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; }