Android异步处理三：Handler+Looper+MessageQueue深入详解

Android Loop&Handle学习总结 - New Start - 博客频道 - CSDN.NET

       昨晚偷懒，这篇博客只写了一个标题，今天早晨一看，还有15的阅读量。实在是对不起那些同学.......换了是我，也会BS这样的LZ吧!sorry 啦

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       菜鸟我刚刚接触android源码的时候，当时连java语言都不太熟悉（菜鸟我一直是学C/C++），看到android某个应用的源码的时候，曾为这样的代码迷惑过。

private class MainHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case RESTART_PREVIEW: {
                    restartPreview();
                    if (mJpegPictureCallbackTime != 0) {
                        long now = System.currentTimeMillis();
                        mJpegCallbackFinishTime = now - mJpegPictureCallbackTime;
                        Log.v(TAG, "mJpegCallbackFinishTime = "
                                + mJpegCallbackFinishTime + "ms");
                        mJpegPictureCallbackTime = 0;
                    }
                    break;
                }

                ...

private class MainHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case RESTART_PREVIEW: {
                    restartPreview();
                    if (mJpegPictureCallbackTime != 0) {
                        long now = System.currentTimeMillis();
                        mJpegCallbackFinishTime = now - mJpegPictureCallbackTime;
                        Log.v(TAG, "mJpegCallbackFinishTime = "
                                + mJpegCallbackFinishTime + "ms");
                        mJpegPictureCallbackTime = 0;
                    }
                    break;
                }

                ...

        在同一个文件中不就是想调用restartPreview()函数吗？为什么不直接调用就好，还要发个消息然后在这个handleMessage()中来调用呢？初学菜鸟有没有同样的困惑？还是只有LZ一人太白痴了。

     那么到底为什么要这么做呢。是为了异步，是为了多线程，将一些耗时的操作放到一个子线程里面去，大家应该知道如果在android的应用程序主线程，也就是UI线程里面如果执行耗时操作超过5s就会报ANR（application not respon）错误。所以，为了避免ANR，android应用程序将一些耗时操作放到一个独立的工作线程中去。

      那么android是如何做到这点的呢？那个新的工作线程又是如何建立的？这就是android中Looper和Handler类的功劳。下面菜鸟来分析下首先看个类图：

      

       从这个图看到Handler和HandlerThread类都有一个Looper成员变量。而且，Handler和Looper都有成员变量MessageQueue，下面我们看看Looper类和Handler类的作用。

Looper：

       Looper类，来实现消息循环，内部有一个消息队列。看到Looper类的构造函数是private的，这样外部没法实例化Looper对象，Looper对象的构造只能通过Looper的内部接口，来看一段函数: 

/** Initialize the current thread as a looper.
      * This gives you a chance to create handlers that then reference
      * this looper, before actually starting the loop. Be sure to call
      * {@link #loop()} after calling this method, and end it by calling
      * {@link #quit()}.
      */
    public static final void prepare() {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper());
    }

    /** Initialize the current thread as a looper, marking it as an application's main
     *  looper. The main looper for your application is created by the Android environment,
     *  so you should never need to call this function yourself.
     * {@link #prepare()}
     */

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

/** Initialize the current thread as a looper.
      * This gives you a chance to create handlers that then reference
      * this looper, before actually starting the loop. Be sure to call
      * {@link #loop()} after calling this method, and end it by calling
      * {@link #quit()}.
      */
    public static final void prepare() {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper());
    }

    /** Initialize the current thread as a looper, marking it as an application's main
     *  looper. The main looper for your application is created by the Android environment,
     *  so you should never need to call this function yourself.
     * {@link #prepare()}
     */

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

        其中函数prepareMainLooper()函数只在ActivityThread.java的main()函数中被调用过。这个prepareMainLooper()的作用保证每个调用线程都有一个Looper对象，这样应用程序的主线程就有一个Looper对象了，so应用程序主线程的消息循环建立了。

        看看，perpare()函数中，有这样一句话 sThreadLocal.set(new Looper());其中sThreadLocal是一个ThreadLocal类型的变量。ThreadLocal表示这是一个线程的局部变量，google之看到下面这段解释。

        Implements a thread-local storage, that is, a variable for which each thread has its own value. All threads share the sameThreadLocal object, but each sees a different value when accessing it, and changes
made by one thread do not affect the other threads. The implementation supportsnull values.

