Handler源码分析，Handler,Looper,Message,MessageQueue

Android开发中我们常用到Handler,但是可能会有一些别的原因，没有去仔细分析源码，今天看了一下，在仅涉及的sdk源码中进行分析，未涉及ndk，因为源码并不是很多，所以建议大家可以自己去看一下，如有错误之处欢迎留言。。。

1.Handler常用的使用

    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();
}

Handler解析_主线程Looper对象的创建

Activity中Looper的注册

    attach()方法中会创建一个ActivityThread对象， ActivityThread对象中的main方法户通过 Looper.prepareMainLooper()这个方法创建一个主线程的Looper。

public static void main(String[] args) {
    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
    SamplingProfilerIntegration.start();

    // CloseGuard defaults to true and can be quite spammy.  We
    // disable it here, but selectively enable it later (via
    // StrictMode) on debug builds, but using DropBox, not logs.
    CloseGuard.setEnabled(false);

    Environment.initForCurrentUser();

    // Set the reporter for event logging in libcore
    EventLogger.setReporter(new EventLoggingReporter());

    // Make sure TrustedCertificateStore looks in the right place for CA certificates
    final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
    TrustedCertificateStore.setDefaultUserDirectory(configDir);

    Process.setArgV0("");

    Looper.prepareMainLooper();

    ActivityThread thread = new ActivityThread();
    thread.attach(false);

    if (sMainThreadHandler == null) {
        sMainThreadHandler = thread.getHandler();
    }

    if (false) {
        Looper.myLooper().setMessageLogging(new
                LogPrinter(Log.DEBUG, "ActivityThread"));
    }

    // End of event ActivityThreadMain.
    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
    Looper.loop();

    throw new RuntimeException("Main thread loop unexpectedly exited");
}

Handler解析_Handler重要方法解析

构造方法：主要是Looper，Callback,async的赋值

1）

public Handler() {
    this(null, false);
}

2）

public Handler(boolean async) {
    this(null, async);
}

3）

public Handler(Callback callback, boolean async) {
    //安全检查
   if (FIND_POTENTIAL_LEAKS) {
        final Classextends 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());
        }
    }
//重点 各个参数的赋值
    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 = callback;
    mAsynchronous = async;
}

4）

public Handler(Looper looper) {
    this(looper, null, false);
}

5）

public Handler(Looper looper, Callback callback) {
    this(looper, callback, false);
}

6）

public Handler(Looper looper, Callback callback, boolean async) {
    mLooper = looper;
    mQueue = looper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

Callback（重要）用于非Handler类中的消息的接收，返回true代表无需进一步处理，也就是说Handler中的handleMessage()方法不会接收到消息

用法：
public class A implements Callback{

        public boolean handleMessage(Message msg){

                                  return false；

          }

}

public interface Callback {
    public boolean handleMessage(Message msg);
}

如果Callback中的handleMessage的返回true，那么Handler中的handleMessage则不会收到消息了

obtainMessage，消息的创建的方式

1）

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

2）

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

3）

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

4）

public final Message obtainMessage(int what, int arg1, int arg2)
{
    return Message.obtain(this, what, arg1, arg2);
}

5）

public final Message obtainMessage(int what, int arg1, int arg2, Object obj)
{
    return Message.obtain(this, what, arg1, arg2, obj);
}

Message obtain()

public static Message obtain(Handler h, int what, 
        int arg1, int arg2, Object obj) {
    Message m = obtain();
    m.target = h;
    m.what = what;
    m.arg1 = arg1;
    m.arg2 = arg2;
    m.obj = obj;

    return m;
}

消息发送的方式（常用） 

sendXXX系列

1）

public final boolean sendEmptyMessage(int what)
{
    return sendEmptyMessageDelayed(what, 0);
}

2）

public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
    Message msg = Message.obtain();
    msg.what = what;
    return sendMessageDelayed(msg, delayMillis);
}

3）

public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
    Message msg = Message.obtain();
    msg.what = what;
    return sendMessageDelayed(msg, delayMillis);
}

4）对消息延时处理，系统当前时间+延迟时间（毫秒数）SystemClock.uptimeMillis() + delayMillis

public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
    if (delayMillis < 0) {
        delayMillis = 0;
    }
    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

5）

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
    MessageQueue queue = mQueue;
    if (queue == null) {
        RuntimeException e = new RuntimeException(
                this + " sendMessageAtTime() called with no mQueue");
        Log.w("Looper", e.getMessage(), e);
        return false;
    }
    return enqueueMessage(queue, msg, uptimeMillis);
}

enqueueMessage消息入队列，对message进行处理，调用MessageQueue 的enqueueMessage（）方法，入队

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this;
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
}

