Android Handler机制5之Message简介与消息对象对象池

    /**
     * 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;

    /**
     * An arbitrary object to send to the recipient.  When using
     * {@link Messenger} to send the message across processes this can only
     * be non-null if it contains a Parcelable of a framework class (not one
     * implemented by the application).   For other data transfer use
     * {@link #setData}.
     * 
     * <p>Note that Parcelable objects here are not supported prior to
     * the {@link android.os.Build.VERSION_CODES#FROYO} release.
     */
    public Object obj;

    // 回复跨进程的Messenger 
    public Messenger replyTo;

    // Messager发送这的Uid
    public int sendingUid = -1;

    // 正在使用的标志值 表示当前Message 正处于使用状态，当Message处于消息队列中、处于消息池中或者Handler正在处理Message的时候，它就处于使用状态。
    /*package*/ static final int FLAG_IN_USE = 1 << 0;

    // 异步标志值 表示当前Message是异步的。
    /*package*/ static final int FLAG_ASYNCHRONOUS = 1 << 1;

    // 消息标志值 在调用copyFrom()方法时，该常量将会被设置，其值其实和FLAG_IN_USE一样
    /*package*/ static final int FLAGS_TO_CLEAR_ON_COPY_FROM = FLAG_IN_USE;

    // 消息标志，上面三个常量 FLAG 用在这里
    /*package*/ int flags;

    // 用于存储发送消息的时间点，以毫秒为单位
    /*package*/ long when;

    // 用于存储比较复杂的数据
    /*package*/ Bundle data;
    
    // 用于存储发送当前Message的Handler对象，前面提到过Handler其实和Message相互持有引用的
    /*package*/ Handler target;
    
    // 用于存储将会执行的Runnable对象，前面提到过除了handlerMessage(Message msg)方法，你也可以使用Runnable执行操作，要注意的是这种方法并不会创建新的线程。
    /*package*/ Runnable callback;
 
    // 指向下一个Message，也就是线程池其实是一个链表结构
    /*package*/ Message next;

    // 该静态变量仅仅是为了给同步块提供一个锁而已
    private static final Object sPoolSync = new Object();

    //该静态的Message是整个线程池链表的头部，通过它才能够逐个取出对象池的Message
    private static Message sPool;

    // 该静态变量用于记录对象池中的Message的数量，也就是链表的长度
    private static int sPoolSize = 0;
  
    // 设置了对象池中的Message的最大数量，也就是链表的最大长度
    private static final int MAX_POOL_SIZE = 50;

     //该版本系统是否支持回收标志位
    private static boolean gCheckRecycle = true;

    /** Constructor (but the preferred way to get a Message is to call {@link #obtain() Message.obtain()}).
    */
    public Message() {
    }

    /**
     * Return a new Message instance from the global pool. Allows us to
     * avoid allocating new objects in many cases.
     */
    public static Message obtain() {
       // 保证线程安全
        synchronized (sPoolSync) {
            if (sPool != null) {
                Message m = sPool;
                sPool = m.next;
                m.next = null;
                 // flags为移除使用标志
                m.flags = 0; // clear in-use flag
                sPoolSize--;
                return m;
            }
        }
        return new Message();
    }

   private static Message sPool;

   /**
     * Recycles a Message that may be in-use.
     * Used internally by the MessageQueue and Looper when disposing of queued Messages.
     */
    void recycleUnchecked() {
        // Mark the message as in use while it remains in the recycled object pool.
        // Clear out all other details.
        // 添加正在使用标志位，其他情况就除掉
        flags = FLAG_IN_USE;
        what = 0;
        arg1 = 0;
        arg2 = 0;
        obj = null;
        replyTo = null;
        sendingUid = -1;
        when = 0;
        target = null;
        callback = null;
        data = null;
        //拿到同步锁，以避免线程不安全
        synchronized (sPoolSync) {
            if (sPoolSize < MAX_POOL_SIZE) {
                next = sPool;
                sPool = this;
                sPoolSize++;
            }
        }
    }

void recycleUnchecked() {
                ...
        if (sPoolSize < MAX_POOL_SIZE) {
                // 第一步
                next = sPool;
                 // 第二步
                sPool = this;
                 // 第三步
                sPoolSize++;
                 ...
         }
    }

public static Message obtain() {
    synchronized (sPoolSync) {
        //第一步
        if (sPool != null) {
            // 第二步
            Message m = sPool;
            // 第三步
            sPool = m.next;
            // 第四步
            m.next = null;
            // 第五步
            m.flags = 0; 
            // 第六步
            sPoolSize--;
            return m;
        }
    }
}

