Android 源碼解析- Handler 實現機制

源碼是Android API 25

Handler 、 Looper 、Message 這三者到底有什麼樣的關係？什麼叫異步消息處理線程呢？本文將在源碼層面進行分析。

Android UI是線程不安全的，如果在子線程中嘗試進行UI操作，程序就有可能出現ANR。解決的方法是創建一個Message對象，然後通過Handler發送出去，之後在Handler的handleMessage()方法中獲得剛纔發送的Message對象，然後在這裏進行UI操作就不會再出現崩潰了。

Android應用程序是通過消息來驅動的，系統爲每一個應用程序維護一個消息隊例，應用程序的主線程不斷地從這個消息隊例中獲取消息，然後對這些消息進行處理，這樣就實現了通過消息來驅動應用程序的執行。這樣做的好處就是消息的發送方只要把消息發送到應用程序的消息隊列中去就行了，它可以馬上返回去處理別的事情，而不需要等待消息的接收方去處理完這個消息才返回，這樣就可以提高系統的併發性。實質上，這就是一種異步處理機制。Android應用程序的消息處理機制也是由消息循環、消息發送和消息處理這三個部分組成的，接下來，我們就詳細解析這三個過程。

1、 消息循環

應用程序的主線程是圍繞消息隊列進入一個無線循環的，如果消息隊列中有消息，主線程會把它取出來並交給相應的Handler 進行處理， 在消息隊列沒有消息的情況下，主線程會進入空想等待狀態， 直到下一個消息的到來。Android 應用程序中的的消息循環是通過Looper 類來實現的，Looper.java 位於 frameworks/base/core/java/android/os/Looper.java 中， 接下來我們分析下Android 應用程序是如何進入消息循環的。我們都知道，應用程序啓動時會在進程中加載ActiityThread類，從類的main 函數開始執行，下面我們來解析ActivityThread 類， 該類位於frameworks/base/core/java/android/app 目錄下。

ActivityThread.java main 方法的代碼如下：

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("<pre-initialized>");

    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");
}

該函數首先創建了 一個 ActivityThread 實例， 然後通過Looper 類使得應用進入消息循環中，如果 Looper 中出錯，就會拋出 “Main thread loop unexpectedly exited” ，下面我麼來分析Looper 是如何讓應用進入消息循環的， Looper 類位於frameworks/base/core/java/android/os/Looper.java 文件中。

public final class Looper {
    /*
     * API Implementation Note:
     *
     * This class contains the code required to set up and manage an event loop
     * based on MessageQueue.  APIs that affect the state of the queue should be
     * defined on MessageQueue or Handler rather than on Looper itself.  For example,
     * idle handlers and sync barriers are defined on the queue whereas preparing the
     * thread, looping, and quitting are defined on the looper.
     */

    private static final String TAG = "Looper";

    // sThreadLocal.get() will return null unless you've called prepare().
    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
    private static Looper sMainLooper;  // guarded by Looper.class

    final MessageQueue mQueue;
    final Thread mThread;

    private Printer mLogging;
    private long mTraceTag;

     /** 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);
    }

    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));
    }

    /**
     * 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.  See also: {@link #prepare()}
     */
    public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

    /**
     * Returns the application's main looper, which lives in the main thread of the application.
     */
    public static Looper getMainLooper() {
        synchronized (Looper.class) {
            return sMainLooper;
        }
    }

    /**
     * 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();
        }
    }

    /**
     * Return the Looper object associated with the current thread.  Returns
     * null if the calling thread is not associated with a Looper.
     */
    public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

    /**
     * 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.
     */
    public static @NonNull MessageQueue myQueue() {
        return myLooper().mQueue;
    }

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

    /**
     * Returns true if the current thread is this looper's thread.
     */
    public boolean isCurrentThread() {
        return Thread.currentThread() == mThread;
    }

    /**
     * Control logging of messages as they are processed by this Looper.  If
     * enabled, a log message will be written to <var>printer</var>
     * at the beginning and ending of each message dispatch, identifying the
     * target Handler and message contents.
     *
     * @param printer A Printer object that will receive log messages, or
     * null to disable message logging.
     */
    public void setMessageLogging(@Nullable Printer printer) {
        mLogging = printer;
    }

    /** {@hide} */
    public void setTraceTag(long traceTag) {
        mTraceTag = traceTag;
    }

