Looper用于在android线程中进行消息处理，默认情况下,一个线程并不和任何Looper绑定。当我们调用Looper.prepare()时，如果当前线程还没有和任何Looper绑定，那么将创建一个Looper让它和当前线程绑定。当我们调用Looper.loop()时，它将对当前线程所对应的Looper的消息进行处理，从消息队列里取消息，处理消息，一直循环直到对该Looper调用 quit（） 函数。
注意：Looper.loop()中是个while循环，只有对它所在线程的Looper调用了quit()函数，Looper.loop()函数才能完成，其后的代码才能得以运行。一个线程对应一个Looper，一个Looper对应一个消息队列MessageQueue。
对于Handler，其实只是把消息发送到其对应的Looper的消息队列MessageQueue中，最后的处理还是在Looper.loop()的while循环中进行的。一个Looper可以用于构造多个Handler。因为Looper.loop()函数是个while循环，会让当前线程一直在那里处理进行循环，直到对该线程的Looper调用了quit()函数，所以 ， 如果想对该Handler发送消息或添加Runnable以进行事务处理，要么在别的线程中进行，要么在该Handler在处理消息时或在Runnable（）的run()函数中进行事务处理时进行。
注意1：Handler的构造函数Handler()和Handler(Handler.Callback callback)，虽然没有Looper参数，但是它实际上是通过Looper.myLooper()来获取当前线程中的Looper的。
注意2：如果你使用的Looper来自于你自己创建的线程，那么在不使用其对应的Handler时候，一定要通过 handler.getLooper().quit()来使其线程退出。
以下是Android API中的一个典型的Looper thread实现
示例1：
class LooperThread extends Thread
{
public Handler mHandler
;
public void run()
{
Looper.prepare();
mHandler = new Handler()
{
public void handleMessage(Message msg)
{
// process incoming messages here
}
};
Looper.loop();
}
注意：默认情况下，线程是没有Looper的，所以要调用 Looper.prepare()来给线程创建消息循环，然后再通过，Looper.loop()来使消息循环起作用。
另外，Activity的MainUI线程已经新建并绑定了个Looper(在源码1的，Main函数中你可以看到)。所以在Activity中新建Handler时，不需要先调用Looper.prepare()。
关于
Activity的MainUI线程的更多内容可以查看源码文件
ActivityThread.java,
它位于
android\frameworks\base\core\java\android\app
下
源码1
public final class ActivityThread {
//省略
final ApplicationThread mAppThread = new ApplicationThread();
final Looper mLooper = Looper.myLooper();
final H mH = new H();
//省略
private final class H extends Handler
{
//省略
}
//省略
public static final void main(String[] args) {
SamplingProfilerIntegration.start();
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
if (sMainThreadHandler == null) {
sMainThreadHandler = new Handler();
}
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
if (Process.supportsProcesses()) {
throw new RuntimeException("Main thread loop unexpectedly exited");
}
thread.detach();
String name = (thread.mInitialApplication != null)
? thread.mInitialApplication.getPackageName()
: "<unknown>";
Slog.i(TAG, "Main thread of " + name + " is now exiting");
}
还Activity的生命周期函数（如onCreate(),onResume）本身也是通过 一个基于其MainUI线程的Looper（即ActivityThread的Main函数线程）的Handler（即ActivityThread的H mH这个Handler
）来调用的.也就说这些函数都是在Looper.loop()中被执行的。当然如果我们创建了一个基于Activity本身的MainUI线程的Looper的Handler的话，该Handler的事务处理也是在Looper.loop()中进行的。我们从测试实例1也可以看到这一点。
测试实例1
：
package com.teleca.robin;
import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.util.Log;
public class MainActivity extends Activity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
Log.i(tag,"-----------------------------------------------");
StackTraceElement st[]= Thread.currentThread().getStackTrace();
for(int i=0;i<st.length;i++)
Log.i(tag,i+":"+st[i]);
Log.i(tag,"------------------------------------------------");
handler=new Handler();
handler.postDelayed(r, 100);
handler2.postDelayed(r, 200);
}
final static String tag="robin";
Handler handler;
Handler handler2=new Handler();
Runnable r=new Runnable(){
public void run()
{
StackTraceElement st[]= Thread.currentThread().getStackTrace();
for(int i=0;i<st.length;i++)
Log.i(tag,i+":"+st[i]);
Log.i(tag,"----------------------------------------------");
}
};
}
：
07-20 22:10:18.799: INFO/robin(7844): -----------------------------------------------
07-20 22:10:18.819: INFO/robin(7844): 0:dalvik.system.VMStack.getThreadStackTrace(Native Method)
07-20 22:10:18.819: INFO/robin(7844): 1:java.lang.Thread.getStackTrace(Thread.java:788)
07-20 22:10:18.819: INFO/robin(7844): 2:com.teleca.robin.MainActivity.onCreate(MainActivity.java:15)
07-20 22:10:18.819: INFO/robin(7844): 3:android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
07-20 22:10:18.819: INFO/robin(7844): 4:android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2633)
07-20 22:10:18.819: INFO/robin(7844): 5:android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2685)
07-20 22:10:18.819: INFO/robin(7844): 6:android.app.ActivityThread.access$2300(ActivityThread.java:126)
07-20 22:10:18.819: INFO/robin(7844): 7:android.app.ActivityThread$H.handleMessage(ActivityThread.java:2038)
