Android 消息机制Handler

Android的消息机制主要是指Handler的运行机制以及Handler所附带的MessageQueue和Looper的工作过程。Handler的主要作用是将一个任务切换到某个指定的线程中去执行，常用来更新UI。

Handler的简单使用

public class MainActivity extends Activity {    private Handler handler;    private int i = 1;    private TextView textView;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        textView = (TextView) findViewById(R.id.tv_process_id);        handler = new Handler() {            @Override            public void handleMessage(Message message) {                Log.e("id -->", message.arg1+"");                textView.setText(message.arg1+"");            }        };        findViewById(R.id.bt_add_process).setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                send(i);                i++;            }        });    }    //在新的线程中可以执行如下载之类的耗时任务，然后把结果发送给Handler，在Handler中更新UI    private void send(final int id) {        new Thread(new Runnable() {            @Override            public void run() {                try {                    Log.e("thread id is:" ,Thread.currentThread().getId()+"");                    Message message = new Message();                    message.arg1 = id;                    handler.sendMessage(message);                } catch (Exception e) {                    e.printStackTrace();                }            }        }).start();    }}

原理分析

Handler的发送消息

• 消息发送通过post的一系列方法以及send的一些列方法实现，post的一系列方法最终是通过send的一系列方法实现。

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

• 由以上的源码可以发现，Handler发送消息的过程仅仅只是向消息队列（也就是MessageQueue)插入一条消息

MessageQueue插入消息

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;    }

• MessageQueue的enqueueMessage主要操作是单链表的插入操作

MessageQueue的next()方法从消息队列中取出一条消息并从消息列表中删除

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();            }            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) {                    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 {                        // 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;        }    }

• 从上面next()的方法中可以看出，next()是一个无限循环的方法，如果消息队列中没有消息，next()方法就会一直阻塞在这里

MessageQueue的quit()方法退出消息队列

void quit(boolean safe) {        if (!mQuitAllowed) {            throw new IllegalStateException("Main thread not allowed to quit.");        }        synchronized (this) {            if (mQuitting) {                return;            }            mQuitting = true;            if (safe) {                removeAllFutureMessagesLocked();            } else {                removeAllMessagesLocked();            }            // We can assume mPtr != 0 because mQuitting was previously false.            nativeWake(mPtr);        }    }

Looper的工作过程

• Looper会不停地从MessageQueue中查看是否有新消息，如果有新消息就立即处理，否则一直阻塞在那里
• Looper在构造方法中会创建一个MessageQueue

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

• Looper主要有2个方法，Looper.prepare()方法主要是为当前线程创建Looper，Looper.loop()开启消息循环，只有调用了loop()方法后，消息循环系统才开始起作用

 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();        }    }

• 从上面的源码可以看出，loop方法是一个死循环，一直不停查看MessageQueue中是否有消息，唯一跳出循环的条件是MessageQueue的next()方法返回null。
• 从MessageQueue的next()获得消息后，调用msg.target.dispatchMessage(msg)处理消息，msg.target就是发送消息的Handler对象，可以看出Handler的dispatchMessage()方法是在Looper线程中执行。
• Looper的停止

public void quit() {        mQueue.quit(false);    }public void quitSafely() {        mQueue.quit(true);    }

• Looper的quit和quitSafely调用的是MessageQueue的quit方法，不同之处在于，quit()方法直接退出Looper，quitSafely()只是设定 一个退出标记，把消息队列的已有消息处理完毕后才安全退出。MessageQueue的quit方法被调用后，next()方法就会返回null，loop方法就退出阻塞状态，所以Looper必须退出。

消息的处理

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

• 处理的优先级是Message的callback,然后是mCallback ，最后再是Handler的handleMessage，也就是我们需要重写的处理消息的逻辑。

简要分析一下开篇例子中的处理过程

• 首先当主线程中需要执行耗时任务时，开启新的子线程执行任务
• 子线程执行完任务后通过handler将处理的结果通过消息发送给主线程
• 主线程的MessageQueue收到消息，Looper（主线程中自带Looper)获取消息并通过msg.target.dispatchMessage方法调用handler的handleMessage方法执行我们的逻辑，比如更新UI