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

2013-01-17 15:47 591 查看

Android异步处理一：使用Thread+Handler实现非UI线程更新UI界面

Android异步处理二：使用AsyncTask异步更新UI界面

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

Android异步处理四：AsyncTask的实现原理

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

[java]
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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

[html]
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<?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

[java]
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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

[java]
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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

[java]
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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

[java]
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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

[java]
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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。

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