   /**
     * Same as {@link #obtain()}, but copies the values of an existing
     * message (including its target) into the new one.
     * @param orig Original message to copy.
     * @return A Message object from the global pool.
     */
    public static Message obtain(Message orig) {
        Message m = obtain();
        m.what = orig.what;
        m.arg1 = orig.arg1;
        m.arg2 = orig.arg2;
        m.obj = orig.obj;
        m.replyTo = orig.replyTo;
        m.sendingUid = orig.sendingUid;
        if (orig.data != null) {
            m.data = new Bundle(orig.data);
        }
        m.target = orig.target;
        m.callback = orig.callback;

        return m;
    }

     /**
     * Same as {@link #obtain()}, but sets the value for the <em>target</em> member on the Message returned.
     * @param h  Handler to assign to the returned Message object's <em>target</em> member.
     * @return A Message object from the global pool.
     */
    public static Message obtain(Handler h) {
        Message m = obtain();
        m.target = h;

        return m;
    }

    /**
     * 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;
    }

    /**
     * Same as {@link #obtain()}, but sets the values for both <em>target</em> and
     * <em>what</em> members on the Message.
     * @param h  Value to assign to the <em>target</em> member.
     * @param what  Value to assign to the <em>what</em> member.
     * @return A Message object from the global pool.
     */
    public static Message obtain(Handler h, int what) {
        Message m = obtain();
        m.target = h;
        m.what = what;

        return m;
    }

    /**
     * Same as {@link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>, and <em>obj</em>
     * members.
     * @param h  The <em>target</em> value to set.
     * @param what  The <em>what</em> value to set.
     * @param obj  The <em>object</em> method to set.
     * @return  A Message object from the global pool.
     */
    public static Message obtain(Handler h, int what, Object obj) {
        Message m = obtain();
        m.target = h;
        m.what = what;
        m.obj = obj;

        return m;
    }

    /**
     * Same as {@link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>, 
     * <em>arg1</em>, and <em>arg2</em> members.
     * 
     * @param h  The <em>target</em> value to set.
     * @param what  The <em>what</em> value to set.
     * @param arg1  The <em>arg1</em> value to set.
     * @param arg2  The <em>arg2</em> value to set.
     * @return  A Message object from the global pool.
     */
    public static Message obtain(Handler h, int what, int arg1, int arg2) {
        Message m = obtain();
        m.target = h;
        m.what = what;
        m.arg1 = arg1;
        m.arg2 = arg2;

        return m;
    }

    /**
     * Same as {@link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>, 
     * <em>arg1</em>, <em>arg2</em>, and <em>obj</em> members.
     * 
     * @param h  The <em>target</em> value to set.
     * @param what  The <em>what</em> value to set.
     * @param arg1  The <em>arg1</em> value to set.
     * @param arg2  The <em>arg2</em> value to set.
     * @param obj  The <em>obj</em> value to set.
     * @return  A Message object from the global pool.
     */
    public static Message obtain(Handler h, int what, 
            int arg1, int arg2, Object obj) {
        Message m = obtain();
        m.target = h;
        m.what = what;
        m.arg1 = arg1;
        m.arg2 = arg2;
        m.obj = obj;

        return m;
    }

    /**
     * Make this message like o.  Performs a shallow copy of the data field.
     * Does not copy the linked list fields, nor the timestamp or
     * target/callback of the original message.
     */
    public void copyFrom(Message o) {
        this.flags = o.flags & ~FLAGS_TO_CLEAR_ON_COPY_FROM;
        this.what = o.what;
        this.arg1 = o.arg1;
        this.arg2 = o.arg2;
        this.obj = o.obj;
        this.replyTo = o.replyTo;
        this.sendingUid = o.sendingUid;

        if (o.data != null) {
            this.data = (Bundle) o.data.clone();
        } else {
            this.data = null;
        }
    }