转： Android中Message机制的灵活应用

原文出自：http://qaohao.javaeye.com/blog/509145

1.活用Android线程间通信的Message机制
1.1.Message
代码在frameworks/base/core/java/android/Os/Message.java中。
Message.obtain函数：有多个obtain函数，主要功能一样，只是参数不一样。作用是从Message Pool中取出一个Message，如果Message Pool中已经没有Message可取则新建一个Message返回，同时用对应的参数给得到的Message对象赋值。
Message Pool：大小为10个；通过Message.mPool->（Message并且Message.next）-> （Message并且Message.next）-> （Message并且Message.next）...构造一个Message Pool。Message Pool的第一个元素直接new出来，然后把Message.mPool（static类的static变量）指向它。其他的元素都是使用完的 Message通过Message的recycle函数清理后放到Message Pool（通过Message Pool最后一个Message的next指向需要回收的Message的方式实现）。下图为Message Pool的结构：

1.2.MessageQueue
MessageQueue里面有一个收到的Message的对列：
MessageQueue.mMessages(static变量)->( Message并且Message.next)-> ( Message并且Message.next)->...，下图为接收消息的消息队列：

上层代码通过Handler的sendMessage等函数放入一个message到MessageQueue里面时最终会调用MessageQueue的 enqueueMessage函数。enqueueMessage根据上面的接收的Message的队列的构造把接收到的Message放入队列中。
MessageQueue的removeMessages函数根据上面的接收的Message的队列的构造把接收到的Message从队列中删除，并且调用对应Message对象的recycle函数把不用的Message放入Message Pool中。
1.3.Looper
Looper对象的创建是通过prepare函数，而且每一个Looper对象会和一个线程关联

Java代码

public static final void prepare() {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper());
}

public static final void prepare() {    if (sThreadLocal.get() != null) {        throw new RuntimeException("Only one Looper may be created per thread");    }    sThreadLocal.set(new Looper());}

Looper对象创建时会创建一个MessageQueue，主线程默认会创建一个Looper从而有MessageQueue，其他线程默认是没有 MessageQueue的不能接收Message，如果需要接收Message则需要通过prepare函数创建一个MessageQueue。具体操作请见示例代码。

Java代码

private Looper() {
mQueue = new MessageQueue();
mRun = true;
mThread = Thread.currentThread();
}

private Looper() {    mQueue = new MessageQueue();    mRun = true;    mThread = Thread.currentThread();}

prepareMainLooper函数只给主线程调用（系统处理，程序员不用处理），它会调用prepare建立Looper对象和MessageQueue。

Java代码

public static final void prepareMainLooper() {
prepare();
setMainLooper(myLooper());
if (Process.supportsProcesses()) {
myLooper().mQueue.mQuitAllowed = false;
}
}

public static final void prepareMainLooper() {    prepare();    setMainLooper(myLooper());    if (Process.supportsProcesses()) {        myLooper().mQueue.mQuitAllowed = false;    }}

Loop函数从MessageQueue中从前往后取出Message，然后通过Handler的dispatchMessage函数进行消息的处理（可见消息的处理是Handler负责的），消息处理完了以后通过Message对象的recycle函数放到Message Pool中，以便下次使用，通过Pool的处理提供了一定的内存管理从而加速消息对象的获取。至于需要定时处理的消息如何做到定时处理，请见 MessageQueue的next函数，它在取Message来进行处理时通过判断MessageQueue里面的Message是否符合时间要求来决定是否需要把Message取出来做处理，通过这种方式做到消息的定时处理。

Java代码

public static final void loop() {
Looper me = myLooper();
MessageQueue queue = me.mQueue;
while (true) {
Message msg = queue.next(); // might block
//if (!me.mRun) {
// break;
//}
if (msg != null) {
if (msg.target == null) {
// No target is a magic identifier for the quit message
return;
}
if (me.mLogging!= null)
me.mLogging.println(">>>>> Dispatching to " + msg.target + " "+ msg.callback + ": " + msg.what);
msg.target.dispatchMessage(msg);
if (me.mLogging!= null)
me.mLogging.println("<<<<< Finished to" + msg.target + " "+ msg.callback);
msg.recycle();
}
}
}

public static final void loop() {    Looper me = myLooper();    MessageQueue queue = me.mQueue;    while (true) {        Message msg = queue.next(); // might block        //if (!me.mRun) {        //    break;        //}        if (msg != null) {            if (msg.target == null) {                // No target is a magic identifier for the quit message                return;            }            if (me.mLogging!= null)                 me.mLogging.println(">>>>> Dispatching to " + msg.target + " "+ msg.callback + ": " + msg.what);            msg.target.dispatchMessage(msg);            if (me.mLogging!= null)                 me.mLogging.println("<<<<< Finished to" + msg.target + " "+ msg.callback);            msg.recycle();        }    }}