MessageQueue 中的enqueueMessage()方法

boolean enqueueMessage(Message msg, long when) {
    if (msg.target == null) {
        throw new IllegalArgumentException("Message must have a target.");
    }
    if (msg.isInUse()) {
        throw new IllegalStateException(msg + " This message is already in use.");
    }

    synchronized (this) {
        if (mQuitting) {
            IllegalStateException e = new IllegalStateException(
                    msg.target + " sending message to a Handler on a dead thread");
            Log.w(TAG, e.getMessage(), e);
            msg.recycle();
            return false;
        }

        msg.markInUse();
        msg.when = when;
        Message p = mMessages;
        boolean needWake;
        if (p == null || when == 0 || when < p.when) {
            // New head, wake up the event queue if blocked.
            msg.next = p;
            mMessages = msg;
            needWake = mBlocked;
        } else {
            // Inserted within the middle of the queue.  Usually we don't have to wake
            // up the event queue unless there is a barrier at the head of the queue
            // and the message is the earliest asynchronous message in the queue.
            needWake = mBlocked && p.target == null && msg.isAsynchronous();
            Message prev;
            for (;;) {
                prev = p;
                p = p.next;
                if (p == null || when < p.when) {
                    break;
                }
                if (needWake && p.isAsynchronous()) {
                    needWake = false;
                }
            }
            msg.next = p; // invariant: p == prev.next
            prev.next = msg;
        }

        // We can assume mPtr != 0 because mQuitting is false.
        if (needWake) {
            nativeWake(mPtr);
        }
    }
    return true;
}

postXXX系列  本质上还是调用的sendXXX系列中的方法

1)

public final boolean post(Runnable r)
{
   return  sendMessageDelayed(getPostMessage(r), 0);
}

2)

public final boolean postAtTime(Runnable r, long uptimeMillis)
{
    return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}

3)

public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)
{
    return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}

4)

public final boolean postDelayed(Runnable r, long delayMillis)
{
    return sendMessageDelayed(getPostMessage(r), delayMillis);
}

5)

public final boolean postAtFrontOfQueue(Runnable r)
{
    return sendMessageAtFrontOfQueue(getPostMessage(r));
}

6)

public final boolean sendMessageAtFrontOfQueue(Message msg) {
    MessageQueue queue = mQueue;
    if (queue == null) {//异常检查
        RuntimeException e = new RuntimeException(
            this + " sendMessageAtTime() called with no mQueue");
        Log.w("Looper", e.getMessage(), e);
        return false;
    }
    return enqueueMessage(queue, msg, 0);//调用 Handler的enqueueMessage（）同sendMessage
}

getPostMessage(r) 创建message并且赋值callback

private static Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r;
    return m;
}

dispatchMessage（重要），消息分配的关键方法，在Lopper中的loop方法中被调用

/**
 * Handle system messages here.
 */
public void dispatchMessage(Message msg) {
    if (msg.callback != null) {//msg.callback 是一个线程，如果不为空就运行
        handleCallback(msg);
    } else {
        if (mCallback != null) { //判断Callback是否为空，非空就调用Callback的handleMessage方法处理，
                                 //mCallback.handleMessage(msg)返回true的话Handler自带的handleMessage
                                 //就不会收到消息，上面讲过
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

private static void handleCallback(Message message) {
    message.callback.run();
}

移除方法： 本质上调用的Message.Queue中的removeXXX()方法
removeCallbacks

/**
 * Remove any pending posts of Runnable r that are in the message queue.
 */
public final void removeCallbacks(Runnable r)
{
    mQueue.removeMessages(this, r, null);
}

/**
 * Remove any pending posts of Runnable r with Object
 * token that are in the message queue.  If token is null,
 * all callbacks will be removed.
 */
public final void removeCallbacks(Runnable r, Object token)
{
    mQueue.removeMessages(this, r, token);
}

removeMessages

/**
 * 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);
}

/**
 * Remove any pending posts of messages with code 'what' and whose obj is
 * 'object' that are in the message queue.  If object is null,
 * all messages will be removed.
 */
public final void removeMessages(int what, Object object) {
    mQueue.removeMessages(this, what, object);
}

Handler解析_Looper重要方法解析

官方用法

*  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();
*      }

Looper.prepare();主要的作用通过是ThreadLocal这个类进行判断，创建Looper

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));
}

Looper中创建MessageQueue

private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed);
    mThread = Thread.currentThread();
}

关键步骤Looper的轮询，Loop.loop(); 