07-20 22:10:18.819: INFO/robin(7844): 8:android.os.Handler.dispatchMessage(Handler.java:99)
07-20 22:10:18.819: INFO/robin(7844): 9:android.os.Looper.loop(Looper.java:123)
07-20 22:10:18.819: INFO/robin(7844): 10:android.app.ActivityThread.main(ActivityThread.java:4633)
07-20 22:10:18.819: INFO/robin(7844): 11:java.lang.reflect.Method.invokeNative(Native Method)
07-20 22:10:18.819: INFO/robin(7844): 12:java.lang.reflect.Method.invoke(Method.java:521)
07-20 22:10:18.819: INFO/robin(7844): 13:com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
07-20 22:10:18.819: INFO/robin(7844): 14:com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
07-20 22:10:18.819: INFO/robin(7844): 15:dalvik.system.NativeStart.main(Native Method)
07-20 22:10:18.819: INFO/robin(7844): ------------------------------------------------
07-20 22:10:18.969: INFO/robin(7844): 0:dalvik.system.VMStack.getThreadStackTrace(Native Method)
07-20 22:10:18.969: INFO/robin(7844): 1:java.lang.Thread.getStackTrace(Thread.java:788)
07-20 22:10:18.969: INFO/robin(7844): 2:com.teleca.robin.MainActivity$1.run(MainActivity.java:29)
07-20 22:10:18.969: INFO/robin(7844): 3:android.os.Handler.handleCallback(Handler.java:587)
07-20 22:10:18.969: INFO/robin(7844): 4:android.os.Handler.dispatchMessage(Handler.java:92)
07-20 22:10:18.969: INFO/robin(7844): 5:android.os.Looper.loop(Looper.java:123)
07-20 22:10:18.969: INFO/robin(7844): 6:android.app.ActivityThread.main(ActivityThread.java:4633)
07-20 22:10:18.969: INFO/robin(7844): 7:java.lang.reflect.Method.invokeNative(Native Method)
07-20 22:10:18.969: INFO/robin(7844): 8:java.lang.reflect.Method.invoke(Method.java:521)
07-20 22:10:18.969: INFO/robin(7844): 9:com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
07-20 22:10:18.969: INFO/robin(7844): 10:com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
07-20 22:10:18.969: INFO/robin(7844): 11:dalvik.system.NativeStart.main(Native Method)
07-20 22:10:18.969: INFO/robin(7844): ----------------------------------------------
07-20 22:10:19.019: INFO/robin(7844): 0:dalvik.system.VMStack.getThreadStackTrace(Native Method)
07-20 22:10:19.019: INFO/robin(7844): 1:java.lang.Thread.getStackTrace(Thread.java:788)
07-20 22:10:19.019: INFO/robin(7844): 2:com.teleca.robin.MainActivity$1.run(MainActivity.java:29)
07-20 22:10:19.019: INFO/robin(7844): 3:android.os.Handler.handleCallback(Handler.java:587)
07-20 22:10:19.019: INFO/robin(7844): 4:android.os.Handler.dispatchMessage(Handler.java:92)
07-20 22:10:19.019: INFO/robin(7844): 5:android.os.Looper.loop(Looper.java:123)
07-20 22:10:19.019: INFO/robin(7844): 6:android.app.ActivityThread.main(ActivityThread.java:4633)
07-20 22:10:19.019: INFO/robin(7844): 7:java.lang.reflect.Method.invokeNative(Native Method)
07-20 22:10:19.019: INFO/robin(7844): 8:java.lang.reflect.Method.invoke(Method.java:521)
07-20 22:10:19.019: INFO/robin(7844): 9:com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
07-20 22:10:19.019: INFO/robin(7844): 10:com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
07-20 22:10:19.019: INFO/robin(7844): 11:dalvik.system.NativeStart.main(Native Method)
07-20 22:10:19.019: INFO/robin(7844): ----------------------------------------------
另外同过查看ActivityThread.java源码，你可以看到系统还会通过ApplicationThread mAppThread向该Handler发送消息，以便进行事务处理。
测试实例2
MyService.java
package com.lenovo.robin.test;
import android.app.Service;
import android.content.Intent;
import android.os.IBinder;
import android.util.Log;
public class MyService extends Service {
String tag="robin";
@Override
public void onCreate() {
Log.i(tag,"MyService is oncreate");
}
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
StackTraceElement st[]= Thread.currentThread().getStackTrace();
for(int i=0;i<st.length;i++)
Log.i(tag,i+":"+st[i]);
return START_STICKY;
}
@Override
public void onDestroy() {
Log.i(tag,"OnDestory");
}
@Override
public IBinder onBind(Intent arg0) {
return null;
}
}
测试结果
08-12 22:11:30.950: I/robin(26133): MyService is oncreate
08-12 22:11:30.960: I/robin(26133): 0:dalvik.system.VMStack.getThreadStackTrace(Native Method)
08-12 22:11:30.960: I/robin(26133): 1:java.lang.Thread.getStackTrace(Thread.java:745)
08-12 22:11:30.960: I/robin(26133): 2:
com.lenovo.robin.test.MyService
.
