Handler异步消息传递机制（四）Handler发送消息流程，源码（Android 9.0）解析

前言

上篇文章我们从源码角度分析了如何在主线程、子线程创建Handler对象。可参考：Handler异步消息传递机制（三）在主线程、子线程中创建Handler，源码（Android 9.0）彻底解析 

那么创建Handler之后，如何发送消息呢？这个流程相信大家也已经非常熟悉了，我们继续以文章 Handler异步消息传递机制（一）Handler常用实现方式 的demo为例，然后进行源码跟踪解析！

 

Handler对象 在新启动的子线程发送消息（源码跟踪）

下面是 Handler对象：在新启动的子线程发送消息  的代码：

public class DownLoadAppFile {
    public void download(String urlPath, Handler handler, ProgressBar pb) {

        try {
            //下载apk的代码，这里用线程睡眠模拟
            Thread.currentThread().sleep(3*1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        Message msg = Message.obtain();
        msg.what =1;//成功
        //msg.what =2;//失败
        handler.sendMessage(msg);//发送消息

    }
}

Handler到底是把Message发送到哪里去了呢？为什么之后又可以在Handler的handleMessage()方法中重新得到这条Message呢？接下来我们来看一下发送消息的源码：

    public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }

它里面调用了sendMessageDelayed方法，

往下追踪

  public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

我们可以看到sendMessageDelayed方法，其中msg参数就是我们发送的Message对象，而delayMillis参数则表示延迟发送消息的时间（毫秒），这里默认传入的为0

往下追踪

    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);
    }

我们可以看到sendMessageAtTime()方法同样接收两个参数，其中msg参数就是我们发送的Message对象，而uptimeMillis参数则表示发送消息的时间，SystemClock.uptimeMillis() + delayMillis 即它的值等于自系统开机到当前时间的毫秒数再加上延迟时间。

然后对 MessageQueue对象queue进行了赋值，这个MessageQueue又是什么东西呢？学过java基础的可能会马上想到Queue，基本上一个队列就是一个先入先出（FIFO）的数据结构。同样在这里MessageQueue  直译过来就是 消息队列 的意思，用于将所有收到的消息以队列的形式进行排列，并提供入队和出队的方法。

那么enqueueMessage()方法就是入队的方法了，我们来看下这个方法的源码：

    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

msg.target赋值为this，也就是把当前所在类handler，作为msg的target属性。然后将这三个参数都传递到MessageQueue的enqueueMessage()方法中，也就是说handler发出的消息，最终会保存到消息队列中去。

调用sendMessage方法其实最后是调用了类MessageQueueen消息队列的queueMessage入队方法。

那么有了MessageQueueen消息队列的queueMessage入队方法。它必然有相对应的出队方法。

Handler对象在新启动的子线程发送消息以后，接下来在主线程中，Handler类处理消息的方法handleMessage被自动回调。

那么接下来在主线程中，回调handleMessage方法的流程是怎样的呢？

Android的主线程就是ActivityThread，主线程的入口方法为main，我们继续来看一下ActivityThread类中main方法的源代码：

public static void main(String[] args) {
        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
 
        // 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("");
 
        Looper.prepareMainLooper();
 
        // Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line.
        // It will be in the format "seq=114"
        long startSeq = 0;
        if (args != null) {
            for (int i = args.length - 1; i >= 0; --i) {
                if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) {
                    startSeq = Long.parseLong(
                            args[i].substring(PROC_START_SEQ_IDENT.length()));
                }
            }
        }
        ActivityThread thread = new ActivityThread();
        thread.attach(false, startSeq);
 
        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");
    }

第47行调用了Looper.loop()方法，它执行了执行消息循环，我们来看下它的源码：

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();

        // Allow overriding a threshold with a system prop. e.g.
        // adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
        final int thresholdOverride =
                SystemProperties.getInt("log.looper."
                        + Process.myUid() + "."
                        + Thread.currentThread().getName()
                        + ".slow", 0);

        boolean slowDeliveryDetected = false;

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

            final long traceTag = me.mTraceTag;
            long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
            long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
            if (thresholdOverride > 0) {
                slowDispatchThresholdMs = thresholdOverride;
                slowDeliveryThresholdMs = thresholdOverride;
            }
            final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
            final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);

            final boolean needStartTime = logSlowDelivery || logSlowDispatch;
            final boolean needEndTime = logSlowDispatch;

            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }

            final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
            final long dispatchEnd;
            try {
                msg.target.dispatchMessage(msg);
                dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            if (logSlowDelivery) {
                if (slowDeliveryDetected) {
                    if ((dispatchStart - msg.when) <= 10) {
                        Slog.w(TAG, "Drained");
                        slowDeliveryDetected = false;
                    }
                } else {
                    if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
                            msg)) {
                        // Once we write a slow delivery log, suppress until the queue drains.
                        slowDeliveryDetected = true;
                    }
                }
            }
            if (logSlowDispatch) {
                showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", 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.recycleUnchecked();
        }
    }

我们可以看到前面几行代码，首先对Looper、MessageQueue对象进行了赋值，然后第23行进入了一个死循环for( ; ; ){ }，然后不断地调用的MessageQueue的next()方法，Message msg = queue.next() 很明显这个next()方法就是消息队列的出队方法。

接下来你会发现第57行执行了msg.target.dispatchMessage(msg); 其中msg.target就是指的Handler对象，你回看一下上面enqueueMessage()方法就可以看出来。接下来当然就要看一看Handler中dispatchMessage()方法的源码了，如下：

    /**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

在第8行进行判断，如果mCallback不为空，则调用mCallback的handleMessage()方法，否则直接调用Handler的handleMessage方法，并将消息对象作为参数传递过去。这样我相信大家就都明白了为什么handleMessage()方法中可以获取到之前发送的消息了吧！