       菜鸟斗胆来翻译下，ThreadLocal实现了一种线程的局部存储，ThreadLocal代表一种线程局部变量，对于这种变量来说ThreadLocal保证每个线程都有一个独立的值(value)，所有线程都共有一个ThreadLocal对象，但是每个线程在访问这些变量的时候能得到不同的值，每个线程可以更改这些变量并且不会影响其他的线程。ThreadLocal支持NULL值。/*那位大牛有更好的翻译，分享下呗*/

       其中ThreadLocal开放了两个接口：

           public Tget()：获取调用线程的局部变量

       public void set(T value) ：设置调用线程的局部变量

       perpare()函数的这种处理方式保证在每个调用该函数的线程都有一个独立的Looper对象。

      

Handler：    

       Handler的作用是处理消息，他封装了消息的投递和对消息的处理。通过上面的图可以看到Handler类有一个Looper和MessageQueue成员变量，Handler类有4个不同的构造函数如下:

public Handler() {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }
        <span style="color:#000000;">/*Handler默认的构造函数，使用当前线程（调用线程）的Looper对象给Handler成员变量mLooper赋值*/</span>
        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        <span style="color:#000000;">/*使用当前线程（调用线程）的消息队列给Handler的mQueue赋值*/</span>
        mQueue = mLooper.mQueue;
        mCallback = null;
    }

    /**
     * Constructor associates this handler with the queue for the
     * current thread and takes a callback interface in which you can handle
     * messages.
     */
    public Handler(Callback callback) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }

        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        <span style="color:#000000;">/*跟默认构造函数差不多，只不过带了一个callback*/</span>
        mCallback = callback;
    }

    /**
     * Use the provided queue instead of the default one.
     */
    public Handler(Looper looper) {
        <span style="color:#000000;">/*使用指定的Looper来给Handler类的mLooper赋值*/</span>
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = null;
    }

    /**
     * Use the provided queue instead of the default one and take a callback
     * interface in which to handle messages.
     */
    public Handler(Looper looper, Callback callback) {
        <span style="color:#000000;">/*使用指定的Looper和CallBack来初始化Handler*/</span>
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = callback;
    }

public Handler() {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }
        /*Handler默认的构造函数，使用当前线程（调用线程）的Looper对象给Handler成员变量mLooper赋值*/
        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        /*使用当前线程（调用线程）的消息队列给Handler的mQueue赋值*/
        mQueue = mLooper.mQueue;
        mCallback = null;
    }

    /**
     * Constructor associates this handler with the queue for the
     * current thread and takes a callback interface in which you can handle
     * messages.
     */
    public Handler(Callback callback) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }

        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        /*跟默认构造函数差不多，只不过带了一个callback*/
        mCallback = callback;
    }

    /**
     * Use the provided queue instead of the default one.
     */
    public Handler(Looper looper) {
        /*使用指定的Looper来给Handler类的mLooper赋值*/
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = null;
    }

    /**
     * Use the provided queue instead of the default one and take a callback
     * interface in which to handle messages.
     */
    public Handler(Looper looper, Callback callback) {
        /*使用指定的Looper和CallBack来初始化Handler*/
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = callback;
    }

     

       可以看到Handler类的成员变量MessageQueue mQueue都会指向Looper类的的MessageQueue，Handler为什么要这么做呢？

      下面看看Handler类提供那些接口：

sendEmptyMessage

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

/**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

/**
     * Sends a Message containing only the what value.
     *
     * @return Returns true if the message was successfully placed in to the
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */
    public final boolean sendEmptyMessage(int what)
    {
        return sendEmptyMessageDelayed(what, 0);
    }

/**
     * Remove any pending posts of messages with code 'what' that are in the
     * message queue.
     */
    public final void removeMessages(int what) {
        mQueue.removeMessages(this, what, null, true);
    }

     /**
     *<span style="color:#ff0000;">注意这个函数，函数将有子类实现</span>
     * Subclasses must implement this to receive messages.
     */
    public void handleMessage(Message msg) {
    }

