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。