Frameworks层handler分析(二)

3． Handler源码分析
为了能够进一步了解Handler的消息控制处理细节，需要研究相关源码（frameworks/base/core/java/android/os/）。
首先，在应用中使用的post和sendMessage方法最终都是调用了Handler.java中sendMessageAtTime方法。

public boolean sendMessageAtTime(Message msg, long uptimeMillis)    {        boolean sent = false;        MessageQueue queue = mQueue;        if (queue != null) {  //将handler实例与Message对象绑定， 然后将消息加入队列之中            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;}

最重要的部分用注释标出，它的作用是将handler实例与Message对象绑定， 然后将消息加入队列之中（MessageQueue.java的enqueueMessage方法）。

final boolean enqueueMessage(Message msg, long when) {        if (msg.isInUse()) {            throw new AndroidRuntimeException(msg                    + " This message is already in use.");        }        if (msg.target == null && !mQuitAllowed) {            throw new RuntimeException("Main thread not allowed to quit");        }        final boolean needWake;        synchronized (this) {            if (mQuiting) {                RuntimeException e = new RuntimeException(                    msg.target + " sending message to a Handler on a dead thread");                Log.w("MessageQueue", e.getMessage(), e);                return false;            } else if (msg.target == null) {                mQuiting = true;            }            msg.when = when;            //Log.d("MessageQueue", "Enqueing: " + msg);            Message p = mMessages;            if (p == null || when == 0 || when < p.when) {                msg.next = p;                mMessages = msg;                needWake = mBlocked; // new head, might need to wake up            } else {                Message prev = null;                while (p != null && p.when <= when) {                    prev = p;                    p = p.next;                }                msg.next = prev.next;                prev.next = msg;                needWake = false; // still waiting on head, no need to wake up            }        }        if (needWake) {            nativeWake(mPtr);        }        return true;    }

Looper运行使消息到达消息队列头部。（Looper.java的loop方法）

public static void loop() {        Looper me = myLooper();        if (me == null) {            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");        }        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 (msg != null) {                if (msg.target == null) {                    // No target is a magic identifier for the quit message.                    return;                }                long wallStart = 0;                long threadStart = 0;                // 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);                    wallStart = SystemClock.currentTimeMicro();                    threadStart = SystemClock.currentThreadTimeMicro();                }        //调用了消息对应handler的dispatchMessage方法                msg.target.dispatchMessage(msg);                if (logging != null) {                    long wallTime = SystemClock.currentTimeMicro() - wallStart;                    long threadTime = SystemClock.currentThreadTimeMicro() - threadStart;                    logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);                    if (logging instanceof Profiler) {                        ((Profiler) logging).profile(msg, wallStart, wallTime,                                threadStart, threadTime);                    }                }                // 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.recycle();            }        }    }

注释部分是消息处理的过程，本质是调用了消息对应handler的dispatchMessage方法。

public void dispatchMessage(Message msg) {        if (msg.callback != null) {            handleCallback(msg);        } else {            if (mCallback != null) {                if (mCallback.handleMessage(msg)) {                    return;                }            }            handleMessage(msg);        }}

这段代码的说明引用邓凡平的《深入理解Android》中的分析。
如果msg本身设置了callback，则直接交给这个callback处理了；
如果该handler的callback有的话，则交给这个callback处理了---相当于集中处理；     
否则交给派生处理,基类默认处理是什么都不干。  

分析到这里我们已经明白了handler的消息设定和处理的流程了，但还是有一个问题困扰我们Looper实例是怎么创建的呢。
首先根据google的API 我们知道每一个线程都可以创建一个looper实例。下面附上google的API中的样例代码。

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();      }  }

那应用主线程运行的Looper实例是什么时间创建的呢？答案就是在Activity创建时由ActivityThread对象创建。

public static void main(String[] args) {        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);        Process.setArgV0("<pre-initialized>");    //创建主looper对象        Looper.prepareMainLooper();    //创建Handler对象并将其和主looper对象绑定        if (sMainThreadHandler == null) {            sMainThreadHandler = new Handler();        }        ActivityThread thread = new ActivityThread();        thread.attach(false);        if (false) {            Looper.myLooper().setMessageLogging(new                    LogPrinter(Log.DEBUG, "ActivityThread"));        }        Looper.loop();        throw new RuntimeException("Main thread loop unexpectedly exited");    }}

注释标示的代码就是创建主looper对象并将Handler和主looper对象绑定。调用的Looper.java中的方法和Handler.java的构造方法罗列如下。
Looper.java

public static void prepareMainLooper() {        prepare();        setMainLooper(myLooper());        myLooper().mQueue.mQuitAllowed = false;    }  public static void prepare() {        if (sThreadLocal.get() != null) {            throw new RuntimeException("Only one Looper may be created per thread");        }        sThreadLocal.set(new Looper());    } private synchronized static void setMainLooper(Looper looper) {        mMainLooper = looper;    }

Handler.java
 

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());            }        }  //关联looper对象        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;    }