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

/**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

/**
     * Sends a Message containing only the what value.
     *
     * @return Returns true if the message was successfully placed in to the
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */
    public final boolean sendEmptyMessage(int what)
    {
        return sendEmptyMessageDelayed(what, 0);
    }

/**
     * Remove any pending posts of messages with code 'what' that are in the
     * message queue.
     */
    public final void removeMessages(int what) {
        mQueue.removeMessages(this, what, null, true);
    }

     /**
     *注意这个函数，函数将有子类实现
     * Subclasses must implement this to receive messages.
     */
    public void handleMessage(Message msg) {
    }

        其中sendMessage函数都会调用到sendMessageAtTime函数，下面看下sendMessageAtTime(Message msg, long uptimeMillis)函数的实现

public boolean sendMessageAtTime(Message msg, long uptimeMillis)
    {
        boolean sent = false;
        MessageQueue queue = mQueue;
        if (queue != null) {
            <span style="color:#ff0000;">/*看到Message的target变量指向当前这个Handler类*/</span>
            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) {
            /*看到Message的target变量指向当前这个Handler类*/
            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;
    }

       msg.target指明了这个msg将有谁来处理，在这里msg.target=this，Handler类除了封装消息添加外还封装了消息处理的接口。

Looper和Handler的联系：

           本菜鸟不才不知道上面的分析有没有错误，有没有让很多初学者犯迷糊，下面我将分析以下Looper和Handler类的联系，希望可以把上面的知识点串联起来。

           当调用Looper类的loop()函数的时候当前线程就进入到一个消息循环中去。看看这个loop()@Looper.java函数先.

<span style="font-size:18px;">/**
     *  Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static final void loop() {
        Looper me = myLooper();
        MessageQueue queue = me.mQueue;

        // Make sure the identity of this thread is that of the local process,
        // and keep track of what that identity token actually is.
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();

        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
                        );
                <span style="color:#ff0000;">/*下面的函数将Looper类和Handler类联系起来*/</span>
                msg.target.dispatchMessage(msg);
                if (me.mLogging!= null) me.mLogging.println(
                        "<<<<< Finished to    " + msg.target + " "
                        + msg.callback);

                // Make sure that during the course of dispatching the
                // identity of the thread wasn't corrupted.
                final long newIdent = Binder.clearCallingIdentity();
                if (ident != newIdent) {
                    Log.wtf("Looper", "Thread identity changed from 0x"
                            + Long.toHexString(ident) + " to 0x"
                            + Long.toHexString(newIdent) + " while dispatching to "
                            + msg.target.getClass().getName() + " "
                            + msg.callback + " what=" + msg.what);
                }

                msg.recycle();
            }
        }
    }</span>

/**
     *  Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static final void loop() {
        Looper me = myLooper();
        MessageQueue queue = me.mQueue;

        // Make sure the identity of this thread is that of the local process,
        // and keep track of what that identity token actually is.
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();

        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
                        );
                /*下面的函数将Looper类和Handler类联系起来*/
                msg.target.dispatchMessage(msg);
                if (me.mLogging!= null) me.mLogging.println(
                        "<<<<< Finished to    " + msg.target + " "
                        + msg.callback);

                // Make sure that during the course of dispatching the
                // identity of the thread wasn't corrupted.
                final long newIdent = Binder.clearCallingIdentity();
                if (ident != newIdent) {
                    Log.wtf("Looper", "Thread identity changed from 0x"
                            + Long.toHexString(ident) + " to 0x"
                            + Long.toHexString(newIdent) + " while dispatching to "
                            + msg.target.getClass().getName() + " "
                            + msg.callback + " what=" + msg.what);
                }

                msg.recycle();
            }
        }
    }

        可以看到当前线程不断的从消息队列中取出消息，如果消息的target成员变量为null，就表示要退出消息循环了，否则的话就要调用这个target对象的dispatchMessage成员函数来处理这个消息，这个target对象的类型为Handler。

           

                 其实这个Looper和Handler类中消息传递的机制还是很复杂到，用到了Linux中Pipe的相关知识，我太菜还不能做出更深的分析，以后有时间会好好学习下，在完善下这部份的知识。