1.4.Handler
Handler的构造函数表示Handler会有成员变量指向Looper和MessageQueue，后面我们会看到没什么需要这些引用；至于callback是实现了Callback接口的对象，后面会看到这个对象的作用。

Java代码

public Handler(Looper looper, Callback callback) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
}
public interface Callback {
public boolean handleMessage(Message msg);
}

public Handler(Looper looper, Callback callback) {    mLooper = looper;    mQueue = looper.mQueue;    mCallback = callback;}public interface Callback {    public boolean handleMessage(Message msg);}

获取消息：直接通过Message的obtain方法获取一个Message对象。

Java代码

public final Message obtainMessage(int what, int arg1, int arg2, Object obj){
return Message.obtain(this, what, arg1, arg2, obj);
}

public final Message obtainMessage(int what, int arg1, int arg2, Object obj){    return Message.obtain(this, what, arg1, arg2, obj);}

发送消息：通过MessageQueue的enqueueMessage把Message对象放到MessageQueue的接收消息队列中

Java代码

public boolean sendMessageAtTime(Message msg, long uptimeMillis){
boolean sent = false;
MessageQueue queue = mQueue;
if (queue != null) {
msg.target = this;
sent = queue.enqueueMessage(msg, uptimeMillis);
} else {
RuntimeException e = new RuntimeException(this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
}
return sent;
}

public boolean sendMessageAtTime(Message msg, long uptimeMillis){    boolean sent = false;    MessageQueue queue = mQueue;    if (queue != null) {        msg.target = this;    sent = queue.enqueueMessage(msg, uptimeMillis);    } else {        RuntimeException e = new RuntimeException(this + " sendMessageAtTime() called with no mQueue");        Log.w("Looper", e.getMessage(), e);    }    return sent;}

线程如何处理MessageQueue中接收的消息：在Looper的loop函数中循环取出MessageQueue的接收消息队列中的消息，然后调用 Hander的dispatchMessage函数对消息进行处理，至于如何处理（相应消息）则由用户指定（三个方法，优先级从高到低：Message里面的Callback，一个实现了Runnable接口的对象，其中run函数做处理工作；Handler里面的mCallback指向的一个实现了 Callback接口的对象，里面的handleMessage进行处理；处理消息Handler对象对应的类继承并实现了其中 handleMessage函数，通过这个实现的handleMessage函数处理消息）。

Java代码

public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}

public void dispatchMessage(Message msg) {    if (msg.callback != null) {        handleCallback(msg);    } else {        if (mCallback != null) {            if (mCallback.handleMessage(msg)) {                return;            }        }        handleMessage(msg);    }}

Runnable说明：Runnable只是一个接口，实现了这个接口的类对应的对象也只是个普通的对象，并不是一个Java中的Thread。Thread类经常使用Runnable，很多人有误解，所以这里澄清一下。
从上可知以下关系图：

其中清理Message是Looper里面的loop函数指把处理过的Message放到Message的Pool里面去，如果里面已经超过最大值10个，则丢弃这个Message对象。
调用Handler是指Looper里面的loop函数从MessageQueue的接收消息队列里面取出消息，然后根据消息指向的Handler对象调用其对应的处理方法。
1.5.代码示例
下面我们会以android实例来展示对应的功能，程序界面于下：

