Android Handler MessageQueue Looper 消息機制原理

提到Android裏的消息機制，便會提到Message、Handler、Looper、MessageQueue這四個類，我先簡單介紹如下這4個類
之間的愛恨情仇。

Message

消息的封裝類，裏邊存儲了消息的詳細信息，以及要傳遞的數據

Handler

主要用在消息的發送上，有即時消息，有延遲消息，內部還提供了享元模式封裝了消息對象池，可以有效的減小重複對象的建立，留更多的內存作其餘的事，

Looper

這個類內部持有一個MessageQueue對象，當建立Looper的時候，同時也會建立一個MessageQueue，而後Looper的主要工做就不斷的輪訓MessageQueue，輪到天荒地老的那種

MessageQueue

內部持有一個Message對象，採用單項鍊表的形式來維護消息列隊。而且提供了入隊，出隊的基礎操做

舉個現實中的栗子，Message就至關於包裝好的快遞盒子，Handler就至關於傳送帶，MessageQueue就至關於快遞車，Looper就至關於快遞員，聯想一下，來個快遞盒子，biu丟到傳送帶上，傳送帶很智能，直接傳送到快遞三輪車裏，而後快遞小哥送一波～，日夜交替，不分晝夜的工做，好傢伙，007工做制

消息機制的初始化

好，咱們把這4個傢伙從頭到位分析一遍，要想使用Android的消息，首先要建立Looper對象，Android系統已經幫咱們在UI線程內建立好了一個，咱們能夠看一下

public final class ActivityThread extends ClientTransactionHandler {
    /**
     * The main entry point from zygote.
     */
    public static void main(String[] args) {
        Looper.prepareMainLooper();

        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");
    }
}

ActivityThread這個類你們應該不陌生吧，沒錯，他就是咱們App的主線程管理類，咱們看到他調用了 prepareMainLooper 來初始化，而後 loop，天荒地老的那種loop，這個loop，咱們最後聊

咱們看一下Looper內部提供的 prepareMainLooper 實現

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

上邊涉及到了3個方法，我都貼出來了，首先 quitAllowed 這個參數表明該Looper是否能夠退出，咱們主線程內的Looper是不容許退出的，因此封裝了 prepareMainLooper 方法和 prepare 方法已作區分，咱們項目中平時用的都是 prepare 方法，由於是子線程，因此容許退出Looper，你們在子線程內用完記得調用quit哦~
這裏咱們看Looper內部是經過ThreadLocal維護的Looper對象，也就是說每一個線程都是相互獨立的。並且Looper作了限制，每一個線程內部只能存在一個Looper對象，等同於每一個線程內只能有一個MessageQueue
最後在Looper的構造方法內，建立了一個MessageQueue對象，整個Looper的初始化就結束了

建立消息

咱們準備好了Looper和MessageQueue後，就能夠建立消息啦，接下來咱們建立一個消息吧

//直接new對象，不推薦的方式
Message msg = new Message();
//推薦：內部是一個複用對象池
Message message = handler.obtainMessage();
message.what = 1;
message.obj = "hello world";

發送消息（入隊）

咱們發送消息的時候，都是會藉助Handler的sendMessage就能夠把消息發送到列隊裏了，咱們往下看是如何完成的入隊操做吧，首先咱們平時都是建立一個Handler，而後調用sendMessage就能夠了

Handler handler = new Handler();
handler.sendMessage(message);

咱們先看一下Handler的構造方法

public Handler() {
    this(null, false);
}
public Handler(@Nullable Callback callback, boolean async) {
    //FIND_POTENTIAL_LEAKS一直都是false，因此不用關心這個邏輯
    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());
        }
    }
    //獲得當前線程下的Looper對象
    mLooper = Looper.myLooper();
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread " + Thread.currentThread()
                    + " that has not called Looper.prepare()");
    }
    //從Loopper內部獲取一個列隊
    mQueue = mLooper.mQueue;
    // 回調對象，咱們平時寫的時候，通常都是用類集成的方式重寫 handleMessage 方法
    mCallback = callback;
    //標示當前Handler是否支持異步消息
    mAsynchronous = async;
}

其實構造方法很簡單吶，就是獲取Looper對象，而後初始化列隊和回調對象就完事了，咱們繼續看sendMessage而後看消息的入隊吧

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

經過內部的重載方法，一直調用到sendMessageAtTime方法，在這裏獲得Handler內部的MessageQueue對象，而後調用了 enqueueMessage 方法準備入隊

private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
        long uptimeMillis) {
    msg.target = this;
    msg.workSourceUid = ThreadLocalWorkSource.getUid();