          好了，这篇blog就说到这里吧。下篇blog我会试着分析下，Handler和Looper的同步问题。届时会顺便学习一下HandlerThread类。

              忘了说，如果哪位大牛发现我这篇blog有什么错误，敬请指正，免得误导了跟我一样的菜鸟

Android异步处理三：Handler+Looper+MessageQueue深入详解 - lzc的专栏 - 博客频道 - CSDN.NET

在《Android异步处理一：使用Thread+Handler实现非UI线程更新UI界面》中，我们讲到使用Thread+Handler的方式来实现界面的更新，其实是在非UI线程发送消息到UI线程，通知UI线程进行界面更新，这一篇我们将深入学习Android线程间通讯的实现原理。

概述：Android使用消息机制实现线程间的通信，线程通过Looper建立自己的消息循环，MessageQueue是FIFO的消息队列，Looper负责从MessageQueue中取出消息，并且分发到消息指定目标Handler对象。Handler对象绑定到线程的局部变量Looper，封装了发送消息和处理消息的接口。

例子：在介绍原理之前，我们先介绍Android线程通讯的一个例子，这个例子实现点击按钮之后从主线程发送消息"hello"到另外一个名为” CustomThread”的线程。

代码下载

LooperThreadActivity.java

package com.zhuozhuo;

import android.app.Activity;
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;

public class LooperThreadActivity extends Activity{
    /** Called when the activity is first created. */

    private final int MSG_HELLO = 0;
    private Handler mHandler;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        new CustomThread().start();//新建并启动CustomThread实例

        findViewById(R.id.send_btn).setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {//点击界面时发送消息
                String str = "hello";
                Log.d("Test", "MainThread is ready to send msg:" + str);
                mHandler.obtainMessage(MSG_HELLO, str).sendToTarget();//发送消息到CustomThread实例

            }
        });

    }





    class CustomThread extends Thread {
        @Override
        public void run() {
            //建立消息循环的步骤
            Looper.prepare();//1、初始化Looper
            mHandler = new Handler(){//2、绑定handler到CustomThread实例的Looper对象
                public void handleMessage (Message msg) {//3、定义处理消息的方法
                    switch(msg.what) {
                    case MSG_HELLO:
                        Log.d("Test", "CustomThread receive msg:" + (String) msg.obj);
                    }
                }
            };
            Looper.loop();//4、启动消息循环
        }
    }
}

package com.zhuozhuo;

import android.app.Activity;
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;

public class LooperThreadActivity extends Activity{
    /** Called when the activity is first created. */
	
	private final int MSG_HELLO = 0;
    private Handler mHandler;
	
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        new CustomThread().start();//新建并启动CustomThread实例
        
        findViewById(R.id.send_btn).setOnClickListener(new OnClickListener() {
			
			@Override
			public void onClick(View v) {//点击界面时发送消息
				String str = "hello";
		        Log.d("Test", "MainThread is ready to send msg:" + str);
				mHandler.obtainMessage(MSG_HELLO, str).sendToTarget();//发送消息到CustomThread实例
				
			}
		});
        
    }
    
    
    
    
    
    class CustomThread extends Thread {
    	@Override
    	public void run() {
    		//建立消息循环的步骤
    		Looper.prepare();//1、初始化Looper
    		mHandler = new Handler(){//2、绑定handler到CustomThread实例的Looper对象
    			public void handleMessage (Message msg) {//3、定义处理消息的方法
    				switch(msg.what) {
    				case MSG_HELLO:
    					Log.d("Test", "CustomThread receive msg:" + (String) msg.obj);
    				}
    			}
    		};
    		Looper.loop();//4、启动消息循环
    	}
    }
}

main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
<TextView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/hello"
    />
<Button android:text="发送消息" android:id="@+id/send_btn" android:layout_width="wrap_content" android:layout_height="wrap_content"></Button>
</LinearLayout>

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
<TextView  
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="@string/hello"
    />
<Button android:text="发送消息" android:id="@+id/send_btn" android:layout_width="wrap_content" android:layout_height="wrap_content"></Button>
</LinearLayout>

Log打印结果：

原理：

我们看到，为一个线程建立消息循环有四个步骤：

1、  初始化Looper

2、  绑定handler到CustomThread实例的Looper对象

3、  定义处理消息的方法

4、  启动消息循环

下面我们以这个例子为线索，深入Android源代码，说明Android Framework是如何建立消息循环，并对消息进行分发的。

1、  初始化Looper : Looper.prepare()

