handler源码分析

handler类里面有个dispatchMessage()方法,是handler处理消息的方法

    /**     * 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);        }    }

消息拦截的顺序msg.callback->handler构造的传进来的mCallback->调用handler成员方法handlerMessage(msg)

Message的callback

    /**     * Same as {@link #obtain(Handler)}, but assigns a callback Runnable on     * the Message that is returned.     * @param h  Handler to assign to the returned Message object's <em>target</em> member.     * @param callback Runnable that will execute when the message is handled.     * @return A Message object from the global pool.     */    public static Message obtain(Handler h, Runnable callback) {        Message m = obtain();        m.target = h;        m.callback = callback;        return m;    }

这个message的callback就是一个Runnable

再看handler的成员mCallback

     *     * @param callback The callback interface in which to handle messages, or null.     */    public Handler(Callback callback) {        this(callback, false);    }

所以为什么创建handler时候如果传递一个callBack,重写的handleMessage方法就无效的原因

handler类的CallBack接口

    /**     * Callback interface you can use when instantiating a Handler to avoid     * having to implement your own subclass of Handler.     *     * @param msg A {@link android.os.Message Message} object     * @return True if no further handling is desired     */    public interface Callback {        public boolean handleMessage(Message msg);    }

再看参数最多的这个构造方法

     * @param looper The looper, must not be null.     * @param callback The callback interface in which to handle messages, or null.     * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for     * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.     *     * @hide     */    public Handler(Looper looper, Callback callback, boolean async) {        mLooper = looper;        mQueue = looper.mQueue;        mCallback = callback;        mAsynchronous = async;    }

可以看到looper是必须要传进去的,那我们平时用的handler在主线程序创建为什么没有传进去looper呢,找到其他构造方法

     * @param callback The callback interface in which to handle messages, or null.     * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for     * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.     *     * @hide     */    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;    }

看到我们主线程创建handler的时候其实会去调用mLooper = Looper.myLooper();来获取一个looper对象

    /**     * Return the Looper object associated with the current thread.  Returns     * null if the calling thread is not associated with a Looper.     */    public static @Nullable Looper myLooper() {        return sThreadLocal.get();    }

ThreadLocal其实就是一个全局的Map对象,保存的key就是当前线程对象,value就是looper对象,这样在哪个线程调用就取得是哪个线程对应的looper,那么在这个地方调用是要从主线程获取looper了

    // sThreadLocal.get() will return null unless you've called prepare().    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();    private static Looper sMainLooper;  // guarded by Looper.class

可以看到这个主线程的sMainLooper 是静态的成员变量,也就是说每个aplication的生命周期内会创建且存在一个looper对象
那么什么时候创建的呢,在Looper对象类中找不到,其实Activity在被创建的时候，框架会帮我们初始化一个Looper对象，因此在主线程中，我们不必去调用Looper.prepare()去初始化Looper对象。

我们经常调用的post(runnable)就能实现任务调度处理,又是怎么实现的呢

    public final boolean post(Runnable r)    {       return  sendMessageDelayed(getPostMessage(r), 0);    }

–>

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

其实还是创建了一个Message对象等到处理消息导读的时候在message的callback回调
继续跟进

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

在跟进

     *         occurs then the message will be dropped.     */    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);    }

如果消息队列对象不存在,打印异常,返回false
再跟进

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

queue.enqueueMessage(msg, uptimeMillis)就是把消息对象添加到queue里面,并且重点是msg的target绑定是当前handler对象,等looper轮训到这个msg的时候就可以拿出来msg的target对象并且调用他的处理方法,继续跟进到messageQueue类中

    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) {            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;            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;                    }                }                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;    }

—-1>

        if (msg.target == null) {            throw new IllegalArgumentException("Message must have a target.");        }

添加进来的msg必须要有对应的handler对象,否则抛出异常

—-2>

Message p = mMessages;

mMessages代表当前队列(实际就是一个链表,接下来我用链表代替),如果p==null证明这个链表是空的,需要初始化链表操作

—-3>

            if (p == null || when == 0 || when < p.when) {                // New head, wake up the event queue if blocked.                msg.next = p;                mMessages = msg;                needWake = mBlocked;

那么此时的msg就是链表的head,因为此时只有一个所以msg.next指向p,也就是null

—-4>

                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;                    }                }                msg.next = p; // invariant: p == prev.next                prev.next = msg;

用一个prev指向链表最后一个节点,然后让p=p.next(即null),p==null;break
msg.next指向p,让prev即链表的最后一个元素的next指向msg,就完成了链表添加

那么此时就完成了handler的创建,发送message到添加到messageQueue中等待Looper去轮询

那么很明了,我们创建一个handler(消息发送处理器),对应必须要有一个(looper消息轮询器)和一个MessageQueue(消息队列|消息链表),如何说明对应一个MessageQueue呢

handler中

        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;

跟进到looper中

    private Looper(boolean quitAllowed) {        mQueue = new MessageQueue(quitAllowed);        mThread = Thread.currentThread();    }

真相明了,每创建一个looper对象就会创建一个queue,至于lopper的轮询放在下一篇
queue的轮询任务是looper的loop()方法实现的

 /**     * Run the message queue in this thread. Be sure to call     * {@link #quit()} to end the 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 traceTag = me.mTraceTag;            if (traceTag != 0) {                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));            }            try {                msg.target.dispatchMessage(msg);            } 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();        }    }

一个while true循环不停的进行轮询
之前有人问我当消息队列为空时,还进行轮询么

        for (;;) {            Message msg = queue.next(); // might block            if (msg == null) {                // No message indicates that the message queue is quitting.                return;            }

取出来一个msg,当没有消息的时候msg==null,这时候return就跳出轮询,所以答案是否

            try {                msg.target.dispatchMessage(msg);            } finally {                if (traceTag != 0) {                    Trace.traceEnd(traceTag);                }            }

拿到msg的target就是handler对象,调用dispatchMessage(msg)方法
就来到了最开始我们分析的handler处理消息的方法