    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
}

這裏調用了MessageQueue的enqueueMessage方法真正入隊，咱們繼續看一下

boolean enqueueMessage(Message msg, long when) {
    if (msg.target == null) {
        throw new IllegalArgumentException("Message must have a target.");
    }
    if (msg.isInUse()) {
        throw new IllegalStateException(msg + " This message is already in use.");
    }

    synchronized (this) {
        //若是當前退出狀態，則回收消息，並返回消息入隊失敗
        if (mQuitting) {
            IllegalStateException e = new IllegalStateException(
                    msg.target + " sending message to a Handler on a dead thread");
            Log.w(TAG, e.getMessage(), e);
            msg.recycle();
            return false;
        }

        msg.markInUse();
        msg.when = when;
        Message p = mMessages;
        boolean needWake;
        //若是鏈表是空的，或者當前消息的when小於表頭的when的時候，便會從新設置表頭
      //這裏能夠得知，消息的順序是按照延遲時間，從小往大排序的
        if (p == null || when == 0 || when < p.when) {
            // New head, wake up the event queue if blocked.
            msg.next = p;
            mMessages = msg;
            needWake = mBlocked;
        } else {
            // Inserted within the middle of the queue.  Usually we don't have to wake
            // up the event queue unless there is a barrier at the head of the queue
            // and the message is the earliest asynchronous message in the queue.
            needWake = mBlocked && p.target == null && msg.isAsynchronous();
            Message prev;
            for (;;) {
                prev = p;
                p = p.next;
                if (p == null || when < p.when) {
                    break;
                }
                if (needWake && p.isAsynchronous()) {
                    needWake = false;
                }
            }
            //把msg放到鏈表最後
            msg.next = p; // invariant: p == prev.next
            prev.next = msg;
        }

        // We can assume mPtr != 0 because mQuitting is false.
        if (needWake) {
            nativeWake(mPtr);
        }
    }
    return true;
}

經過這個方法，咱們瞭解到MessageQueue是經過Message的單鏈結構存儲的，而後每次入隊的時候，都會
經過這個enqueueMessage方法向鏈表的最末尾添加數據。

最後咱們聊一下Looper下的loop方法吧

接下來咱們看一下

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();

    // 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 (;;) {
        //queue的next會阻塞
        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);
        }
        // Make sure the observer won't change while processing a transaction.
        final Observer observer = sObserver;

        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;
        Object token = null;
        if (observer != null) {
            token = observer.messageDispatchStarting();
        }
        long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
        try {
            //派發消息，執行回調handleMessage
            msg.target.dispatchMessage(msg);
            if (observer != null) {
                observer.messageDispatched(token, msg);
            }
            dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
        } catch (Exception exception) {
            if (observer != null) {
                observer.dispatchingThrewException(token, msg, exception);
            }
            throw exception;
        } finally {
            ThreadLocalWorkSource.restore(origWorkSource);
            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();
    }
}

Looper內的loop方法別看這麼多，大多數都是日誌相關的處理。其實他就兩件事
第一件事就是從列隊中經過next取出Message對象
第二件事就是經過Message對象上綁定的target對象dispatchMessage方法，來分發消息
咱們接下來看一下dispatchMessage方法，而後在看MessageQueue的next

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

灰常簡單，判斷CallBack對象。而後調用handleMessage就完事了，咱們的Activity就收到數據了。
接下來咱們看看MessageQueue的next是怎麼獲取列隊內的消息的把。

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();
        }
        //沒有消息的時候，或者有延遲消息的時候會進行睡眠
        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) {
                //當前時間小於消息內記錄的時間，而後計算一個睡眠時間，跳出循環執行睡眠
                if (now < msg.when) {
                    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;
    }
}

首先MessageQueue的消息是用單鏈表的形式存儲，而後next函數作的事情就是死循環獲取消息，
在獲取消息的時候判斷一下消息是否符合執行時間，若是不符合執行時間，就進入睡眠狀態等待消息。
若是符合執行時間就直接返回Message給Looper進行分發，若是Message鏈表都爲空。則睡眠時間是-1
表明無休止的睡眠。在無休止睡眠的狀態下，enqueueMessage的nativeWake方法，會進行一次喚醒，喚醒後next函數繼續執行，判斷返回消息給Looper執行消息分發