程序代码如下，后面部分有代码说明：

Java代码

package com.android.messageexample;
import android.app.Activity;
import android.content.Context;
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.widget.Button;
import android.widget.LinearLayout;
import android.widget.TextView;
public class MessageExample extends Activity implements OnClickListener {
private final int WC = LinearLayout.LayoutParams.WRAP_CONTENT;
private final int FP = LinearLayout.LayoutParams.FILL_PARENT;
public TextView tv;
private EventHandler mHandler;
private Handler mOtherThreadHandler=null;
private Button btn, btn2, btn3, btn4, btn5, btn6;
private NoLooperThread noLooerThread = null;
private OwnLooperThread ownLooperThread = null;
private ReceiveMessageThread receiveMessageThread =null;
private Context context = null;
private final String sTag = "MessageExample";
private boolean postRunnable = false;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
context = this.getApplicationContext();
LinearLayout layout = new LinearLayout(this);
layout.setOrientation(LinearLayout.VERTICAL);
btn = new Button(this);
btn.setId(101);
btn.setText("message from main thread self");
btn.setOnClickListener(this);
LinearLayout.LayoutParams param =
new LinearLayout.LayoutParams(250,50);
param.topMargin = 10;
layout.addView(btn, param);
btn2 = new Button(this);
btn2.setId(102);
btn2.setText("message from other thread to main thread");
btn2.setOnClickListener(this);
layout.addView(btn2, param);
btn3 = new Button(this);
btn3.setId(103);
btn3.setText("message to other thread from itself");
btn3.setOnClickListener(this);
layout.addView(btn3, param);
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, param);
btn5 = new Button(this);
btn5.setId(105);
btn5.setText("main thread's message to other thread");
btn5.setOnClickListener(this);
layout.addView(btn5, param);
btn6 = new Button(this);
btn6.setId(106);
btn6.setText("exit");
btn6.setOnClickListener(this);
layout.addView(btn6, param);
tv = new TextView(this);
tv.setTextColor(Color.WHITE);
tv.setText("");
LinearLayout.LayoutParams param2 =
new LinearLayout.LayoutParams(FP, WC);
param2.topMargin = 10;
layout.addView(tv, param2);
setContentView(layout);
//主线程要发送消息给other thread，这里创建那个other thread
receiveMessageThread = new ReceiveMessageThread();
receiveMessageThread.start();
}
//implement the OnClickListener interface
@Override
public void onClick(View v) {
switch(v.getId()){
case 101:
//主线程发送消息给自己
Looper looper;
looper = Looper.myLooper(); //get the Main looper related with the main thread
//如果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);
//mHandler = new EventHandler();
// 清除整个MessageQueue里的消息
mHandler.removeMessages(0);
String obj = "This main thread's message and received by itself!";
//得到Message对象
Message m = mHandler.obtainMessage(1, 1, 1, obj);
// 将Message对象送入到main thread的MessageQueue里面
mHandler.sendMessage(m);
break;
case 102:
//other线程发送消息给主线程
postRunnable = false;
noLooerThread = new NoLooperThread();
noLooerThread.start();
break;
case 103:
//other thread获取它自己发送的消息
