Handler 机制学习之相关类

1.Handler类

/**
 * A Handler allows you to send and process {@link Message} and Runnable
 * objects associated with a thread's {@link MessageQueue}.  Each Handler
 * instance is associated with a single thread and that thread's message
 * queue.  When you create a new Handler, it is bound to the thread /
 * message queue of the thread that is creating it -- from that point on,
 * it will deliver messages and runnables to that message queue and execute
 * them as they come out of the message queue.
 * 
 * <p>There are two main uses for a Handler: (1) to schedule messages and
 * runnables to be executed as some point in the future; and (2) to enqueue
 * an action to be performed on a different thread than your own.
 * <p>When posting or sending to a Handler, you can either
 * allow the item to be processed as soon as the message queue is ready
 * to do so, or specify a delay before it gets processed or absolute time for
 * it to be processed.  The latter two allow you to implement timeouts,
 * ticks, and other timing-based behavior.
 * 
 * <p>When a
 * process is created for your application, its main thread is dedicated to
 * running a message queue that takes care of managing the top-level
 * application objects (activities, broadcast receivers, etc) and any windows
 * they create.  You can create your own threads, and communicate back with
 * the main application thread through a Handler.  This is done by calling
 * the same <em>post</em> or <em>sendMessage</em> methods as before, but from
 * your new thread.  The given Runnable or Message will then be scheduled
 * in the Handler's message queue and processed when appropriate.
 */

体验下原汁原味~

/**
     * Subclasses must implement this to receive messages.  子类实现这个方法来处理消息
     */
    public void handleMessage(Message msg) {
    }

/**
     * Default constructor associates this handler with the {@link Looper} for the
     * current thread.
     *如果线程没有一个Looper,则不能接受消息，否则会有异常
     * If this thread does not have a looper, this handler won't be able to receive messages
     * so an exception is thrown.
     */
    public Handler() {
        this(null, false);
    }

最终，调用的构造函数：

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();  //得到当前Looper
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;      //初始化MessageQueue对象
        mCallback = callback;         //初始化Callback对象
        mAsynchronous = async;        //是否异步
    }

/**
     * Returns a new {@link android.os.Message Message} from the global message pool. More efficient than
     * creating and allocating new instances. The retrieved message has its handler set to this instance (Message.target == this).
     *  If you don't want that facility, just call Message.obtain() instead.
     */
    public final Message obtainMessage()                    //还有很多重载的方法，就不一一列了
    {
        return Message.obtain(this);
    }

/**
     * Causes the Runnable r to be added to the message queue.
     * The runnable will be run on the thread to which this handler is 
     * attached. 
     *  
     * @param r The Runnable that will be executed.
     * 
     * @return Returns true if the Runnable was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */

public final boolean post(Runnable r)               //把一个Runnable对象加入到消息队列。 
    {
       return  sendMessageDelayed(getPostMessage(r), 0);    //getPostMessage:把Runnable对象转换成Message对象
    }

/**
     * Causes the Runnable r to be added to the message queue, to be run
     * at a specific time given by <var>uptimeMillis</var>.
     * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
     * The runnable will be run on the thread to which this handler is attached.
     *
     */
    public final boolean postAtTime(Runnable r, long uptimeMillis)
    {
        return sendMessageAtTime(getPostMessage(r), uptimeMillis);
    }

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

<pre name="code" class="java">//把Post中传入的参数Runnable对象转换成Message对象。

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

/**
     * Pushes a message onto the end of the message queue after all pending messages
     * before the current time. It will be received in {@link #handleMessage},
     * in the thread attached to this handler.
     *  
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */
    public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }

/**
     * Sends a Message containing only the what value.
     *  
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */
    public final boolean sendEmptyMessage(int what)
    {
        return sendEmptyMessageDelayed(what, 0);
    }

/**
     * Remove any pending posts of messages with code 'what' that are in the
     * message queue.
     */
    public final void removeMessages(int what) {
        mQueue.removeMessages(this, what, null);
    }

/**
     * Check if there are any pending posts of messages with code 'what' in
     * the message queue.
     */
    public final boolean hasMessages(int what) {
        return mQueue.hasMessages(this, what, null);
    }

public final Looper getLooper() {
        return mLooper;
    }

/**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {       //处理Runnable的消息
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {      
                    return;
                }
            }
            handleMessage(msg);      //处理message消息
        }
    }

private static void handleCallback(Message message) {
        message.callback.run();             //对于Runnable消息，不跑handleMessage，而是跑Runnable中的run方法
    }

Handler主要负责发送消息和处理消息。

2.Looper类

/**
  * Class used to run a message loop for a thread.  Threads by default do
  * not have a message loop associated with them; to create one, call
  * {@link #prepare} in the thread that is to run the loop, and then
  * {@link #loop} to have it process messages until the loop is stopped.
  * 
  * <p>Most interaction with a message loop is through the
  * {@link Handler} class.
  * 
  * <p>This is a typical example of the implementation of a Looper thread,
  * using the separation of {@link #prepare} and {@link #loop} to create an
  * initial Handler to communicate with the Looper.
  *
  * <pre>
  *  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();
  *      }
  *  }</pre>
  */

/** Initialize the current thread as a looper.
      * This gives you a chance to create handlers that then reference
      * this looper, before actually starting the loop. Be sure to call
      * {@link #loop()} after calling this method, and end it by calling
      * {@link #quit()}.
      */
    public static void prepare() {
        prepare(true);
    }

    private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {                  //如果已经有一个Looper了的话就会报错。说明一个线程只能有一个Looper
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));      //为当前线程创建一个Looper。
    }

