Android的Handler，Looper源码剖析

之前了解android的消息处理机制，但是源码看的少，现在把Looper，Handler，Message这几个类的源码分析一哈

android的消息处理有三个核心类：Looper,Handler和Message。其实还有一个Message Queue（消息队列），但是MQ被封装到Looper里面了，我们不会直接与MQ打交道，因此我没将其作为核心类

Looper源码：

Looper的字面意思是“循环者”，它被设计用来使一个普通线程变成Looper线程。所谓Looper线程就是循环工作的线程。

使用Looper类创建Looper线程Demo：

public class LooperThread extends Thread {
@Override
public void run() {
// 将当前线程初始化为Looper线程
Looper.prepare();

// ...其他处理，如实例化handler

// 开始循环处理消息队列
Looper.loop();
}
}

1)Looper.prepare()源码

public final class Looper {
private static final String TAG = "Looper";

// sThreadLocal.get() will return null unless you've called prepare().
/*如果没有调用prepare将Looper对象设置为线程的本地变量，则sThreadLocal.get()为空*/
/*// 每个线程中的Looper对象其实是一个ThreadLocal，即线程本地存储(TLS)对象*/
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();//当前线程的本地变量
private static Looper sMainLooper;  // guarded by Looper.class

final MessageQueue mQueue;//Looper维护的消息队列MQ
final Thread mThread;//Looper关联的当前线程

private Printer mLogging;

/** 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 void prepare() {
prepare(true);
}
/* 我们调用该方法会在调用线程的TLS中创建Looper对象*/
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));//就是把Looper对象设置为当前线程的一个本地变量
}

Prepare()之后的的图:



现在你的线程中有一个Looper对象，它的内部维护了一个消息队列MQ。注意，一个Thread只能有一个Looper对象

2）Looper.loop()源码

/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*在当前线程中执行消息队列，确定调用quit()结束循环
*/
public static void loop() {
final Looper me = myLooper();//获得Looper对象
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;//获得Loop对象关联的消息队列

/*没看懂，不影响理解*/
// 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();

/*死循环处理消息队列*/
for (;;) {
Message msg = queue.next(); // might block，从消息队列中获取消息Message
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
/*日志*/
// This must be in a local variable, in case a UI event sets the logger
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}

/*这一句非常重要，将真正的处理工作交给message的target，即后面要讲的handler*/
msg.target.dispatchMessage(msg);

/*日志*/
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
/*没看懂*/
// 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(TAG, "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.recycleUnchecked();   // 回收message资源
}
}

/**
* Return the Looper object associated with the current thread.  Returns
* null if the calling thread is not associated with a Looper.
*返回与当前线程相关联的Looper对象
*/
public static Looper myLooper() {
return sThreadLocal.get();//其实就是从线程的本地变量里面取值
}

/**
* Return the {@link MessageQueue} object associated with the current
* thread.  This must be called from a thread running a Looper, or a
* NullPointerException will be thrown.
* 返回与当前线程相关联的MessageQueue对象
*/
public static MessageQueue myQueue() {
return myLooper().mQueue;
}
/*初始化Looper的两个属性，关联的线程和消息队列*/
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}

调用loop方法后，Looper线程就开始真正工作了，它不断从自己的MQ中取出队头的消息(也叫任务)执行



Looper有了基本的了解，总结几点：

1.每个线程有且最多只能有一个Looper对象，它是一个ThreadLocal就是Looper对象

2.Looper内部有一个消息队列，loop()方法调用后线程开始不断从队列中取出消息执行

3.Looper使一个线程变成Looper线程。

那么，我们如何往MQ上添加消息呢？下面有请Handler

Handler分析：

handler扮演了往MQ上添加消息和处理消息的角色（只处理由自己发出的消息），即通知MQ它要执行一个任务(sendMessage)，并在loop到自己的时候执行该任务(handleMessage)，整个过程是异步的。handler创建时会关联一个looper，默认的构造方法将关联当前线程的looper，不过这也是可以set的

为之前的LooperThread类加入Handler：

public class LooperThread extends Thread {
private Handler handler1;
private Handler handler2;

@Override
public void run() {
// 将当前线程初始化为Looper线程
Looper.prepare();

// 实例化两个handler
handler1 = new Handler();

// 开始循环处理消息队列
Looper.loop();
}
}

加入handler后的效果：



1，Handler发送消息

可以使用

post(Runnable), postAtTime(Runnable, long), postDelayed(Runnable, long), sendEmptyMessage(int), sendMessage(Message), sendMessageAtTime(Message, long)和 sendMessageDelayed(Message, long)这些方法向MQ上发送消息了。光看这些API你可能会觉得handler能发两种消息，一种是Runnable对象，一种是message对象，这是直观的理解，但其实post发出的Runnable对象最后都被封装成message对象

