一、消息循環(huán)過(guò)程
Android應(yīng)用程序進(jìn)程在啟動(dòng)的時(shí)候伟叛，會(huì)在進(jìn)程中加載ActivityThread類略就，并且執(zhí)行這個(gè)類的main函數(shù)，應(yīng)用程序的消息循環(huán)過(guò)程就是在這個(gè)main函數(shù)里面實(shí)現(xiàn)的

public static void main(String[] args) {
        踢星。澳叉。。。成洗。五督。
        Looper.prepareMainLooper();

        ActivityThread thread = new ActivityThread();
        thread.attach(false);
        。瓶殃。充包。。遥椿。基矮。
        Looper.loop();
    }

首先看Looper.prepareMainLooper函數(shù)

    // sThreadLocal.get() will return null unless you've called prepare().
    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
    final MessageQueue mQueue;
    final Thread mThread;

    public static void prepareMainLooper() {
        prepare(false);標(biāo)注1
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();標(biāo)注2
        }
    }

    標(biāo)注1：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));標(biāo)注3
    }

    標(biāo)注3：private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

    標(biāo)注2:public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

然后看Looper.loop()函數(shù)：

    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;標(biāo)注4
        ......
        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }
            ......
            msg.target.dispatchMessage(msg);標(biāo)注5
            ......
            msg.recycleUnchecked();標(biāo)注6
        }
    }

標(biāo)注4:MessageQueue對(duì)象Message隊(duì)列
標(biāo)注5:Message的target是Handler
標(biāo)注6:回收Message

二、消息發(fā)送過(guò)程
ActivityManagerService通過(guò)調(diào)用ApplicationThread類的scheduleLaunchActivity函數(shù)通知應(yīng)用程序冠场，它可以加載應(yīng)用程序的默認(rèn)Activity了（ApplicationThread是ActivityThread的內(nèi)部類）

    private class ApplicationThread extends ApplicationThreadNative {
    @Override
        public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
                ActivityInfo info, Configuration curConfig, Configuration overrideConfig,
                CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor,
                int procState, Bundle state, PersistableBundle persistentState,
                List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents,
                boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) {

            updateProcessState(procState, false);

            ActivityClientRecord r = new ActivityClientRecord();

            r.token = token;
            r.ident = ident;
            r.intent = intent;
            r.referrer = referrer;
            r.voiceInteractor = voiceInteractor;
            r.activityInfo = info;
            r.compatInfo = compatInfo;
            r.state = state;
            r.persistentState = persistentState;

            r.pendingResults = pendingResults;
            r.pendingIntents = pendingNewIntents;

            r.startsNotResumed = notResumed;
            r.isForward = isForward;

            r.profilerInfo = profilerInfo;

            r.overrideConfig = overrideConfig;
            updatePendingConfiguration(curConfig);

            sendMessage(H.LAUNCH_ACTIVITY, r);
        }
    }

    private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
        if (DEBUG_MESSAGES) Slog.v(
            TAG, "SCHEDULE " + what + " " + mH.codeToString(what)
            + ": " + arg1 + " / " + obj);
        Message msg = Message.obtain();
        msg.what = what;
        msg.obj = obj;
        msg.arg1 = arg1;
        msg.arg2 = arg2;
        if (async) {
            msg.setAsynchronous(true);
        }
        mH.sendMessage(msg);
    }

mH是Handler愈捅，Handler.sendMessage最終會(huì)調(diào)用sendMessageAtTime方法：

    final MessageQueue mQueue;

    public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        MessageQueue queue = mQueue;標(biāo)注7
        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);標(biāo)注8
    }

    public Handler(Callback callback, boolean async) {
        mLooper = Looper.myLooper();
        ......
        標(biāo)注7:mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

標(biāo)注8:private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;標(biāo)注9:Handler發(fā)送的Message中的target都是Handler本身
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);標(biāo)注10
    }

標(biāo)注9:Handler發(fā)送的Message中的target都是Handler本身
標(biāo)注10:MessageQueue的enqueueMessage方法就已經(jīng)將Message信息加入到消息隊(duì)列中了

三、消息的處理
在標(biāo)注5處獲取Message的target也就是當(dāng)前Handler（mH）對(duì)象慈鸠，調(diào)用Handler的dispatchMessage方法蓝谨，最終會(huì)調(diào)用mH的handlerMessage方法：

public void handleMessage(Message msg) {
            if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
            switch (msg.what) {
                case LAUNCH_ACTIVITY: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
                    final ActivityClientRecord r = (ActivityClientRecord) msg.obj;

                    r.packageInfo = getPackageInfoNoCheck(
                            r.activityInfo.applicationInfo, r.compatInfo);
                    handleLaunchActivity(r, null);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;