/**
     * Initialize the current thread as a looper, marking it as an
     * application's main looper. The main looper for your application
     * is created by the Android environment, so you should never need
     * to call this function yourself.  See also: {@link #prepare()}
     */
    public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

/** Returns the application's main looper, which lives in the main thread of the application.
     */
    public static Looper getMainLooper() {
        synchronized (Looper.class) {
            return sMainLooper;
        }
    }

/**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static void loop() {                       //是一个死循环，一直取出消息
        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 (;;) {                   //死循环
            Message msg = queue.next(); // might block  取出消息
            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);
            }

            msg.target.dispatchMessage(msg);    //分发消息

            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.recycle();           
        }
    }

/**
     * Return the Looper object associated with the current thread.  Returns
     * null if the calling thread is not associated with a Looper.
     */
    public static Looper myLooper() {              //得到当前线程的Looper
        return sThreadLocal.get();
    }

/**
     * Quits the looper.
     * <p>
     * Causes the {@link #loop} method to terminate without processing any
     * more messages in the message queue.
     * </p><p>
     * Any attempt to post messages to the queue after the looper is asked to quit will fail.
     * For example, the {@link Handler#sendMessage(Message)} method will return false.
     * </p><p class="note">
     * Using this method may be unsafe because some messages may not be delivered
     * before the looper terminates.  Consider using {@link #quitSafely} instead to ensure
     * that all pending work is completed in an orderly manner.
     * </p>
     *
     * @see #quitSafely
     */
    public void quit() {             //结束当前的Looper<span style="font-family: 'Microsoft YaHei UI';">不要在本线程中调用quit()方法，因为永远都无法执行到这里。只能在其他线程调用。</span>
        mQueue.quit(false);
    }

/**
     * Return the {@link MessageQueue} object associated with the current
     * thread.  This must be called from a thread running a Looper, or a
     * NullPointerException will be thrown.
     */
    public static MessageQueue myQueue() {
        return myLooper().mQueue;
    }

private Looper(boolean quitAllowed) {          //创建一个Looper，为私有构造方法，说明在外面不能创建一个Looper对象。
        mQueue = new MessageQueue(quitAllowed);      //创建Looper的时候就会创建MessageQueue。
        mThread = Thread.currentThread();
    }

Looper主要负责管理线程中的消息队列（MessageQueue），取出消息，分发消息。

3.Message类

/**
 * 
 * Defines a message containing a description and arbitrary data object that can be
 * sent to a {@link Handler}.  This object contains two extra int fields and an
 * extra object field that allow you to not do allocations in many cases.  
 *
 * <p class="note">While the constructor of Message is public, the best way to get
 * one of these is to call {@link #obtain Message.obtain()} or one of the
 * {@link Handler#obtainMessage Handler.obtainMessage()} methods, which will pull
 * them from a pool of recycled objects.</p>
 */

public final class Message implements Parcelable               //实现了Parcelable接口

/**
     * User-defined message code so that the recipient can identify 
     * what this message is about. Each {@link Handler} has its own name-space
     * for message codes, so you do not need to worry about yours conflicting
     * with other handlers.
     */
    public int what;

    /**
     * arg1 and arg2 are lower-cost alternatives to using
     * {@link #setData(Bundle) setData()} if you only need to store a
     * few integer values.
     */
    public int arg1; 

    /**
     * arg1 and arg2 are lower-cost alternatives to using
     * {@link #setData(Bundle) setData()} if you only need to store a
     * few integer values.
     */
    public int arg2;

/**
     * Return a new Message instance from the global pool. Allows us to
     * avoid allocating new objects in many cases.
     */
    public static Message obtain() {     //得到一条消息。还有很多重载的 就不贴了
        synchronized (sPoolSync) {
            if (sPool != null) {
                Message m = sPool;
                sPool = m.next;
                m.next = null;
                sPoolSize--;
                return m;
            }
        }
        return new Message();
    }

/**
     * Return a Message instance to the global pool.  You MUST NOT touch
     * the Message after calling this function -- it has effectively been
     * freed.
     */
    public void recycle() {
        clearForRecycle();

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

/** 
     * Obtains a Bundle of arbitrary data associated with this
     * event, lazily creating it if necessary. Set this value by calling
     * {@link #setData(Bundle)}.  Note that when transferring data across
     * processes via {@link Messenger}, you will need to set your ClassLoader
     * on the Bundle via {@link Bundle#setClassLoader(ClassLoader)
     * Bundle.setClassLoader()} so that it can instantiate your objects when
     * you retrieve them.
     * @see #peekData()
     * @see #setData(Bundle)
     */
    public Bundle getData() {
        if (data == null) {
            data = new Bundle();
        }
        
        return data;
    }

/**
     * Sets a Bundle of arbitrary data values. Use arg1 and arg1 members 
     * as a lower cost way to send a few simple integer values, if you can.
     * @see #getData() 
     * @see #peekData()
     */
    public void setData(Bundle data) {
        this.data = data;
    }

Message类主要就是描述一条消息。

4.MessageQueue类

/**
 * Low-level class holding the list of messages to be dispatched by a
 * {@link Looper}.  Messages are not added directly to a MessageQueue,
 * but rather through {@link Handler} objects associated with the Looper.
 * 
 * <p>You can retrieve the MessageQueue for the current thread with
 * {@link Looper#myQueue() Looper.myQueue()}.
 */

MessageQueue类就是消息队列。