tv.setText("please look at the error level log for other thread received message");
ownLooperThread = new OwnLooperThread();
ownLooperThread.start();
break;
case 104:
//other thread通过Post Runnable方式发送消息给主线程
postRunnable = true;
noLooerThread = new NoLooperThread();
noLooerThread.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;
}
}
class EventHandler extends Handler
{
public EventHandler(Looper looper) {
super(looper);
}
public EventHandler() {
super();
}
public void handleMessage(Message msg) {
//可以根据msg.what执行不同的处理，这里没有这么做
switch(msg.what){
case 1:
tv.setText((String)msg.obj);
break;
case 2:
tv.setText((String)msg.obj);
noLooerThread.stop();
break;
case 3:
//不能在非主线程的线程里面更新UI，所以这里通过Log打印收到的消息
Log.e(sTag, (String)msg.obj);
ownLooperThread.stop();
break;
default:
//不能在非主线程的线程里面更新UI，所以这里通过Log打印收到的消息
Log.e(sTag, (String)msg.obj);
break;
}
}
}
//NoLooperThread
class NoLooperThread extends Thread{
private EventHandler mNoLooperThreadHandler;
public void run() {
Looper myLooper, mainLooper;
myLooper = Looper.myLooper();
mainLooper = Looper.getMainLooper(); //这是一个static函数
String obj;
if(myLooper == null){
mNoLooperThreadHandler = new EventHandler(mainLooper);
obj = "NoLooperThread has no looper and handleMessage function executed in main thread!";
}
else {
mNoLooperThreadHandler = new EventHandler(myLooper);
obj = "This is from NoLooperThread self and handleMessage function executed in NoLooperThread!";
}
mNoLooperThreadHandler.removeMessages(0);
if(false == postRunnable){
//send message to main thread
Message m = mNoLooperThreadHandler.obtainMessage(2, 1, 1, obj);
mNoLooperThreadHandler.sendMessage(m);
Log.e(sTag, "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
//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
//您可以在run上加断点然后在eclipse调试中看它在哪个线程中执行
mNoLooperThreadHandler.post(new Runnable(){
@Override
public void run() {
tv.setText("update UI through handler post runnalbe mechanism!");
noLooerThread.stop();
}
});
}
}
}
//OwnLooperThread has his own message queue by execute Looper.prepare();
class OwnLooperThread extends Thread{
private EventHandler mOwnLooperThreadHandler;
public void run() {
Looper.prepare();
Looper myLooper, mainLooper;
myLooper = Looper.myLooper();
mainLooper = Looper.getMainLooper(); //这是一个static函数
String obj;
if(myLooper == null){
mOwnLooperThreadHandler = new EventHandler(mainLooper);
obj = "OwnLooperThread has no looper and handleMessage function executed in main thread!";
}
else {
mOwnLooperThreadHandler = new EventHandler(myLooper);
obj = "This is from OwnLooperThread self and handleMessage function executed in NoLooperThread!";
}
mOwnLooperThreadHandler.removeMessages(0);
//给自己发送消息
Message m = mOwnLooperThreadHandler.obtainMessage(3, 1, 1, obj);
mOwnLooperThreadHandler.sendMessage(m);
Looper.loop();
}
}
//ReceiveMessageThread has his own message queue by execute Looper.prepare();
class ReceiveMessageThread extends Thread{
public void run() {
Looper.prepare();
mOtherThreadHandler = new Handler(){
public void handleMessage(Message msg) {
Log.e(sTag, (String)msg.obj);
}
};
Looper.loop();
}
}
}

