カスタムRecyclerView.LayoutManager

public class PaddingLayoutManager extends RecyclerView.LayoutManager {

    /**
     * constant
     */
    private static final String TAG = "PaddingLayoutManager";
    private static final int ITEM_WIDTH = DeviceUtil.getScreenWidth() / 5;//child view    

    /**
     * data
     */
    private SparseArray mAllRectList = new SparseArray<>();//    item     
    private SparseBooleanArray mAllStateList = new SparseBooleanArray();//    item           
    private int mTotalWidth = 0;
    private int mAllOffset = 0;

    /**
     *          
     */
    public int getMiddlePos() {
        int prePos = mAllOffset / ITEM_WIDTH;
        int restOffset = mAllOffset % ITEM_WIDTH;

        //            
        if (restOffset >= ITEM_WIDTH / 2) {
            prePos += 1;
        }
        return prePos + 2;
    }

    @Override
    public RecyclerView.LayoutParams generateDefaultLayoutParams() {
        return new RecyclerView.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
    }

    @Override
    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
        Log.w(TAG, "onLayoutChildren");
        if (getItemCount() <= 0 || state.isPreLayout()) {
            return;
        }
        //getItemCount()     adapter getItemCount()  ，             ， getChildCount()              View   。
        if (getChildCount() == 0) {
            //     View  RecyclerView detach ，     "Scrap"  ，    View        (    )。     View   Recycler       
            detachAndScrapAttachedViews(recycler);
            calculateChildrenSite(recycler, state);
        }
    }

    /**
     *           ItemView   
     */
    private void calculateChildrenSite(RecyclerView.Recycler recycler, RecyclerView.State state) {
        //    
        for (int i = 0; i < getItemCount(); i++) {
            View view = recycler.getViewForPosition(i);
            addView(view);//     detach  ，      
            //  ItemView   。   measureChildWithMargins(view, 0, 0);
            measureChildWithMargins(view, ITEM_WIDTH, 0);
            //  ItemView   ，   ItemDecorator。
            calculateItemDecorationsForChild(view, new Rect());

            //  child view   
            Rect tempRect = mAllRectList.get(i);
            if (tempRect == null) {
                tempRect = new Rect();
            }
            int width = getDecoratedMeasuredWidth(view);
            tempRect.set(mTotalWidth, 0, mTotalWidth + width, getHeight());
            mTotalWidth += width;

            //  ItemView     ,     detachAndScrapAttachedViews(recycler)，  item     
            mAllRectList.put(i, tempRect);
            mAllStateList.put(i, false);
        }
        //    
        recycleAndFillView(recycler, state);
    }

    @Override
    public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
        return super.scrollVerticallyBy(dy, recycler, state);
    }

    @Override
    public int scrollHorizontallyBy(int dx, RecyclerView.Recycler recycler, RecyclerView.State state) {
        //            View，    recycleAndFillView()   addView()。
        detachAndScrapAttachedViews(recycler);
        //    dy  ，    dx  
        int travel = dx;//       

        //     
        if (mAllOffset + dx < 0) {
            travel = -mAllOffset;
            //     
        } else if (mAllOffset + dx > mTotalWidth - getHorizontalSpace()) {
            travel = mTotalWidth - getHorizontalSpace() - mAllOffset;
        }
        //        view x         
        offsetChildrenHorizontal(-travel);
        recycleAndFillView(recycler, state); //     View
        //          +travel
        mAllOffset += travel;
        return travel;
    }

    /**
     *   item
     */
    private void recycleAndFillView(RecyclerView.Recycler recycler, RecyclerView.State state) {
        if (getItemCount() <= 0 || state.isPreLayout()) {
            return;
        }

        //   scroll offset        
        Rect displayRect = new Rect(mAllOffset, 0, mAllOffset + getHorizontalSpace(), getVerticalSpace());

        //      Items   Recycle   
        Rect childRect = new Rect();
        for (int i = 0; i < getChildCount(); i++) {
            //        RecyclerView  View，    Recycler  View
            //        View
            View child = getChildAt(i);
            //            View          ，  ItemDecorator
            if (child != null) {
                childRect.left = getDecoratedLeft(child);
                childRect.top = getDecoratedTop(child);
                childRect.right = getDecoratedRight(child);
                childRect.bottom = getDecoratedBottom(child);
            }
            //  Item       ，       
            if (!Rect.intersects(displayRect, childRect)) {
                //           View
                if (child != null) {
                    removeAndRecycleView(child, recycler);
                }
                mAllStateList.put(i, false); //   View       
            }
        }

        //             View
        for (int i = 0; i < getItemCount(); i++) {
            //  ItemView              
            if (Rect.intersects(displayRect, mAllRectList.get(i))) {
                //  Recycler    View
                View itemView = recycler.getViewForPosition(i);
                measureChildWithMargins(itemView, ITEM_WIDTH, 0);
                //  View RecyclerView 
                addView(itemView);
                //       ItemView     
                Rect rect = mAllRectList.get(i);
                //   item    ,    item            ，     y        
                int itemMiddle = (rect.left + rect.right - 2 * mAllOffset) / 2;
                int itemXOffSet = Math.abs(DeviceUtil.getScreenWidth() / 2 - itemMiddle);
                int itemYOffset = (int) ((itemXOffSet * 1.0f / (DeviceUtil.getScreenWidth() / 2)) * (getHeight() - 40));
                layoutDecoratedWithMargins(itemView, rect.left - mAllOffset, rect.top - itemYOffset, rect.right - mAllOffset, rect.bottom - itemYOffset);
                mAllStateList.put(i, true); //   View      
            }
        }
    }

    @Override
    public boolean canScrollHorizontally() {
        //  true        
        return true;
    }

    @Override
    public boolean canScrollVertically() {
        //  true        
        return super.canScrollVertically();
    }

    /**
     *   RecyclerView     (    )，    Padding 
     */
    private int getVerticalSpace() {
        return getHeight() - getPaddingBottom() - getPaddingTop();
    }

    /**
     *   RecyclerView     (    )，    Padding 
     */
    private int getHorizontalSpace() {
        return getWidth() - getPaddingLeft() - getPaddingRight();
    }
}