    /**
     * Quits the looper.
     * <p>
     * Causes the {@link #loop} method to terminate without processing any
     * more messages in the message queue.
     * </p><p>
     * 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.
     * </p><p class="note">
     * 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.
     * </p>
     *
     * @see #quitSafely
     */
    public void quit() {
        mQueue.quit(false);
    }

    /**
     * Quits the looper safely.
     * <p>
     * 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.
     * </p><p>
     * 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.
     * </p>
     */
    public void quitSafely() {
        mQueue.quit(true);
    }

    /**
     * Gets the Thread associated with this Looper.
     *
     * @return The looper's thread.
     */
    public @NonNull Thread getThread() {
        return mThread;
    }

    /**
     * Gets this looper's message queue.
     *
     * @return The looper's message queue.
     */
    public @NonNull MessageQueue getQueue() {
        return mQueue;
    }

    /**
     * Dumps the state of the looper for debugging purposes.
     *
     * @param pw A printer to receive the contents of the dump.
     * @param prefix A prefix to prepend to each line which is printed.
     */
    public void dump(@NonNull Printer pw, @NonNull String prefix) {
        pw.println(prefix + toString());
        mQueue.dump(pw, prefix + "  ");
    }

    @Override
    public String toString() {
        return "Looper (" + mThread.getName() + ", tid " + mThread.getId()
                + ") {" + Integer.toHexString(System.identityHashCode(this)) + "}";
    }
}

首先我們分析 prepareMainLooper 方法，該方法主要是在線程中創建一個給sMainLooper 初始化，Looper 類中有一個類型爲ThreadLoacal 的成員變量 sThreadLocal ， 保證每一個線程中有一個獨立的Looper對象，線程中創建Looper 是通過prepare()方法完成的。 Looper 中有一個類型爲MessageQueue的成員變量mQueue， 消息就是存在這個變量中的， MessageQueue 是實現消息循環的重要部分， 下面我們來解析MessageQueue 的源碼， 源碼位於 rameworks/base/core/java/android/os/MessageQueue.java 文件中。

......
private native static long nativeInit();
.......
MessageQueue(boolean quitAllowed) {
    mQuitAllowed = quitAllowed;
    mPtr = nativeInit();
}

構造方法是本地實現的 ， JNI 層主要實現的功能是創建一個消息隊列NativeMessageQueue ， 並且在JNI 層創建一個Looper 對象，把JNI 層的NativeMessageQueue 消息對象保存到Java 層中創建的MessageQueue對象的mPtr成員變量中，JNI 層Looper 的部分代碼如下

int wakeFds[2];  
int result = pipe(wakeFds);  
......  

mWakeReadPipeFd = wakeFds[0];  
mWakeWritePipeFd = wakeFds[1];  

主要是創建了一個管道， 該部分與應用程序主線程在消息隊列中沒有消息時要進入等待狀態以及當消息隊列有消息時要把應用程序主線程喚醒相關， 上面的部分主要做了如下三件事，

第一 、在Java層創建一個Looper對象，它的內部有一個消息隊列MessageQueue對象mQueue
第二、在JNI層創建一個NativeMessageQueue對象，這個NativeMessageQueue對象保存在Java層的消息隊列對象mQueue的成員變量mPtr中；
第三、在C++層創建一個Looper對象，保存在JNI層的NativeMessageQueue對象的成員變量mLooper中，這個對象的作用是，當Java層的消息隊列中沒有消息時，就使Android應用程序主線程進入等待狀態，而當Java層的消息隊列中來了新的消息後，就喚醒Android應用程序的主線程來處理這個消息。上面的工作準備好之後，ActivitThread 函數中的main 方法中調用Looper 中的loop() 方法 代碼如下：

/**
 * 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();
    }
}

通過該方法就進入到消息循環中了，不斷的讀取mQueue 中的消息msg ， 如果msg 爲null for 循環結束，如果不爲空調用，msg.target.dispatchMessage(msg) 來處理消息。該函數最關鍵的部分是Message msg = queue.next(); // might block ，即MessageQueue.next函數，方法的代碼如下：

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;
                    }
                    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;
    }
}

有兩種情況會使線程進入等待狀態，一種是消息隊列中沒有消息，一種是消息隊列中有消息但是沒有到指定的執行時間， 執行nativePollOnce(ptr, nextPollTimeoutMillis); 看看當前消息隊列中有沒有消息。該方法中有兩個參數，一個是ptr，指向的是JNI 層創建的NativeMessageQueue， nextPollTimeoutMillis表示等待的時間， 開始的時候傳入的是0 表示不等待。該方法返回後，就去檢查消息隊列中有無消息，該函數來檢測消息隊列中有無消息， 關鍵代碼如下：

