Handler源码分析

每个Handler都关联了一个线程，每个线程都维护了一个消息队列MessageQueue,这样Handler实际上就关联了一个消息队列。
可以通过handler将message和runnable对象发送到该handler所关联线程的messagequeue中，然后该消息队列一直在循环拿出一个message，并对其处理。

当创建handler时，该handler就绑定了当前创建handler的线程。

一.在子线程使用Handler方式为：

Runnable runnable  = new Runnable(){

public void run(){

Looper.prepare();

Handler handler = new Handler();

Looper.loop();

}

};

我们来分析一下，为什么要这么使用：

1. Looper.prepare();我们看看源码是怎么实现的;

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));    }

它实例化了一个Looper，并且和当前线程绑定，也就是实例化一个Looper绑定到当前子线程。

2.Handler handler = new Handler();

public Handler(Callback callback, boolean async) {        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());            }        }        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;    }

这一段是实例化Handler。

3. Looper.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            Printer logging = me.mLogging;            if (logging != null) {                logging.println(">>>>> Dispatching to " + msg.target + " " +                        msg.callback + ": " + msg.what);            }            msg.target.dispatchMessage(msg);            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.recycle();        }    }

我们可以发现for(;;)的死循环，queue.next()从MessageQueue队列中取消息，

msg.target.dispatchMessage(msg); 其中msg.target一般是handler（下文会提到），然后由handler来分发消息。

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

这里我看看到了熟悉的代码handleMessage()；先不看msg.callback和mCallback

当我们实例化handler，重写handleMessage（），会在handlerMessage里面处理自己的逻辑（比如更新Ui等）

二：下面我们看看在UI线程使用handler的方式

Handler handler = new Handler();

handler.sendMessage()；

这里有一个问题：为什么UI线程没有调用 Looper.prepare()和Looper.loop()；

带着这个问题，我们来看看UI线程的源码；

下面是ActivityThread的主函数入口：

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);        Environment.initForCurrentUser();        // Set the reporter for event logging in libcore        EventLogger.setReporter(new EventLoggingReporter());        Security.addProvider(new AndroidKeyStoreProvider());        Process.setArgV0("<pre-initialized>");        Looper.prepareMainLooper();        ActivityThread thread = new ActivityThread();        thread.attach(false);        if (sMainThreadHandler == null) {            sMainThreadHandler = thread.getHandler();        }        AsyncTask.init();        if (false) {            Looper.myLooper().setMessageLogging(new                    LogPrinter(Log.DEBUG, "ActivityThread"));        }        Looper.loop();        throw new RuntimeException("Main thread loop unexpectedly exited");    }

我们可以看到主线程已经创建了Looper，因此，在主线程使用handler时不需要Looper.prepare();

三 new Handler(Looper.getMainLooper()) 这种情况是，使用主线程的Looper

比如在子线程里面初始化handler，需要更新ui，那么就可以这么用。