package com.android.messageexample;import android.app.Activity;import android.content.Context;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.widget.Button;import android.widget.LinearLayout;import android.widget.TextView;public class MessageExample extends Activity implements OnClickListener { private final int WC = LinearLayout.LayoutParams.WRAP_CONTENT; private final int FP = LinearLayout.LayoutParams.FILL_PARENT; public TextView tv; private EventHandler mHandler; private Handler mOtherThreadHandler=null; private Button btn, btn2, btn3, btn4, btn5, btn6; private NoLooperThread noLooerThread = null; private OwnLooperThread ownLooperThread = null; private ReceiveMessageThread receiveMessageThread =null; private Context context = null; private final String sTag = "MessageExample"; private boolean postRunnable = false;  /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); context = this.getApplicationContext(); LinearLayout layout = new LinearLayout(this); layout.setOrientation(LinearLayout.VERTICAL); btn = new Button(this); btn.setId(101); btn.setText("message from main thread self"); btn.setOnClickListener(this); LinearLayout.LayoutParams param = new LinearLayout.LayoutParams(250,50); param.topMargin = 10; layout.addView(btn, param); btn2 = new Button(this); btn2.setId(102); btn2.setText("message from other thread to main thread"); btn2.setOnClickListener(this); layout.addView(btn2, param); btn3 = new Button(this); btn3.setId(103); btn3.setText("message to other thread from itself"); btn3.setOnClickListener(this); layout.addView(btn3, param); 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, param); btn5 = new Button(this); btn5.setId(105); btn5.setText("main thread's message to other thread"); btn5.setOnClickListener(this); layout.addView(btn5, param); btn6 = new Button(this); btn6.setId(106); btn6.setText("exit"); btn6.setOnClickListener(this); layout.addView(btn6, param); tv = new TextView(this); tv.setTextColor(Color.WHITE); tv.setText(""); LinearLayout.LayoutParams param2 = new LinearLayout.LayoutParams(FP, WC); param2.topMargin = 10; layout.addView(tv, param2); setContentView(layout); //主线程要发送消息给other thread， 这里创建那个other thread receiveMessageThread = new ReceiveMessageThread(); receiveMessageThread.start(); }  //implement the OnClickListener interface @Override public void onClick(View v) { switch(v.getId()){ case 101: //主线程发送消息给自己 Looper looper; looper = Looper.myLooper(); //get the Main looper related with the main thread //如果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); //mHandler = new EventHandler(); // 清除整个MessageQueue里的消息 mHandler.removeMessages(0); String obj = "This main thread's message and received by itself!"; //得到Message对象 Message m = mHandler.obtainMessage(1, 1, 1, obj); // 将Message对象送入到main thread的MessageQueue里面 mHandler.sendMessage(m); break; case 102: //other线程发送消息给主线程 postRunnable = false; noLooerThread = new NoLooperThread(); noLooerThread.start(); break; case 103: //other thread获取它自己发送的消息 tv.setText("please look at the error level log for other thread received message"); ownLooperThread = new OwnLooperThread(); ownLooperThread.start(); break; case 104: //other thread通过Post Runnable方式发送消息给主线程 postRunnable = true; noLooerThread = new NoLooperThread(); noLooerThread.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; } } class EventHandler extends Handler { public EventHandler(Looper looper) { super(looper); } public EventHandler() { super(); } public void handleMessage(Message msg) { //可以根据msg.what执行不同的处理，这里没有这么做 switch(msg.what){ case 1: tv.setText((String)msg.obj); break; case 2: tv.setText((String)msg.obj); noLooerThread.stop(); break; case 3: //不能在非主线程的线程里面更新UI，所以这里通过Log打印收到的消息 Log.e(sTag, (String)msg.obj); ownLooperThread.stop(); break; default: //不能在非主线程的线程里面更新UI，所以这里通过Log打印收到的消息 Log.e(sTag, (String)msg.obj); break; } } } //NoLooperThread class NoLooperThread extends Thread{ private EventHandler mNoLooperThreadHandler; public void run() { Looper myLooper, mainLooper; myLooper = Looper.myLooper(); mainLooper = Looper.getMainLooper(); //这是一个static函数 String obj; if(myLooper == null){ mNoLooperThreadHandler = new EventHandler(mainLooper); obj = "NoLooperThread has no looper and handleMessage function executed in main thread!"; } else { mNoLooperThreadHandler = new EventHandler(myLooper); obj = "This is from NoLooperThread self and handleMessage function executed in NoLooperThread!"; } mNoLooperThreadHandler.removeMessages(0); if(false == postRunnable){ //send message to main thread Message m = mNoLooperThreadHandler.obtainMessage(2, 1, 1, obj); mNoLooperThreadHandler.sendMessage(m); Log.e(sTag, "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 //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 //您可以在run上加断点然后在eclipse调试中看它在哪个线程中执行 mNoLooperThreadHandler.post(new Runnable(){ @Override public void run() { tv.setText("update UI through handler post runnalbe mechanism!"); noLooerThread.stop(); } }); } } }  //OwnLooperThread has his own message queue by execute Looper.prepare(); class OwnLooperThread extends Thread{ private EventHandler mOwnLooperThreadHandler; public void run() { Looper.prepare(); Looper myLooper, mainLooper; myLooper = Looper.myLooper(); mainLooper = Looper.getMainLooper(); //这是一个static函数 String obj; if(myLooper == null){ mOwnLooperThreadHandler = new EventHandler(mainLooper); obj = "OwnLooperThread has no looper and handleMessage function executed in main thread!"; } else { mOwnLooperThreadHandler = new EventHandler(myLooper); obj = "This is from OwnLooperThread self and handleMessage function executed in NoLooperThread!"; } mOwnLooperThreadHandler.removeMessages(0); //给自己发送消息 Message m = mOwnLooperThreadHandler.obtainMessage(3, 1, 1, obj); mOwnLooperThreadHandler.sendMessage(m); Looper.loop(); } }  //ReceiveMessageThread has his own message queue by execute Looper.prepare(); class ReceiveMessageThread extends Thread{ public void run() { Looper.prepare(); mOtherThreadHandler = new Handler(){ public void handleMessage(Message msg) { Log.e(sTag, (String)msg.obj); } }; Looper.loop(); } } }

