Looper,Handler,MessageQuene

关于Looper,Handler,MessageQuenu的关系，查看了一下源码，在此做个记录。有描述不准确的欢迎大家多多指教。

概述

Message： Handler接收和处理的消息对象。
MessageQueue：消息队列，用来管理Message。
Looper：每个线程只能有一个Looper，使用loop方法循环读取MessageQueue中的消息，把消息发送给Handler进行处理。
Handler：发送消息和处理消息。

分析

我们都知道对于使用子线程Handler，需要调用Looper.prepare()方法创建Looper对象，调用Looper.loop()方法启动该线程的消息队列。使用如下：

class MyThread extends Thread{
        @Override
        public void run() {
            Looper.prepare();
            threadHandler = new Handler(){
                @Override
                public void handleMessage(Message msg) {
                    switch (msg.what){
                        case(100):
                            Log.d("xxx", "success");
                            break;
                    }
                }
            };
            Looper.loop();
        }
    }

Looper.prepare()方法创建Looper对象，并将其放在ThreadLocal中，源码如下：

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));
    }

但是为什么主线程就不需要呢？
在主线程中调用ActivityThread.java的main()方法，源码如下：

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();//创建Looper对象

        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();//开启循环读取消息队列

        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

关键代码是在main()中调用了Looper.prepareMainLooper();创建Looper对象，并将Looper对象放在ThreadLocal中，源码如下：

public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

在main()中调用Looper.loop()方法开启循环读取消息队列，并将消息传递给handler进行消息处理。
但是消息队列中的消息从何而来呢？
handler通过调用handler.sendMessage(msg)或是handler.post(new Runnable())方法发送消息，将消息添加到消息队列中。
这两个发送消息的方法最终调用sendMessageAtTime方法，源码如下：

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, uptimeMillis);
    }

不同的是，对于post方法，还会调用getPostMessage方法，会将参数Runnable对象添加到Message中作为m.callback：

private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;
        return m;
    }

发送消息后，需要循环读取消息，handler接收消息调用handleMessage方法进行处理，此时需要Looper.loop()方法。源码如下：

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 (;;) {//循环读取队列中的Message
            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 {
                msg.target.dispatchMessage(msg);//处理消息
            } 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();
        }
    }

从源码可以看出循环读取消息队列中的Message对象，调用queue.next()方法获取Message:

Message next() {
        // Return here if the message loop has already quit and been disposed.
        // This can happen if the application tries to restart a looper after quit
        // which is not supported.
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }

        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }

            nativePollOnce(ptr, nextPollTimeoutMillis);

            synchronized (this) {
                // Try to retrieve the next message.  Return if found.
                final long now = SystemClock.uptimeMillis();
                Message prevMsg = null;
                Message msg = mMessages;
                if (msg != null && msg.target == null) {
                    // Stalled by a barrier.  Find the next asynchronous message in the queue.
                    do {
                        prevMsg = msg;
                        msg = msg.next;
                    } while (msg != null && !msg.isAsynchronous());
                }
                if (msg != null) {
                    if (now < msg.when) {
                        // Next message is not ready.  Set a timeout to wake up when it is ready.
                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                    } else {
                        // Got a message.
                        mBlocked = false;
                        if (prevMsg != null) {
                            prevMsg.next = msg.next;
                        } else {
                            mMessages = msg.next;
                        }
                        msg.next = null;
                        if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                        msg.markInUse();
                        return msg;
                    }
                } else {
                    // No more messages.
                    nextPollTimeoutMillis = -1;
                }

                // Process the quit message now that all pending messages have been handled.
                if (mQuitting) {
                    dispose();
                    return null;
                }

                // If first time idle, then get the number of idlers to run.
                // Idle handles only run if the queue is empty or if the first message
                // in the queue (possibly a barrier) is due to be handled in the future.
                if (pendingIdleHandlerCount < 0
                        && (mMessages == null || now < mMessages.when)) {
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                if (pendingIdleHandlerCount <= 0) {
                    // No idle handlers to run.  Loop and wait some more.
                    mBlocked = true;
                    continue;
                }

                if (mPendingIdleHandlers == null) {
                    mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
                }
                mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
            }

            // Run the idle handlers.
            // We only ever reach this code block during the first iteration.
            for (int i = 0; i < pendingIdleHandlerCount; i++) {
                final IdleHandler idler = mPendingIdleHandlers[i];
                mPendingIdleHandlers[i] = null; // release the reference to the handler

                boolean keep = false;
                try {
                    keep = idler.queueIdle();
                } catch (Throwable t) {
                    Log.wtf(TAG, "IdleHandler threw exception", t);
                }

                if (!keep) {
                    synchronized (this) {
                        mIdleHandlers.remove(idler);
                    }
                }
            }

            // Reset the idle handler count to 0 so we do not run them again.
            pendingIdleHandlerCount = 0;

            // While calling an idle handler, a new message could have been delivered
            // so go back and look again for a pending message without waiting.
            nextPollTimeoutMillis = 0;
        }
    }

可以看出，如果queue中没有msg, 或者msg没到可执行的时间,那么现在线程就处于空闲时间, 就可以执行IdleHandler,这里出现了IdleHander的概念，使开发者调用可以在空闲时间做一些其他事情。
关于IdleHandler的详细说明请参考
http://blog.youkuaiyun.com/AwayEagle/article/details/50152013。
调用msg.target.dispatchMessage(msg)这个方法处理消息，msg.target是Handler对象，此方法将消息传递给handler,调用Handler.java的dispatchMessage(msg)方法进行消息处理。

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

这里根据msg.callback判断根据发送消息的方法进行处理。如果是通过sendMessage方法发送消息，则执行handler的handleMessage(msg)方法；如果是通过post方法发送消息则执行handleCallback(msg)方法，也就是实现Runnable的run()方法。

总结

创建Looper对象，通过handler发送消息，将Message添加到消息队列中。调用Looper.loop方法开启消息队列，循环读取消息队列中的Message,并将消息传递给handler,由handler进行处理。