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

　

 2 
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