Looper.java

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

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

一个线程在调用Looper的静态方法prepare()时，这个线程会新建一个Looper对象，并放入到线程的局部变量中，而这个变量是不和其他线程共享的（关于ThreadLocal的介绍）。下面我们看看Looper()这个构造函数：

Looper.java

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

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

可以看到在Looper的构造函数中，创建了一个消息队列对象mQueue，此时，调用Looper. prepare()的线程就建立起一个消息循环的对象（此时还没开始进行消息循环）。

2、  绑定handler到CustomThread实例的Looper对象 : mHandler= new Handler()

Handler.java

final MessageQueue mQueue;
 final Looper mLooper;
public Handler() {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }

        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = null;
}

final MessageQueue mQueue;
 final Looper mLooper;
public Handler() {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }

        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = null;
}

Handler通过mLooper = Looper.myLooper();绑定到线程的局部变量Looper上去，同时Handler通过mQueue =mLooper.mQueue;获得线程的消息队列。此时，Handler就绑定到创建此Handler对象的线程的消息队列上了。

3、定义处理消息的方法：Override public void handleMessage (Message msg){}

     子类需要覆盖这个方法，实现接受到消息后的处理方法。

4、启动消息循环 ： Looper.loop()

      所有准备工作都准备好了，是时候启动消息循环了！Looper的静态方法loop()实现了消息循环。

Looper.java

public static final void loop() {
       Looper me = myLooper();
       MessageQueue queue = me.mQueue;

       // Make sure the identity of this thread is that of the local process,
       // and keep track of what that identity token actually is.
       Binder.clearCallingIdentity();
       final long ident = Binder.clearCallingIdentity();

       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);

               // Make sure that during the course of dispatching the
               // identity of the thread wasn't corrupted.
               final long newIdent = Binder.clearCallingIdentity();
               if (ident != newIdent) {
                   Log.wtf("Looper", "Thread identity changed from 0x"
                           + Long.toHexString(ident) + " to 0x"
                           + Long.toHexString(newIdent) + " while dispatching to "
                           + msg.target.getClass().getName() + " "
                           + msg.callback + " what=" + msg.what);
               }

               msg.recycle();
           }
       }
   }

 public static final void loop() {
        Looper me = myLooper();
        MessageQueue queue = me.mQueue;
        
        // Make sure the identity of this thread is that of the local process,
        // and keep track of what that identity token actually is.
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();
        
        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);
                
                // Make sure that during the course of dispatching the
                // identity of the thread wasn't corrupted.
                final long newIdent = Binder.clearCallingIdentity();
                if (ident != newIdent) {
                    Log.wtf("Looper", "Thread identity changed from 0x"
                            + Long.toHexString(ident) + " to 0x"
                            + Long.toHexString(newIdent) + " while dispatching to "
                            + msg.target.getClass().getName() + " "
                            + msg.callback + " what=" + msg.what);
                }
                
                msg.recycle();
            }
        }
    }

while(true)体现了消息循环中的“循环“，Looper会在循环体中调用queue.next()获取消息队列中需要处理的下一条消息。当msg != null且msg.target != null时，调用msg.target.dispatchMessage(msg);分发消息，当分发完成后，调用msg.recycle();回收消息。

msg.target是一个handler对象，表示需要处理这个消息的handler对象。Handler的void dispatchMessage(Message msg)方法如下：

Handler.java

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);
        }
}

可见，当msg.callback== null 并且mCallback == null时，这个例子是由handleMessage(msg);处理消息，上面我们说到子类覆盖这个方法可以实现消息的具体处理过程。

 

总结：从上面的分析过程可知，消息循环的核心是Looper，Looper持有消息队列MessageQueue对象，一个线程可以把Looper设为该线程的局部变量，这就相当于这个线程建立了一个对应的消息队列。Handler的作用就是封装发送消息和处理消息的过程，让其他线程只需要操作Handler就可以发消息给创建Handler的线程。由此可以知道，在上一篇《Android异步处理一：使用Thread+Handler实现非UI线程更新UI界面》中，UI线程在创建的时候就建立了消息循环(在ActivityThread的public static final void main(String[] args)方法中实现)，因此我们可以在其他线程给UI线程的handler发送消息，达到更新UI的目的。