Handler的源碼分析

最近複習到了Handler一直只知道怎麼用，卻沒有仔細分析過源碼，這下就來看看

爲何要使用handler呢？

由於在安卓應用中ui操做是線程安全的，只能ui線程去操做ui組件，可是在現實開發中，可能有多個線程去併發操做ui，因此將會致使線程不安全，因此就用到了handler

 

Looper1.主要負責建立message Queue和自身的建立    2.    消息的循環

public static void prepare() {
    prepare(true);
}

private static void prepare(boolean quitAllowed) {
//判斷當前的對象是否爲空 ，即prepare不能調用兩次
    if (sThreadLocal.get() != null) {
        throw new RuntimeException("Only one Looper may be created per thread");
    }
    建立looper對象放入線程中

    sThreadLocal.set(new Looper(quitAllowed));
}

 

private Looper(boolean quitAllowed) {
        //looper對象建立了messagequeue
    mQueue = new MessageQueue(quitAllowed);
    mThread = Thread.currentThread();
}

2.looper.loop

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 (;;) {
        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 slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;

        final long traceTag = me.mTraceTag;
        if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
            Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
        }
        final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
        final long end;
        try {
//吧msg交給msg.target實際就是綁定的handler去處理這個方法中調用了handlemessage因此在建立handler
的時候要重寫handlemessage方法

            msg.target.dispatchMessage(msg);
            end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
        } finally {
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }
        if (slowDispatchThresholdMs > 0) {
            final long time = end - start;
            if (time > slowDispatchThresholdMs) {
                Slog.w(TAG, "Dispatch took " + time + "ms on "
                        + Thread.currentThread().getName() + ", h=" +
                        msg.target + " cb=" + msg.callback + " msg=" + msg.what);
            }
        }

        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的做用就是建立message隊列，而後吧隊列中的消息派發給handler，而後handler去處理消息

而後咱們在去看下Handler的源碼

public Handler(Callback callback, boolean async) {
    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());
        }
    }

    mLooper = Looper.myLooper();
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

 

能夠看出handler建立的時候就會綁定他的looper並找見他的隊列

handler有兩種發送消息的方法 send和post

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

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

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);
}

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this;
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
//看出來最後調用了隊列的方法，也就是handler發出的消息，最後會保存到對應的隊列中
    return queue.enqueueMessage(msg, uptimeMillis);
}

 

因此就是一個應用的建立，系統會自動給你生成一個主線程，並在這個主線程中生成一個Looper對象來讓你和子線程之間進行消息的傳遞，Looper會建立一個消息隊列，當咱們在子線程中給主線程發消息的時候就須要用到handler，handler建立的時候就會去找到這個對應的looper。looper不斷的執行loop方法去發消息給隊列中 ，沒當handler去發送一個消息的時候，就吧消息加入到隊列中 ，而後隊列的loop方法就會接收到這個消息，而後消息去調用dispatchmsg方法，而後dispatchmsg方法就會調用handler中的handlemessage方法，咱們就能接收到這個消息