Pool：小對象緩存or複用

對象複用

使用鏈表做爲pool來保存要複用的對象。

• pool字段
• obtain
• recycle

案例1

android.os.Message

private static Message sPool;
 Message msg = mHandler.obtainMessage(MSG_PROCESS_INPUT_EVENTS);

   /**
     * Return a new Message instance from the global pool. Allows us to
     * avoid allocating new objects in many cases.
     */
    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();
    }

案例2

android.view.ViewRootImpl

private QueuedInputEvent mQueuedInputEventPool;

QueuedInputEvent q = obtainQueuedInputEvent(event, receiver, flags);


    private QueuedInputEvent obtainQueuedInputEvent(InputEvent event,
            InputEventReceiver receiver, int flags) {
        QueuedInputEvent q = mQueuedInputEventPool;
        if (q != null) {
            mQueuedInputEventPoolSize -= 1;
            mQueuedInputEventPool = q.mNext;
            q.mNext = null;
        } else {
            q = new QueuedInputEvent();
        }

        q.mEvent = event;
        q.mReceiver = receiver;
        q.mFlags = flags;
        return q;
    }

案例3

android.view.MotionEvent

private static MotionEvent gRecyclerTop;

  static private MotionEvent obtain() {
        final MotionEvent ev;
        synchronized (gRecyclerLock) {
            ev = gRecyclerTop;
            if (ev == null) {
                return new MotionEvent();
            }
            gRecyclerTop = ev.mNext;
            gRecyclerUsed -= 1;
        }
        ev.mNext = null;
        ev.prepareForReuse();
        return ev;
    }

    /**
     * Recycle the MotionEvent, to be re-used by a later caller.  After calling
     * this function you must not ever touch the event again.
     */
    @Override
    public final void recycle() {
        super.recycle();

        synchronized (gRecyclerLock) {
            if (gRecyclerUsed < MAX_RECYCLED) {
                gRecyclerUsed++;
                mNext = gRecyclerTop;
                gRecyclerTop = this;
            }
        }
    }