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;
            }
            msg.next = null;
            if (DEBUG) Log.v(TAG, "Returning message: " + msg);
            msg.markInUse();
            return msg;
        }
    } else {
        // No more messages.
        nextPollTimeoutMillis = -1;
    }

如果消息隊列中有消息，並且大於消息中的執行時間，直接返回msg 處理，否則的話就要等待到消息的執行時間：等待是的時長爲nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);如果無消息 nextPollTimeoutMillis = -1 就會無限的等待。。。
上面部分已經將消息的循環部分講完了 ， 下面要分析的是消息的發送。

2、消息發送

消息發送的代碼如下：

Handler callBackHandler = new Handler(new Handler.Callback() {
    @Override
    public boolean handleMessage(Message msg) {
        return false;
    }
});

Message msg = Message.obtain();
msg.what = 1;
callBackHandler.sendMessage(msg);

callBackHandler.sendMessage函數把這個消息對象msg加入到應用程序的消息隊列中去，然後通 handleMessage 方法來處理消息
下面我們來分析下Handler 的構造方法

public Handler(Callback callback, boolean async) {

    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;
}

mLooper 與Queue 是構造方法中最重要的成名變量， myLooper 是一個靜態方法，通過該方法可以獲取到一個Looper 對象，如果獲取到對象爲空，說明還沒有調用Loopepr.prepare() 方法。通過Looper 對象可以獲取到mQeue 對象。進而可以通過Looper.mQueue來訪問應用程序的消息隊列。

下面我我們來分析如何將消息發送到消息隊列的， 首先分析的是handler 的sendMessage(Message msg) 方法

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

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);
}

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

msg.target = this , 這麼目標由誰來處理，這裏賦值爲this， 表示這個消息最終由這個Handler 來處理，消息發送最關鍵的一個函數是 queue.enqueueMessage(msg, uptimeMillis); 源代碼如下

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;
}

將消息加入到消息隊列時，有兩種情況， 第一種是消息隊列爲空，只需要把消息加入到消息隊列頭即可，

   if (p == null || when == 0 || when < p.when) {
            // New head, wake up the event queue if blocked.
            msg.next = p;
            mMessages = msg;
            needWake = mBlocked;
        } 

第二種是消息隊列中有消息，並且我們發送的消息指定了消息的發送時間，這個時候需要需要根據發送的時間進行排序。核心代碼如下：

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;

消息按照排序時間排好之後就會調用 nativeWake(mPtr); 該函數的主要操作是喚醒主線程來處理消息。這個時候就會返回到Java層中的MessageQueue.next函數，主線程開始處理消息。

到這裏消息的發送已經分析完了。下面分析消息的處理。

3、消息處理

消息的處理是在Looper 的loop() 中開始的代碼如下：

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;
        }

        try {
            msg.target.dispatchMessage(msg);
        } finally {
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }


        msg.recycleUnchecked();
    }
}

msg 不爲null 時，就會調用它的target成員變量的dispatchMessage函數來處理這個消息，這個消息對象msg的成員變量target是在發送消息的時候設置好的，一般就通過哪個Handler來發送消息，就通過哪個Handler來處理消息。

我們發消息的時候可以是一下幾種方法

  Handler callBackHandler = new Handler(new Handler.Callback() {
        @Override
        public boolean handleMessage(Message msg) {  // 使用Handler 的Callback 方法
            return false;
        }
    });


    Handler overrideHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {  //重寫handleMessage方法
            super.handleMessage(msg);
        }
    };


    Message message = Message.obtain(callBackHandler, new Runnable() {
        @Override
        public void run() {  // 傳入Runnable 參數

        }
    });
}

Handler 中的 dispatchMessage 方法如下：

public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        handleCallback(msg);  //優先是Message 中的Callback
    } else {
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) { //其次是handler 中的Callback
                return;
            }
        }
        handleMessage(msg); //最後是重寫的方法
    }
}

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

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

這裏的消息對象msg的callback成員變量和Handler類的mCallBack成員變量一般都爲null，於是，就會調用Handler類的handleMessage函數來處理這個消息

分析到這裏已經把Handler 分析完了 。