原文地址：https://blog.csdn.net/LogicTeamLeader/article/details/51165375

　　　　　https://blog.csdn.net/logicteamleader/article/details/46591499

Handler:

　　 要理解Handler,必然要綜合理解Looper,Messagequeue,HandlerThread。

　　　Looper:其中有一個Messagequeue，looper負責輪詢Messagequeue,取出其中的Message交給Handler處理

　　　Handler:負責處理Message。Handler的創建需要Looper對象，它創建時會自動匹配該線程的looper對象，若在子線程創建，需要looper.prepare()創建looper對象。（主線程本身就存在looper）

　　　HandlerThread:顧名思義，Handler存在的線程，Handler在哪個線程創建就存在於哪個線程中。這個大部分時間都在執行looper.loop()方法。getlooper()可以獲取當前線程的looper。

　　　關系圖：

　　具體的用法：

(1)主線程創建Handler

package com.example.wxbhandlertest;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import
 
 android.os.MessageQueue;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
/**
 * @author wxb
 * Android中的消息處理實例之一
 * * 一、在主線程內發送消息
 * 1.使用post
 * 2.使用postDelay
 * 3.使用sendMessage
 * 4.使用Message.sentToTarget
 * 二、在子線程中使用Handler
 * 1.使用post
 * 2.使用postDelay
 * 3.使用sendMessage
 * 4.使用Message.sentToTarget
 
 
*/
public class MainActivity extends Activity {
    private Runnable runnable=null;
    private Runnable runnableDelay=null;
    private Runnable runnableInThread=null;
    private Runnable runnableDelayInThread=null;
    private static TextView tv;
    private static TextView tvOnOtherThread;

    //自定義Message類型
    public final static int MESSAGE_WXB_1 = 1;
    public final static int MESSAGE_WXB_2 = 2;
    public final static int MESSAGE_WXB_3 = 3;
    public final static int MESSAGE_WXB_4 = 4;
    public final static int MESSAGE_WXB_5 = 5;

    private static Handler mHandler=new Handler(){
        @Override
        public void handleMessage(Message msg) {
            switch(msg.what){
            case MESSAGE_WXB_1:
                tv.setText("invoke sendMessage in main thread");
                break;
            case MESSAGE_WXB_2:
                tv.setText("Message.sendToTarget in main thread");
                break;
            case MESSAGE_WXB_3:
                tvOnOtherThread.setText("invoke sendMessage in other thread");
                break;
            case MESSAGE_WXB_4:
                tvOnOtherThread.setText("Message.sendToTarget in other thread");
                break;  
            }
            super.handleMessage(msg);
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv = (TextView) this.findViewById(R.id.tvOnMainThread);
        tvOnOtherThread = (TextView) this.findViewById(R.id.tvOnOtherThread);

        //方法1.post
        runnable = new Runnable(){
            public void run(){
                tv.setText(getString(R.string.postRunnable));
            }
        };
        Button handler_post = (Button) this.findViewById(R.id.btnHandlerpost);
        handler_post.setOnClickListener(new OnClickListener() {     
            @Override
            public void onClick(View v) {
                mHandler.post(runnable);
            }
        });

        //方法2：postDelay
        runnableDelay = new Runnable(){
            public void run(){
                tv.setText(getString(R.string.postRunnableDelay));
            }
        };

        Button handler_post_delay = (Button) this.findViewById(R.id.btnHandlerPostdelay);
        handler_post_delay.setOnClickListener(new OnClickListener() {       
            @Override
            public void onClick(View v) {
                mHandler.postDelayed(runnableDelay, 1000);  //1秒後執行
            }
        });

        //方法3：sendMessage
        Button btnSendMessage = (Button) this.findViewById(R.id.btnSendMessage);
        btnSendMessage.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                Message msg = mHandler.obtainMessage();
                msg.what = MESSAGE_WXB_1;
                mHandler.sendMessage(msg);
            }
        });

        //方法4：Message.sendToTarget
        Button btnSendtoTarget = (Button) this.findViewById(R.id.btnSendtoTarget);
        btnSendtoTarget.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                Message msg = mHandler.obtainMessage();
                msg.what = MESSAGE_WXB_2;
                msg.sendToTarget();
            }
        });

       //在其他線程中發送消息
        //1.post
        runnableInThread = new Runnable(){
            public void run(){
                tvOnOtherThread.setText(getString(R.string.postRunnableInThread));
            }
        };

        Button btnPost = (Button) this.findViewById(R.id.btnPost);
        btnPost.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                new Thread(){
                    @Override
                    public void run() {
                        super.run();
                        mHandler.post(runnableInThread);
                    }
                }.start();
            }
        });

        //2.postDelay
        runnableDelayInThread = new Runnable(){
            public void run(){
                tvOnOtherThread.setText(getString(R.string.postRunnableDelayInThread));
            }
        };

        Button btnPostDelay = (Button) this.findViewById(R.id.btnPostDelay);
        btnPostDelay.setOnClickListener(new OnClickListener() {             
            @Override
            public void onClick(View v) {
                new Thread(){
                    @Override
                    public void run() {
                        super.run();
                        mHandler.postDelayed(runnableDelayInThread, 1000);
                    }
                }.start();
            }
        });

        //3.sendMessage
        Button btnSendMessage2 = (Button) this.findViewById(R.id.btnSendMessage2);
        btnSendMessage2.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                new Thread(){
                    @Override
                    public void run() {
                        super.run();
                        Message msg = mHandler.obtainMessage();
                        msg.what = MESSAGE_WXB_3;
                        mHandler.sendMessage(msg);
                    }
                }.start();
            }
        });

      //方法4：Message.sendToTarget
        Button btnSendToTarget2 = (Button) this.findViewById(R.id.btnSendToTarget2);
        btnSendToTarget2.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                new Thread(){
                    @Override
                    public void run() {
                        super.run();
                        mHandler.obtainMessage(MESSAGE_WXB_4).sendToTarget();
                    }
                }.start();
            }
        });
    }
}

(2)子線程創建Handler

package com.example.wxbloopinthread;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity {
    private static TextView tv = null;

    //自定義Message類型
    public final static int MESSAGE_WXB_1 = 1;

    //主線程中創建Handler
    private static Handler mHandler = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            switch(msg.what){
            case MESSAGE_WXB_1:
                tv.setText("主線程發送，子線程接收消息後回發，主線程修改UI");
                break;
            }
            super.handleMessage(msg);
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);     
        setContentView(R.layout.activity_main);
        tv = (TextView) this.findViewById(R.id.textView1);

        //創建子線程
        new LooperThread().start();

        //點擊按鈕向子線程發送消息
        Button btn = (Button) this.findViewById(R.id.btnSendMessage);
        btn.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                LooperThread.sHandler.sendEmptyMessage(MESSAGE_WXB_1);
            }
        });
    }

    //定義子線程
    static class LooperThread extends Thread {
        public static Handler sHandler = null;

        public void run() {
            //創建消息循環
            Looper.prepare();

            sHandler = new Handler() {
                public void handleMessage(Message msg) {
                    switch(msg.what){
                    case MESSAGE_WXB_1:
                        mHandler.sendEmptyMessage(MESSAGE_WXB_1);
                        break;
                    }
                }
            };
            //開啟消息循環
            Looper.loop();
        }
    }
}

Handler和AsyncTask