RecyclerView：SnapHelper了解一下（基于RecyclerView的Viewpager效果）

本效果基于SnapHelper的源码的基础上进行修改，区分处理了onFiling事件和onScroll事件：

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上代码：

private final RecyclerView.OnScrollListener mScrollListener = new RecyclerView.OnScrollListener() {
       boolean mScrolled = false;

       @Override
       public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
           super.onScrollStateChanged(recyclerView, newState);
// SCROLL_STATE_IDLE是当屏幕停止滚动时 0

   //SCROLL_STATE_TOUCH_SCROLL是当用户在以触屏方式滚动屏幕并且手指仍然还在屏幕上时 1

   //SCROLL_STATE_FLING是当用户由于之前划动屏幕并抬起手指，屏幕产生惯性滑动时 2
           if (newState != RecyclerView.SCROLL_STATE_IDLE) {

               return;
           }
//当滚动停止且产生位移的时候执行对齐方法
           if (mScrolled) {
               mScrolled = false;
               snapToTargetExistingView();
               return;
           }
//当不在位移的时候将当前的位置提供出来
           if (mPageListener != null) {
               mPageListener.onPageSelector(mCurrentPageIndex);
           }
       }

Scroll事件的处理逻辑

    /**
     * 获取当前应该显示第几页
     */
    private int getPagerIndex() {
        final int verticalScrollOffset = mRecyclerView.computeVerticalScrollOffset();
        final int horizontalScrollOffset = mRecyclerView.computeHorizontalScrollOffset();

        final int currentVerticalScrollOffset = mCurrentPageIndex * mRecyclerView.getMeasuredHeight();
        final int currentHorizontalScrollOffset = mCurrentPageIndex * mRecyclerView.getMeasuredWidth();

        int index = 0;
        if (mRecyclerView.getLayoutManager().canScrollVertically()) {

            //除掉整页距离之后的距离
            final float offset = verticalScrollOffset * 1.f % mRecyclerView.getMeasuredHeight();
            final float page = verticalScrollOffset * 1.f / mRecyclerView.getMeasuredHeight();//前面还有多少页

            index = (int) Math.floor(page);//前面还有多少页, 取整
            if (offset == 0) {
                return index;
            }

            if (currentVerticalScrollOffset <= verticalScrollOffset) {
                //向上滚动
                if (offset >= mRecyclerView.getMeasuredHeight() / 2) {
                    //超过屏幕 1/2 就开始滑动
                    index = mCurrentPageIndex + 1;
                } else {
                    index = mCurrentPageIndex;
                }

            } else {
                //向下滚动
                if (offset >= mRecyclerView.getMeasuredHeight() / 2) {
                    //超过屏幕 1/2 就开始滑动
                    index = mCurrentPageIndex;
                } else {
                    index = mCurrentPageIndex - 1;
                }
            }

        } else if (mRecyclerView.getLayoutManager().canScrollHorizontally()) {
            final float offset = horizontalScrollOffset * 1.f % mRecyclerView.getMeasuredWidth();
            final float page = horizontalScrollOffset * 1.f / mRecyclerView.getMeasuredWidth();
            index = (int) Math.floor(page);
            if (offset == 0) {
                return index;
            }

            if (currentHorizontalScrollOffset <= horizontalScrollOffset) {
                //向左滚动
                if (offset >= mRecyclerView.getMeasuredWidth() / 2) {
                    //超过一半的距离
                    index = mCurrentPageIndex + 1;
                } else {
                    index = mCurrentPageIndex;
                }

            } else {
                //向右滚动
                if (offset >= mRecyclerView.getMeasuredWidth() / 2) {
                    //超过一半的距离
                    index = mCurrentPageIndex;
                } else {
                    index = mCurrentPageIndex - 1;
                }
            }
        }
        return index;
    }

fling事件的处理逻辑

 /**
     * 当fling事件执行后找到fling最终的position
     *
     * @param layoutManager
     * @param velocityX
     * @param velocityY
     * @return
     */
    private int findTargetSnapPosition(LayoutManager layoutManager, int velocityX, int velocityY) {
        final int itemCount = layoutManager.getItemCount();
        if (itemCount == 0) {
            return RecyclerView.NO_POSITION;
        }

        int index = RecyclerView.NO_POSITION;

        if (layoutManager.canScrollHorizontally()) {
            if (velocityX > 0) {
                return mCurrentPageIndex + 1;
            } else {
                return mCurrentPageIndex - 1;
            }
        }

        if (layoutManager.canScrollVertically()) {
            if (velocityY > 0) {
                return mCurrentPageIndex + 1;
            } else {
                return mCurrentPageIndex - 1;
            }
        }
        return index;
    }

最终snap逻辑：

    /**
     * 滑动事件的真正执行
     *
     * @param layoutManager
     * @param position
     */
    private void handleScrollToPosition(LayoutManager layoutManager, int position) {

        int itemCount = layoutManager.getItemCount();

        if (itemCount == 0) {
            return;
        }

        if (position >= itemCount) {
            return;
        }

        mCurrentPageIndex = position;

        int[] snapDistance = calculateDistanceToFinalSnap(layoutManager);
        if (snapDistance[0] != 0 || snapDistance[1] != 0) {
            mRecyclerView.smoothScrollBy(snapDistance[0], snapDistance[1]);
        }
    }

计算真正需要滑动的距离：

    @SuppressWarnings("WeakerAccess")
    @Nullable
    public int[] calculateDistanceToFinalSnap(@NonNull LayoutManager layoutManager) {

        int[] out = new int[2];
        if (layoutManager.canScrollHorizontally()) {
            out[0] = mCurrentPageIndex * mRecyclerView.getMeasuredWidth() - mRecyclerView.computeHorizontalScrollOffset();
        } else {
            out[0] = 0;
        }

        if (layoutManager.canScrollVertically()) {
            out[1] = mCurrentPageIndex * mRecyclerView.getMeasuredHeight() - mRecyclerView.computeVerticalScrollOffset();
        } else {
            out[1] = 0;
        }
        return out;
    }