = new Runnable() {

             @Override
             public void run() {
                 Log.i("sub_Thread","子线程2启动,id:"+Thread.currentThread().getId());

                 try {
                     Thread.sleep(6000);
                 } catch (InterruptedException e) {
                     // TODO Auto-generated catch block
                     e.printStackTrace();
                 }

                 Message msg = new Message();
                 //对消息一个识别号，便于handler能够识别
                 msg.what = 2;
                 //handler.sendMessage(msg);
                 handler.sendMessageDelayed(msg, 500);

                 //访问互联网，下载最新的，更新data,但不碰界面
                 for (User user : userList) {
                     user.setSex("不男不女");
                 }

                 Log.i("sub_Thread","子线程2已经发送消息给handler");
             }
         };

     }
 }

效果图：

　　点击button的时候如下图

上面就是一个小例子；但是上面的代码写的还有很多需要改进的地方，详情请参考（此链接为转载）：

Android中使用Handler造成内存泄露的分析和解决http://www.linuxidc.com/Linux/2013-12/94065.htm