/**
 * Run the message queue in this thread. Be sure to call
 * {@link #quit()} to end the loop.
 */
public static void loop() {
    final Looper me = myLooper();
    if (me == null) {
        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
    final 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();

    for (;;) {
        Message msg = queue.next(); // might block
        if (msg == null) {
            // No message indicates that the message queue is quitting.
            return;
        }

        // This must be in a local variable, in case a UI event sets the logger
        final Printer logging = me.mLogging;
        if (logging != null) {
            logging.println(">>>>> Dispatching to " + msg.target + " " +
                    msg.callback + ": " + msg.what);
        }

        final long traceTag = me.mTraceTag;
        if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
            Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
        }
        try {
            msg.target.dispatchMessage(msg);
        } finally {
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }

        if (logging != null) {
            logging.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(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();
    }
}

Looper的退出

/**
 * Quits the looper.
 * 

 * Causes the {@link #loop} method to terminate without processing any
 * more messages in the message queue.
 * 

 * Any attempt to post messages to the queue after the looper is asked to quit will fail.
 * For example, the {@link Handler#sendMessage(Message)} method will return false.
 * 

 * Using this method may be unsafe because some messages may not be delivered
 * before the looper terminates.  Consider using {@link #quitSafely} instead to ensure
 * that all pending work is completed in an orderly manner.
 * 

 *
 * @see #quitSafely
 */
public void quit() {
    mQueue.quit(false);
}

/**
 * Quits the looper safely.
 * 

 * Causes the {@link #loop} method to terminate as soon as all remaining messages
 * in the message queue that are already due to be delivered have been handled.
 * However pending delayed messages with due times in the future will not be
 * delivered before the loop terminates.
 * 

 * Any attempt to post messages to the queue after the looper is asked to quit will fail.
 * For example, the {@link Handler#sendMessage(Message)} method will return false.
 * 

 */
public void quitSafely() {
    mQueue.quit(true);
}

Handler解析_Message重要方法解析

Message final修饰 Message的链表结构，并非是通过类似 LinkedList形成的数据结构，而是同内部的next变量作为指引下一个Message的指针，来获取下一个Message消息

public final class Message implements Parcelable

创建方式：

1）通过handler的obtainMessage()方法创建，本质上调用的Message的obtainMessage()方法

2） 通过Message的obtainMessage()方法

构造方法 （空实现）

/** Constructor (but the preferred way to get a Message is to call {@link #obtain() Message.obtain()}).
*/
public Message() {
}

常用的变量  what  arg1 arg2  obj  用于判断区分message以及携带数据

/**
 * User-defined message code so that the recipient can identify 
 * what this message is about. Each {@link Handler} has its own name-space
 * for message codes, so you do not need to worry about yours conflicting
 * with other handlers.
 */
public int what;

/**
 * arg1 and arg2 are lower-cost alternatives to using
 * {@link #setData(Bundle) setData()} if you only need to store a
 * few integer values.
 */
public int arg1; 

/**
 * arg1 and arg2 are lower-cost alternatives to using
 * {@link #setData(Bundle) setData()} if you only need to store a
 * few integer values.
 */
public int arg2;

/**
 * An arbitrary object to send to the recipient.  When using
 * {@link Messenger} to send the message across processes this can only
 * be non-null if it contains a Parcelable of a framework class (not one
 * implemented by the application).   For other data transfer use
 * {@link #setData}.
 * 
 * 
Note that Parcelable objects here are not supported prior to
 * the {@link android.os.Build.VERSION_CODES#FROYO} release.
 */
public Object obj;

target 用于判断handler的来源，通过该值调用dispatchMessage()方法分配数据   callback       

// sometimes we store linked lists of these things

    /*package*/ Message next;

/*package*/ Handler target;

/*package*/ Runnable callback;

// sometimes we store linked lists of these things
/*package*/ Message next;

Handler解析_MessageQueue重要方法解析

MessageQueue与其说是消息队列，更不如说会用于操作message的一个类

构造方法：quitAllowed(消息是否允许退出)

MessageQueue(boolean quitAllowed) {
    mQuitAllowed = quitAllowed;
    mPtr = nativeInit();
}

