Handler源码浅析(二)

Looper 大管家

• Handler（消息发送者和接受者）
• Looper（消息循环）
• Message（消息）
• MessageQueue（消息队列）

先看Looper源码里给的Demo

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();
    }
}

大概意思:

• Looper这个类是为线程消息循环工作的，
• 线程默认是没有Looper的 需要用prepare方法创建，然后调用loop方法进行工作直到线程停止。
• 大多数与Handler进行交互

• Class used to run a message loop for a thread. Threads by default do

• not have a message loop associated with them; to create one, call
• {@link #prepare} in the thread that is to run the loop, and then
• {@link #loop} to have it process messages until the loop is stopped.
• <p>Most interaction with a message loop is through the
• {@link Handler} class.

从Looper的创建可以看出:

• Looper是一个单例， 每个线程只能创建一个Looper
• 初始化Looper 的时候初始化一个MessageQueue 并且与当前线程绑定

   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));
    }

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

当你初始化一个Looper它并不能工作，你需要调用Looper.loop();
先获取自己的looper和消息队列
注意里面的for(;;){...

//在死循环里不断重复接收消息队列里的消息
** Message msg = queue.next(); //可能阻塞 **

还记得Handler里的 ** msg.target = this; **
在那里我们的target 得到发送Message的Handler
现在我们在调用dispatchMessage(msg);
** msg.target.dispatchMessage(msg); **
Handler里的疑惑解决了 Looper绑定Handler-> Handler 发送消息->Looper启动消息队列->获取消息调用Handler的分发消息方法->获得消息处理 这应该就是工作流程

msg.recycleUnchecked(); 发送完的消息会回收

}

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 (;;) {
            //这是一个死循环，不断接受消息队里里的消息
            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 {
          //派发消息给Handler
                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();
        }
    }

到这里就有个问题主线程(UI 线程)是如何处理事件，更新UI的，我也没构建Looper啊
我们去看看ActivityThread的代码

在如此复杂的代码中找到了main函数。其中果然有
** Looper.prepareMainLooper(); 和 ** Looper.loop();

 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");
    }

Handler总结