android 消息机制

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m