/**
* Causes the Runnable r to be added to the message queue.
* The runnable will be run on the thread to which this handler is
* attached.
*
* @param r The Runnable that will be executed.
*
* @return Returns true if the Runnable was successfully placed in to the
*         message queue.  Returns false on failure, usually because the
*         looper processing the message queue is exiting.
*/
/*把一个Runnable对象加入消息队列，任务将在当前Handler绑定的线程中执行，说白了就是当前线程执行任务*/
public final boolean post(Runnable r)
{
return  sendMessageDelayed(getPostMessage(r), 0);
}

/**
* Causes the Runnable r to be added to the message queue, to be run
* at a specific time given by <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* Time spent in deep sleep will add an additional delay to execution.
* The runnable will be run on the thread to which this handler is attached.
*
* @param r The Runnable that will be executed.
* @param uptimeMillis The absolute time at which the callback should run,
*         using the {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the Runnable was successfully placed in to the
*         message queue.  Returns false on failure, usually because the
*         looper processing the message queue is exiting.  Note that a
*         result of true does not mean the Runnable will be processed -- if
*         the looper is quit before the delivery time of the message
*         occurs then the message will be dropped.
*/
public final boolean postAtTime(Runnable r, long uptimeMillis)
{
return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}

/**
* Causes the Runnable r to be added to the message queue, to be run
* at a specific time given by <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* Time spent in deep sleep will add an additional delay to execution.
* The runnable will be run on the thread to which this handler is attached.
*
* @param r The Runnable that will be executed.
* @param uptimeMillis The absolute time at which the callback should run,
*         using the {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the Runnable was successfully placed in to the
*         message queue.  Returns false on failure, usually because the
*         looper processing the message queue is exiting.  Note that a
*         result of true does not mean the Runnable will be processed -- if
*         the looper is quit before the delivery time of the message
*         occurs then the message will be dropped.
*
* @see android.os.SystemClock#uptimeMillis
*/
public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)
{
return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}

/**
* Causes the Runnable r to be added to the message queue, to be run
* after the specified amount of time elapses.
* The runnable will be run on the thread to which this handler
* is attached.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* Time spent in deep sleep will add an additional delay to execution.
*
* @param r The Runnable that will be executed.
* @param delayMillis The delay (in milliseconds) until the Runnable
*        will be executed.
*
* @return Returns true if the Runnable was successfully placed in to the
*         message queue.  Returns false on failure, usually because the
*         looper processing the message queue is exiting.  Note that a
*         result of true does not mean the Runnable will be processed --
*         if the looper is quit before the delivery time of the message
*         occurs then the message will be dropped.
*/
public final boolean postDelayed(Runnable r, long delayMillis)
{
return sendMessageDelayed(getPostMessage(r), delayMillis);
}

/**
* Posts a message to an object that implements Runnable.
* Causes the Runnable r to executed on the next iteration through the
* message queue. The runnable will be run on the thread to which this
* handler is attached.
* <b>This method is only for use in very special circumstances -- it
* can easily starve the message queue, cause ordering problems, or have
* other unexpected side-effects.</b>
*
* @param r The Runnable that will be executed.
*
* @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 postAtFrontOfQueue(Runnable r)
{
return sendMessageAtFrontOfQueue(getPostMessage(r));
}

/**
* Pushes a message onto the end of the message queue after all pending messages
* before the current time. It will be received in {@link #handleMessage},
* in the thread attached to this handler.
*
* @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.
*/
/*把一个消息放入消息队列中，返回true*/
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}

/**
* 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.
*/
/*把一个只有what的消息放入到消息队列中*/
public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
}

/**
* Sends a Message containing only the what value, to be delivered
* after the specified amount of time elapses.
* @see #sendMessageDelayed(android.os.Message, long)
*
* @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 sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}

/**
* Sends a Message containing only the what value, to be delivered
* at a specific time.
* @see #sendMessageAtTime(android.os.Message, long)
*
* @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 sendEmptyMessageAtTime(int what, long uptimeMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageAtTime(msg, uptimeMillis);
}

/**
* Enqueue a message into the message queue after all pending messages
* before (current time + delayMillis). You will receive it in
* {@link #handleMessage}, in the thread attached to this handler.
*
* @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.  Note that a
*         result of true does not mean the message will be processed -- if
*         the looper is quit before the delivery time of the message
*         occurs then the message will be dropped.
*/
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

Handler处理消息：

/**
* Subclasses must implement this to receive messages.
*子类必须实现这个方法接收消息
*/
public void handleMessage(Message msg) {
}

/**
* Handle system messages here.
*处理系统的消息， 处理消息，该方法由looper调用   msg.target.dispatchMessage(msg);就是把消息交给Handler来处理
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
// 如果message设置了callback，即runnable消息，处理callback！
handleCallback(msg);
} else {
// 如果handler本身设置了callback，则执行callback
if (mCallback != null) {
/* 这种方法允许让activity等来实现Handler.Callback接口，避免了自己编写handler重写handleMessage方法*/
if (mCallback.handleMessage(msg)) {
return;
}
}
// 如果message没有callback，则调用handler的钩子方法handleMessage
handleMessage(msg);
}
}

相关理论看之前的文章/article/3698383.html