Android Handler 机制

Handler机制是Andrdoid中很常用的线程任务控制，Handler类负责信息传送和分发执行，Looper是控制Message中心，MessageQueue保持和提供Message。

1、Message

顺便提一下Message.obtain()，Message会维护一个信息池，以单列表结构存储，使用它可以减少对象的创建，节省内存，代码如下

 //池默认长度是MAX_POOL_SIZE=50
  public static Message obtain() {
        synchronized (sPoolSync) {
            if (sPool != null) {
                Message m = sPool;
                sPool = m.next;
                m.next = null;
                m.flags = 0; // clear in-use flag
                sPoolSize--;
                return m;
            }
        }
        return new Message();
    }

    //执行完会进行回收
     void recycleUnchecked() {
        flags = FLAG_IN_USE;
        what = 0;
        arg1 = 0;
        arg2 = 0;
        obj = null;
        replyTo = null;
        sendingUid = -1;
        when = 0;
        target = null;
        callback = null;
        data = null;

        synchronized (sPoolSync) {
            if (sPoolSize < MAX_POOL_SIZE) {
                next = sPool;
                sPool = this;
                sPoolSize++;
            }
        }
    }

来看一下Message的结构

public final class Message implements Parcelable{
    public int what;

    public int arg1; 

    public int arg2;

    public Object obj;

    public Messenger replyTo;

    public int sendingUid = -1;

    int flags;

    long when;

    Bundle data;

    Handler target;//执行的Handler

    Runnable callback;//post时传入的灰度，会优先回调

    Message next;//链表结构
}

2、Handler

Handler不管的post方法还是sendMessage，信息都是封装成Message传递执行，最终会调到sendMessageDelayed方法

 public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        //先计算运行的时间点，启动时间+延迟时间
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

最后调用sendMessageAtTime插入到MessageQueue队列中

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);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

Handler分发执行，优先级为：Runnable->Callback->handleMessage

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

    private static void handleCallback(Message message) {
        message.callback.run();
    }

3、MessageQueue

插入的代码会比较复杂，mMessage记录为信息队列的第一个

 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) {
            //判断线程的Looper是否停止
            if (mQuitting) {
                IllegalStateException e = new IllegalStateException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w("MessageQueue", e.getMessage(), e);
                msg.recycle();
                return false;
            }

            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            //空队列，或者执行时刻小于列表头的执行时刻，插入到列表头，
            //
            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 {
                //判断是否要唤醒执行队列，一般不需要唤醒
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                for (;;) {//变量查找插入位置
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {//找到比自己执行时刻小的，将插入到它前面
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                //插入操作
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }

            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }

Looper会循环调用MessageQueue.next，获取Message执行

 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();
            }

            //本地方法，原来阻塞队列并设置唤醒时间
            nativePollOnce(ptr, nextPollTimeoutMillis);

            synchronized (this) {
                // Try to retrieve the next message.  Return if found.
                final long now = SystemClock.uptimeMillis();
                Message prevMsg = null;
                //获取列表头信息
                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) {
                    if (now < msg.when) {
                        // 执行时刻未到，设置唤醒阻塞时间
                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                    } else {
                        // 获取到信息并返回
                        mBlocked = false;
                        if (prevMsg != null) {
                            prevMsg.next = msg.next;
                        } else {
                            mMessages = msg.next;
                        }
                        msg.next = null;
                        if (false) Log.v("MessageQueue", "Returning message: " + msg);
                        return msg;
                    }
                } else {
                    // 没有信息
                    nextPollTimeoutMillis = -1;
                }


                // Process the quit message now that all pending messages have been handled.
                if (mQuitting) {
                    dispose();
                    return null;
                }

                //以下代码是获取和运行空闲程序，暂时不去理解
                // 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("MessageQueue", "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;
        }
    }

4、Looper

以下是我们经常在线程启动Looper的代码，

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();
      }
    }

先看一下prepare方法

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

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));
    }

关键在ThreadLocal，ThreadLocal是线程报错数据的一种方式，Thread.threadLocals会记录保持在ThreadLocalMap中的key，下次要再根据key来获取数据，所以线程不一样，可以也不一样，获取Looper对象也不是同一个，所以其他不同线程使用不用Looper的效果

    //ThreadLocal.java
   public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);//获取当前线程的ThreadLocalMap
        if (map != null)
            map.set(this, value);//保存，
        else
            createMap(t, value);//创建
    }

       void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

创建Handler，会调用myLooper获取当前线程的Looper

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

Looper.loop()是一个无限循环的方法，不断获取MessageQueue中的信息进行执行，代码比较简单

“`
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(); // 从消息队列获取信息，可能会阻塞
        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
        Printer logging = me.mLogging;
        if (logging != null) {
            logging.println(">>>>> Dispatching to " + msg.target + " " +
                    msg.callback + ": " + msg.what);
        }

        //通过Handrle处理信息，
        msg.target.dispatchMessage(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);
        }

        //回收Message
        msg.recycleUnchecked();
    }
}

附

MessageQueue 是在native层使用epoll进行阻塞和唤醒，有兴趣可以看下面的文章
Android消息机制Native