ItemTouchHelper实现部分响应拖动

/**
     * Starts dragging the provided ViewHolder. By default, ItemTouchHelper starts a drag when a
     * View is long pressed. You can disable that behavior by overriding
     * {@link ItemTouchHelper.Callback#isLongPressDragEnabled()}.
     拖动一个ItemTouchHelper绑定的RV的时候，默认实现是拖动，你可以通过重写Callback#isLongPressDragEnabled()
     方法来禁用这种手势响应
     * <p>
     * For this method to work: 下面几条是startDrag的使用前提
     * <ul>
     * <li>The provided ViewHolder must be a child of the RecyclerView to which this
     * ItemTouchHelper 首先：入参vh必须是对应绑定ItemTouchHelper的RV，额~ 废话
     * is attached.</li>
     * <li>{@link ItemTouchHelper.Callback} must have dragging enabled.</li> 
        其次：CallBack必须可拖拉？ 
     * <li>There must be a previous touch event that was reported to the ItemTouchHelper
     * through RecyclerView's ItemTouchListener mechanism. As long as no other ItemTouchListener
     * grabs previous events, this should work as expected.</li>
        最后： touch时间必须能正确得汇报至ItemTouchHelper。也就是说要能正确进入mItemTouchListener的
        onInterceptTouchEvent方法等，确保没有其他RV关联的ItemTouchListener抢占了之前的时间，这方法
        就能正确工作了
     *
     * For example, if you would like to let your user to be able to drag an Item by touching one
     * of its descendants, you may implement it as follows:
       想让你item里的一部分响应点击？ OK 按下面这么重写
     * <pre>
     *     viewHolder.dragButton.setOnTouchListener(new View.OnTouchListener() {
     *         public boolean onTouch(View v, MotionEvent event) {
     *             if (MotionEventCompat.getActionMasked(event) == MotionEvent.ACTION_DOWN) {
     *                 mItemTouchHelper.startDrag(viewHolder);
     *             }
     *             return false;
     *         }
     *     });
     * </pre>
     * <p>
     *
     * @param viewHolder The ViewHolder to start dragging. It must be a direct child of
     *                   RecyclerView.
     * @see ItemTouchHelper.Callback#isItemViewSwipeEnabled()
     */
    public void startDrag(ViewHolder viewHolder) {
        if (!mCallback.hasDragFlag(mRecyclerView, viewHolder)) {
            Log.e(TAG, "Start drag has been called but swiping is not enabled");
            return;
        }
        if (viewHolder.itemView.getParent() != mRecyclerView) {
            Log.e(TAG, "Start drag has been called with a view holder which is not a child of "
                    + "the RecyclerView which is controlled by this ItemTouchHelper.");
            return;
        }
        obtainVelocityTracker();
        mDx = mDy = 0f;
        select(viewHolder, ACTION_STATE_DRAG);
    }