onStartCommand
(MyService.java:18)
08-12 22:11:30.960: I/robin(26133): 3:android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2456)
08-12 22:11:30.960: I/robin(26133): 4:android.app.ActivityThread.access$2800(ActivityThread.java:135)
08-12 22:11:30.960: I/robin(26133): 5:android.app.ActivityThread$H.handleMessage(ActivityThread.java:1127)
08-12 22:11:30.960: I/robin(26133): 6:android.os.Handler.dispatchMessage(Handler.java:99)
08-12 22:11:30.960: I/robin(26133): 7:android.os.Looper.loop(Looper.java:150)
08-12 22:11:30.960: I/robin(26133): 8:android.app
.ActivityThread
.main(
ActivityThread.java:4385
)
08-12 22:11:30.960: I/robin(26133): 9:java.lang.reflect.Method.invokeNative(Native Method)
08-12 22:11:30.960: I/robin(26133): 10:java.lang.reflect.Method.invoke(Method.java:507)
08-12 22:11:30.960: I/robin(26133): 11:com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:849)
08-12 22:11:30.970: I/robin(26133): 12:com.android.internal.os.ZygoteInit.main(ZygoteInit.java:607)
08-12 22:11:30.970: I/robin(26133): 13:dalvik.system.NativeStart.main(Native Method)
测试实例3
文件MainActivity.java
package
com
.
lenovo
.
robin
.
test
;
import
android
.
os
.
Bundle
;
import
android
.
app
.
Activity
;
import
android
.
content
.
Intent
;
import
android
.
util
.
Log
;
import
android
.
view
.
Menu
;
import
android
.
view
.
MenuItem
;
import
android
.
view
.
View
;
import
android
.
support
.
v4
.
app
.
NavUtils
;
public
class
MainActivity
extends
Activity
{
static
boolean
iscreated
=
false
;
final
static
String
TAG
=
"robin"
;
@Override
public
void
onCreate
(
Bundle
savedInstanceState
)
{
super
.
onCreate
(
savedInstanceState
);
setContentView
(
R
.
layout
.
activity_main
);
iscreated
=
true
;
Log
.
i
(
TAG
,
"on create() in MainActivity"
);
this
.
startService
(
new
Intent
(
this
,
MyService
.
class
));
}
@Override
public
void
onDestroy
()
{
Log
.
i
(
TAG
,
"OnDestoryv in main"
);
super
.
onDestroy
();
android
.
os
.
Process
.
killProcess
(
android
.
os
.
Process
.
myPid
());
//System.exit(1);
}
public void onClick(View v)
{
StackTraceElement st[]= Thread.currentThread().getStackTrace();
for(int i=0;i<st.length&&i<20;i++)
Log.w(TAG,i+":"+st[i]);
}
}
文件layout\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:layout_centerHorizontal
=
"true"
android:layout_centerVertical
=
"true"
android:text
=
"@string/hello_world"
tools:context
=
".MainActivity"
/>
<Button
android:id
=
"@+id/button1"
android:layout_width
=
"wrap_content"
android:layout_height
=
"wrap_content"
android:layout_above
=
"@+id/textView1"
android:layout_alignLeft
=
"@+id/textView1"
android:layout_marginBottom
=
"46dp"
android:layout_marginLeft
=
"44dp"
android:text
=
"Button"
android:onClick
=
"onClick"
/>
</RelativeLayout>
运行结果：
08-17 08:22:36.755: W/robin(12413): 0:dalvik.system.VMStack.getThreadStackTrace(Native Method)
08-17 08:22:36.755: W/robin(12413): 1:java.lang.Thread.getStackTrace(Thread.java:745)
08-17 08:22:36.755: W/robin(12413): 2:com.lenovo.robin.test.MainActivity.onClick(MainActivity.java:32)
08-17 08:22:36.755: W/robin(12413): 3:java.lang.reflect.Method.invokeNative(Native Method)
08-17 08:22:36.755: W/robin(12413): 4:java.lang.reflect.Method.invoke(Method.java:507)
08-17 08:22:36.755: W/robin(12413): 5:android.view.View$1.
onClick(
View.java:2187)
08-17 08:22:36.755: W/robin(12413): 6:android.view.View.
performClick
(View.java:2533)
08-17 08:22:36.755: W/robin(12413): 7:android.view.View$PerformClick.run(View.java:9320)
08-17 08:22:36.755: W/robin(12413): 8:android.os.Handler.handleCallback(Handler.java:587)
08-17 08:22:36.755: W/robin(12413): 9:android.os.Handler.dispatchMessage(Handler.java:92)
08-17 08:22:36.755: W/robin(12413): 10:android.os.Looper.loop(Looper.java:150)
08-17 08:22:36.755: W/robin(12413): 11:
