什么是Handler

 

 
handler通俗讲就是在各个进程之间发送数据的处理对象。在任何进程中，只要得到了另外一个进程的handler则能够经过 handler.sendMessage(message)方法向那个进程发送数据。基于这个机制，咱们在处理多线程的时候能够新建一个thread，这 个thread拥有UI线程中的一个handler。当thread处理完一些耗时的操做后经过传递过来的handler向UI线程发送数据，由UI线程去更新界面。
 
Handler类简介
 
在Android平台中，新启动的线程是没法访问Activity里的Widget的，固然也不能将运行状态外送出来，这就须要有Handler机 制进行消息的传递了，Handler类位于android.os包下，主要的功能是完成Activity的Widget与应用程序中线程之间的交互。接下 来对该类中经常使用的方法进行介绍，以下表所示。
 
 Handler类的经常使用方法
 

 

方法签名
 

描     述
 

 

public void handleMessage (Message msg)
 

子类对象经过该方法接收信息
 

 

public final boolean sendEmptyMessage (int what)
 

发送一个只含有 what 值的消息
 

 

public final boolean sendMessage (Message msg)
 

发送消息到 Handler ，
 
经过 handleMessage 方法接收
 

 

public final boolean hasMessages (int what)
 

监测消息队列中是否还
 
有 what 值的消息
 

 

public final boolean post (Runnable r)
 

将一个线程添加到消息队列
 

    开发带有Handler类的程序步骤以下。   在Activity或Activity的Widget中开发Handler类的对象，并重写handleMessage方法。   在新启动的线程中调用sendEmptyMessage或者sendMessage方法向Handler发送消息。   Handler类的对象用handleMessage方法接收消息，而后根据消息的不一样执行不一样的操做。       在Android 中Handler和Message、Thread有着很密切的关系。Handler 主要是负责Message的分发和处理。可是这个Message从哪里来的呢？Message 由一个消息队列进行管理，而消息队列却由一个Looper进行管理。Android系统中Looper负责管理线程的消息队列和消息循环，具体实现请参考Looper的源码。 能够经过Loop.myLooper()获得当前线程的Looper对象，经过Loop.getMainLooper()能够得到当前进程的主线程的 Looper对象。Android系统的消息队列和消息循环都是针对具体线程的，一个线程能够存在（固然也能够不存在）一个消息队列和一个消 息循环（Looper），特定线程的消息只能分发给本线程，不能进行跨线程，跨进程通信。可是建立的工做线程默认是没有消息循环和消息队列的，若是想让该 线程具备消息队列和消息循环，须要在线程中首先调用Looper.prepare()来建立消息队列，而后调用Looper.loop()进入消息循环。   虽然说 特定线程的消息只能分发给本线程，不能进行跨线程通信，可是因为能够经过得到线程的Looper对象来进行曲线的实现不一样线程间消息的传递，代码以下   package com.mytest.handlertest;  import android.app.Activity;  import android.graphics.Color;  import android.os.Bundle;  import android.os.Handler;  import android.os.Looper;  import android.os.Message;  import android.util.Log;  import android.view.View;  import android.view.View.OnClickListener;  import android.view.ViewGroup.LayoutParams;  import android.widget.Button;  import android.widget.LinearLayout;  import android.widget.TextView;  public class HandlerTest extends Activity implements OnClickListener{            private String TAG = "HandlerTest";      private boolean bpostRunnable = false;            private NoLooperThread noLooperThread = null;        private OwnLooperThread ownLooperThread = null;        private ReceiveMessageThread receiveMessageThread =null;            private Handler  mOtherThreadHandler=null;      private EventHandler mHandler = null;             private Button btn1 = null;      private Button btn2 = null;      private Button btn3 = null;      private Button btn4 = null;      private Button btn5 = null;      private Button btn6 = null;      private TextView tv = null;      /** Called when the activity is first created. */      @Override      public void onCreate(Bundle savedInstanceState) {          super.onCreate(savedInstanceState);                   LinearLayout layout = new LinearLayout(this);          LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(250, 50);          layout.setOrientation(LinearLayout.VERTICAL);                    btn1 = new Button(this);          btn1.setId(101);          btn1.setText("message from main thread self");          btn1.setOnClickListener(this);          layout.addView(btn1, params);                    btn2 = new Button(this);          btn2.setId(102);          btn2.setText("message from other thread to main thread");          btn2.setOnClickListener(this);          layout.addView(btn2,params);                    btn3 = new Button(this);          btn3.setId(103);          btn3.setText("message to other thread from itself");          btn3.setOnClickListener(this);          layout.addView(btn3, params);                    btn4 = new Button(this);          btn4.setId(104);          btn4.setText("message with Runnable as callback from other thread to main thread");          btn4.setOnClickListener(this);          layout.addView(btn4, params);                    btn5 = new Button(this);          btn5.setId(105);          btn5.setText("main thread's message to other thread");          btn5.setOnClickListener(this);          layout.addView(btn5, params);                    btn6 = new Button(this);          btn6.setId(106);          btn6.setText("exit");          btn6.setOnClickListener(this);          layout.addView(btn6, params);                    tv = new TextView(this);          tv.setTextColor(Color.WHITE);          tv.setText("");          params = new LinearLayout.LayoutParams(LayoutParams.FILL_PARENT,LayoutParams.WRAP_CONTENT);          params.topMargin=10;          layout.addView(tv, params);                    setContentView(layout);                    receiveMessageThread = new ReceiveMessageThread();          receiveMessageThread.start();                }            class EventHandler extends Handler{                    public EventHandler(Looper looper){              super(looper);          }                    public EventHandler(){              super();          }          @Override          public void handleMessage(Message msg) {              // TODO Auto-generated method stub              super.handleMessage(msg);              Log.e(TAG, "CurrentThread id:----------+>" + Thread.currentThread().getId());              switch(msg.what){              case 1:                  tv.setText((String)msg.obj);                  break;                                case 2:                  tv.setText((String)msg.obj);                  noLooperThread.stop();                  break;                                case 3:                  //不能在非主线程的线程里面更新UI，因此这里经过log打印信息                  Log.e(TAG,(String)msg.obj);                  ownLooperThread.stop();                  break;              default:                  Log.e(TAG,(String)msg.obj);                  break;              }          }                }              //ReceiveMessageThread has his own message queue by execute Looper.prepare();       class ReceiveMessageThread extends Thread {                    @Override          public void run(){              Looper.prepare();              mOtherThreadHandler= new Handler(){                  @Override                  public void handleMessage(Message msg) {                      // TODO Auto-generated method stub                      super.handleMessage(msg);                                            Log.e(TAG,"-------+>"+(String)msg.obj);                      Log.e(TAG, "CurrentThread id:----------+>" + Thread.currentThread().getId());                                        }                                  };               Log.e(TAG, "ReceiveMessageThread id:--------+>" + this.getId());                Looper.loop();          }      }            class NoLooperThread extends Thread {          private EventHandler mNoLooperThreadHandler;          @Override          public void run() {                            Looper myLooper = Looper.myLooper();              Looper mainLooper= Looper.getMainLooper();                            String msgobj;              if(null == myLooper){                  //这里得到的是主线程的Looper,因为NoLooperThread没有本身的looper因此这里确定会被执行                  mNoLooperThreadHandler = new EventHandler(mainLooper);                  msgobj = "NoLooperThread has no looper and handleMessage function executed in main thread!";               } else{                  mNoLooperThreadHandler = new EventHandler(myLooper);                  msgobj = "This is from NoLooperThread self and handleMessage function executed in NoLooperThread!";               }                            mNoLooperThreadHandler.removeMessages(0);                            if(bpostRunnable == false){              //send message to main thread                  Message msg = mNoLooperThreadHandler.obtainMessage(2, 1, 1, msgobj);                  mNoLooperThreadHandler.sendMessage(msg);                  Log.e(TAG, "NoLooperThread id:--------+>" + this.getId());               }else{                  //下面new出来的实现了Runnable接口的对象中run函数是在Main Thread中执行，不是在NoLooperThread中执行 记得 null == myLooper么                  //注意Runnable是一个接口，它里面的run函数被执行时不会再新建一个线程                    //您能够在run上加断点而后在eclipse调试中看它在哪一个线程中执行                                      mNoLooperThreadHandler.post(new Runnable(){                      public void run() {                          // TODO Auto-generated method stub                          tv.setText("update UI through handler post runnalbe mechanism!");                            Log.e(TAG, "update UI id:--------+>" + Thread.currentThread().getId());                           noLooperThread.stop();                        }                                        });              }          }                }            class OwnLooperThread extends Thread{          private EventHandler mOwnLooperThreadHandler = null;                    @Override          public void run() {              Looper.prepare();              Looper myLooper = Looper.myLooper();              Looper mainLooper= Looper.getMainLooper();                            String msgobj;                            if(null == myLooper){                  mOwnLooperThreadHandler = new EventHandler(mainLooper);                  msgobj = "OwnLooperThread has no looper and handleMessage function executed in main thread!";               }else{                  mOwnLooperThreadHandler = new EventHandler(myLooper);                  msgobj = "This is from OwnLooperThread self and handleMessage function executed in NoLooperThread!";              }                            mOwnLooperThreadHandler.removeMessages(0);                            //给本身发送消息              Message msg = mOwnLooperThreadHandler.obtainMessage(3,1,1,msgobj);              mOwnLooperThreadHandler.sendMessage(msg);              Looper.loop();          }      }            public void onClick(View v) {          // TODO Auto-generated method stub          switch(v.getId()){          case 101:              //主线程发送消息给本身              Looper looper = Looper.myLooper();//get the Main looper related with the main thread               //若是不给任何参数的话会用当前线程对应的Looper(这里就是Main Looper)为Handler里面的成员mLooper赋值              mHandler = new EventHandler(looper);              // 清除整个MessageQueue里的消息                mHandler.removeMessages(0);              String obj = "This main thread's message and received by itself!";                            Message msg = mHandler.obtainMessage(1,1,1,obj);              // 将Message对象送入到main thread的MessageQueue里面               mHandler.sendMessage(msg);              break;          case 102:              //other线程发送消息给主线程              bpostRunnable = false;              noLooperThread = new NoLooperThread();                noLooperThread.start();                break;          case 103:               //other thread获取它本身发送的消息                tv.setText("please look at the error level log for other thread received message");                ownLooperThread = new OwnLooperThread();                ownLooperThread.start();                break;            case 104:                    //other thread经过Post Runnable方式发送消息给主线程                bpostRunnable = true;                noLooperThread = new NoLooperThread();                noLooperThread.start();                break;          case 105:              //主线程发送消息给other thread                if(null!=mOtherThreadHandler){                tv.setText("please look at the error level log for other thread received message from main thread");                String msgObj = "message from mainThread";                Message mainThreadMsg = mOtherThreadHandler.obtainMessage(1, 1, 1, msgObj);                mOtherThreadHandler.sendMessage(mainThreadMsg);                }                break;            case 106:                finish();                break;            }      }  }