深入理解Handler消息机制

简介

Handler主要用于是线程之间转换；通过其他线程发送消息，在目标线程消费消息，做到线程之间的切换；
消息机制主要组成部分：
Handler：发送和接受消息（Message）；
MessageQueue:消息队列，将Handler发送的消息存在队列中（实际是单链表）；
Looper:死循环，不断的从MessageQueue中取出要执行的消息，执行Handler的dispatchMessage()分发消息；

创建Handler对象

主线程

主线程中由于ActivityThread中已经初始化了；
public static void main(String[] args) {
	 Looper.prepareMainLooper();
	 Looper.loop();

}


//方式一：
private Handler handler = new Handler(new Handler.Callback() {
    @Override
    public boolean handleMessage(Message msg) {
        return true;

    }
});

//方式二：
MyHandler myHandler = new MyHandler(this);
class MyHandler extends Handler {
    private WeakReference<Activity> weakReference;
    public MyHandler(Activity activity ) {
        weakReference = new WeakReference<Activity>(activity);
    }

    @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);

    }
}

子线程中

class LooperThread extends Thread {
    public Handler mHandler;

    public void run() {
        Looper.prepare();
        mHandler = new Handler() {
            public void handleMessage(Message msg) {
                // process incoming messages here
            }
        };
        Looper.loop();
    }
}

发送消息

send方法

public final boolean sendEmptyMessage(int what)
{
    return sendEmptyMessageDelayed(what, 0);
}

public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
    Message msg = Message.obtain();
    msg.what = what;
    return sendMessageDelayed(msg, delayMillis);
}

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);
}

post方法

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

public final boolean postDelayed(Runnable r, long delayMillis)
{
    return sendMessageDelayed(getPostMessage(r), delayMillis);
}

 public final boolean postAtTime(Runnable r, long uptimeMillis)
{
    return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}

public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)
{
    return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}

无论send方法还是post方法最终都是调用enqueueMessage(),将当前消息message插入到MessageQueue中;

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

MessageQueue的enqueueMessage()方法，向MessageQueue中插入数据；MessageQueue中的消息队列其实就是一个单链表，链表中的节点以执行从小到大排序的（最先执行的放在链表头）；链表头是成员变量mMessages；

 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) {
		//当前Looper已经退出，则不再往队列中插入消息；
        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;
		//当前链表头节点；
        Message p = mMessages;
        boolean needWake;
		//当前对列为空或者插入的消息的执行时间小于头结点的执行时间，将当前消息插入到头结点；
        if (p == null || when == 0 || when < p.when) {
            // New head, wake up the event queue if blocked.
            msg.next = p;
            mMessages = msg;
            needWake = mBlocked;
		//将当前节点插入队列中（非队列头）；
        } 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();
			//上一个节点
            Message prev;
            for (;;) {
                prev = p;
                p = p.next;
				//找到第一个比当前消息的执行时间大的消息或者插入到队列尾；
                if (p == null || when < p.when) {
                    break;
                }
                if (needWake && p.isAsynchronous()) {
                    needWake = false;
                }
            }
			//插入到链表中
            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;
}

消费消息

Looper类的loop()死循环，不断的从MessageQueue中获取消息消费；

 public static void loop() {
	//1：得到当前线程的Looper对象
    final Looper me = myLooper();
	//必须要先调用Looper.prepare()
    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 (;;) {
		//2:从MessageQueue中获取消息；当MessageQueue中存在对应的消息时，返回对应的Message,否则阻塞，一直等待；
        Message msg = queue.next(); // might block
		//消息为null,结束循环；
        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 {
			//3：消费消息,Handler类的dispatchMessage();
            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();
    }
}

1:myLooper()方法，每个线程都对应一个ThreadLocal类，线程私有对象，可以实现线程安全，以空间换时间；

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

public static @Nullable Looper myLooper() {
    return sThreadLocal.get();
}

2:主要看MessageQueue的next()方法，死循环，从MessageQueue中获取消息；只有找到了对应开始执行时间的消息或者主动退出了Looper.loop()方法才会结束死循环；

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;
	//如果当前Looper已经退出，ptr=0;直接返回null,Looper.loop()退出循环；
    if (ptr == 0) {
        return null;
    }

    int pendingIdleHandlerCount = -1; // -1 only during first iteration
    int nextPollTimeoutMillis = 0;
    for (;;) {
        if (nextPollTimeoutMillis != 0) {
            Binder.flushPendingCommands();
        }

        nativePollOnce(ptr, nextPollTimeoutMillis);

        synchronized (this) {
            // Try to retrieve the next message.  Return if found.
            final long now = SystemClock.uptimeMillis();
            Message prevMsg = null;
			//从队列头开始遍历
            Message msg = mMessages;
			//一般情况下msg.target不为null,从MessageQueue类的enqueueMessage()可以知道；
            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());
            }
			//当前队列不为空
            if (msg != null) {
				//当前时间小于队列头消息的执行时间，得到消息执行的等待时间；
                if (now < msg.when) {
                    // Next message is not ready.  Set a timeout to wake up when it is ready.
                    nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                } else {
					//找到了消息，将当前消息从队列中移除，并将下一个消息设置为队列头；并且返回当前消息；
                    // 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 {//否则队列为null;
                // No more messages.
                nextPollTimeoutMillis = -1;
            }
			//正在中断Looper.loop()方法
            // Process the quit message now that all pending messages have been handled.
            if (mQuitting) {
                dispose();
                return null;
            }
			//继续循环，线程阻塞了；
            // 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;
    }
}

Looper类的loop()和MessageQueue的next()方法是最核心的地方；

3:Handler类的dispatchMessage()

public void dispatchMessage(Message msg) {
	//post()发送消息的回调
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
		//直接创建Handle的对象，并且返回true;
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
		//具体实现类的回调
        handleMessage(msg);
    }
}

注意

在退出线程前，调用quit()，quitSafely()；

以上就是Handler消息机制的主要内容，如有问题，请多指教，谢谢！