说明（代码详细解释请见后文）：
使用Looper.myLooper静态方法可以取得当前线程的Looper对象。
使用mHandler = new EevntHandler(Looper.myLooper()); 可建立用来处理当前线程的Handler对象；其中，EevntHandler是Handler的子类。
使用mHandler = new EevntHandler(Looper.getMainLooper()); 可建立用来处理main线程的Handler对象；其中，EevntHandler是Handler的子类。
1.5.1.主线程给自己发送消息示例
主线程发送消息：
在onClick的case 101中创建一个继承自Handler的EventHandler对象，然后获取一个消息，然后通过EventHandler对象调用 sendMessage把消息发送到主线程的MessageQueue中。主线程由系统创建，系统会给它建立一个Looper对象和 MessageQueue，所以可以接收消息。这里只要根据主线程的Looper对象初始化EventHandler对象，就可以通过 EventHandler对象发送消息到主线程的消息队列中。
主线程处理消息：
这里是通过EventHandler的handleMessage函数处理的，其中收到的Message对象中what值为一的消息就是发送给它的，然后把消息里面附带的字符串在TextView上显示出来。
1.5.2.其他线程给主线程发送消息示例
其他线程发送消息（这里是说不使用Runnable作为callback的消息）：
首先 postRunnable设为false，表示不通过Runnable方式进行消息相关的操作。然后启动线程noLooerThread，然后以主线程的Looper对象为参数建立EventHandler的对象mNoLooperThreadHandler，然后获取一个Message并把一个字符串赋值给它的一个成员obj，然后通过mNoLooperThreadHandler把消息发送到主线程的MessageQueue中。
主线程处理消息：
这里是通过EventHandler的handleMessage函数处理的，其中收到的Message对象中what值为二的消息就是上面发送给它的，然后把消息里面附带的字符串在TextView上显示出来。
1.5.3.其他线程给自己发送消息示例
其他线程发送消息：
其他非主线程建立后没有自己的Looper对象，所以也没有MessageQueue，需要给非主线程发送消息时需要建立MessageQueue以便接收消息。下面说明如何给自己建立MessageQueue和Looper对象。从OwnLooperThread的run函数中可以看见有一个 Looper.prepare()调用，这个就是用来建立非主线程的MessageQueue和Looper对象的。
所以这里的发送消息过程是建立线程mOwnLooperThread，然后线程建立自己的Looper和MessageQueue对象，然后根据上面建立的Looper对象建立对应的EventHandler对象mOwnLooperThreadHandler，然后由mOwnLooperThreadHandler建立消息并且发送到自己的MessageQueue里面。
其他线程处理接收的消息：
线程要接收消息需要在run函数中调用Looper.loop()，然后loop函数会从MessageQueue中取出消息交给对应的Handler对象 mOwnLooperThreadHandler处理，在mOwnLooperThreadHandler的handleMessage函数中会把 Message对象中what值为三的消息（上面发送的消息）在Log中打印出来，可以通过Logcat工具查看log。
1.5.4.其他线程以Runnable为消息参数给主线程发送消息示例
其他线程发送消息（这里是说使用Runnable作为callback的消息）：
首先 postRunnable设为true，表示通过Runnable方式进行消息相关的操作。然后启动线程noLooerThread，然后以主线程的Looper对象为参数建立EventHandler的对象mNoLooperThreadHandler，然后获取一个Message并把一个字符串赋值给它的一个成员obj，然后通过mNoLooperThreadHandler把消息发送到主线程的MessageQueue中。
主线程处理消息：
主线程收到上面发送的Message后直接运行上面Runnable对象中的run函数进行相应的操作。run函数通过Log打印一个字符串，可以通过Logcat工具查看log。
1.5.5.主线程给其他线程发送消息示例
主线程发送消息：
这里首先要求线程receiveMessageThread运行（在onCreate函数中完成），并且准备好自己的Looper和 MessageQueue（这个通过ReceiveMessageThread中的run函数中的Looper.prepare()调用完成），然后根据建立的Looper对象初始化Handler对象mOtherThreadHandler。然后在onClick的case 105中由mOtherThreadHandler建立一个消息（消息中有一个字符串对象）并且发送到线程receiveMessageThread中的 MessageQueue中。
其他线程处理接收的消息：
线程要接收消息需要在run函数中调用Looper.loop()，然后loop函数会从MessageQueue中取出消息交给对应的Handler对象mOtherThreadHandler处理，在mOtherThreadHandler的handleMessage函数中会把Message对象中的字符串对象在Log中打印出来，可以通过Logcat工具查看log。

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