重要方法：该类中有多个native方法

enqueueMessage消息的入队列，通过Handler中的sendMessage()方法调用

boolean enqueueMessage(Message msg, long when) {
    if (msg.target == null) {
        throw new IllegalArgumentException("Message must have a target.");
    }
    if (msg.isInUse()) {
        throw new IllegalStateException(msg + " This message is already in use.");
    }

    synchronized (this) {
        if (mQuitting) {
            IllegalStateException e = new IllegalStateException(
                    msg.target + " sending message to a Handler on a dead thread");
            Log.w(TAG, e.getMessage(), e);
            msg.recycle();
            return false;
        }

        msg.markInUse();
        msg.when = when;
        Message p = mMessages;
        boolean needWake;
        if (p == null || when == 0 || when < p.when) {
            // New head, wake up the event queue if blocked.
            msg.next = p;
            mMessages = msg;
            needWake = mBlocked;
        } else {
            // Inserted within the middle of the queue.  Usually we don't have to wake
            // up the event queue unless there is a barrier at the head of the queue
            // and the message is the earliest asynchronous message in the queue.
            needWake = mBlocked && p.target == null && msg.isAsynchronous();
            Message prev;
            for (;;) {
                prev = p;
                p = p.next;
                if (p == null || when < p.when) {
                    break;
                }
                if (needWake && p.isAsynchronous()) {
                    needWake = false;
                }
            }
            msg.next = p; // invariant: p == prev.next
            prev.next = msg;//新消息的赋值，并且唤醒消息队列，进行消息的提取
        }

        // We can assume mPtr != 0 because mQuitting is false.
        if (needWake) {
            nativeWake(mPtr);
        }
    }
    return true;
}

next()方法 Loop.loop()方法会调用

public static void loop() {

for (;;) {
...

Message msg = queue.next();
...

} }

Message next() {
    // Return here if the message loop has already quit and been disposed.
    // This can happen if the application tries to restart a looper after quit
    // which is not supported.
    final long ptr = mPtr;
    if (ptr == 0) {
        return null;
    }

    int pendingIdleHandlerCount = -1; // -1 only during first iteration
    int nextPollTimeoutMillis = 0;
    for (;;) {
        if (nextPollTimeoutMillis != 0) {
            Binder.flushPendingCommands();
        }

        nativePollOnce(ptr, nextPollTimeoutMillis);

        synchronized (this) {
            // Try to retrieve the next message.  Return if found.
            final long now = SystemClock.uptimeMillis();
            Message prevMsg = null;
            Message msg = mMessages;
            if (msg != null && msg.target == null) {
                // Stalled by a barrier.  Find the next asynchronous message in the queue.
                do {
                    prevMsg = msg;
                    msg = msg.next;
                } while (msg != null && !msg.isAsynchronous());
            }
            if (msg != null) {
                if (now < msg.when) {
                    // Next message is not ready.  Set a timeout to wake up when it is ready.
                    nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                } else {
                    // Got a message.
                    mBlocked = false;
                    if (prevMsg != null) {
                        prevMsg.next = msg.next;
                    } else {
                        mMessages = msg.next;
                    }//对消息判断如果是消息还没有发生那么就放在消息队列的后面，如果已经超时了，就提前放在消息的队
                       //列的前面
                    msg.next = null;
                    if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                    msg.markInUse();
                    return msg;
                }
            } else {
                // No more messages.
                nextPollTimeoutMillis = -1;
            }

            // Process the quit message now that all pending messages have been handled.
            if (mQuitting) {
                dispose();
                return null;
            }

            // If first time idle, then get the number of idlers to run.
            // Idle handles only run if the queue is empty or if the first message
            // in the queue (possibly a barrier) is due to be handled in the future.
            if (pendingIdleHandlerCount < 0
                    && (mMessages == null || now < mMessages.when)) {
                pendingIdleHandlerCount = mIdleHandlers.size();
            }
            if (pendingIdleHandlerCount <= 0) {
                // No idle handlers to run.  Loop and wait some more.
                mBlocked = true;
                continue;
            }

            if (mPendingIdleHandlers == null) {
                mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
            }
            mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
        }

        // Run the idle handlers.
        // We only ever reach this code block during the first iteration.
        for (int i = 0; i < pendingIdleHandlerCount; i++) {
            final IdleHandler idler = mPendingIdleHandlers[i];
            mPendingIdleHandlers[i] = null; // release the reference to the handler

            boolean keep = false;
            try {
                keep = idler.queueIdle();
            } catch (Throwable t) {
                Log.wtf(TAG, "IdleHandler threw exception", t);
            }

            if (!keep) {
                synchronized (this) {
                    mIdleHandlers.remove(idler);
                }
            }
        }

        // Reset the idle handler count to 0 so we do not run them again.
        pendingIdleHandlerCount = 0;

        // While calling an idle handler, a new message could have been delivered
        // so go back and look again for a pending message without waiting.
        nextPollTimeoutMillis = 0;
    }
}

先暂时这些吧