这RecyclerView的特效,看了直呼牛批

原创

用户9239674

发布于 2021-12-08 11:53:26

4800

发布于 2021-12-08 11:53:26

文章被收录于专栏：用户9239674的专栏

/ 前言 /

还是老套路,先来看看实现的效果

图片

在写这个效果之前,需要熟悉Rv的回收复用机制,因为实现这个效果,需要自定义LayoutManager()…

众所周知,RecyclerView 是一个可滑动的View,那么他的回收/复用入口一定是在onTouchEvent()事件中

滑动过程中响应的是MotionEvent.ACTION_MOVE事件,所以直接来这里找找看!!

/ 缓存机制 /

onTouchEvent()入口

#RecyclerView.java

 @Override
public boolean onTouchEvent(MotionEvent e) {

    final int action = e.getActionMasked();
     switch (action) {
            ........................................
               ........只展示代码思路,细节请自行查看........
               ........................................

            case MotionEvent.ACTION_MOVE: {
            if (mScrollState == SCROLL_STATE_DRAGGING) {
                    mLastTouchX = x - mScrollOffset[0];
                    mLastTouchY = y - mScrollOffset[1];

                    // 关键代码1
                    if (scrollByInternal(
                            canScrollHorizontally ? dx : 0,
                            canScrollVertically ? dy : 0,
                            vtev)) {
                        getParent().requestDisallowInterceptTouchEvent(true);
                    }
                    if (mGapWorker != null && (dx != 0 || dy != 0)) {
                        mGapWorker.postFromTraversal(this, dx, dy);
                    }
                }
            }
            break;
    }
}

接着找scrollByInternal(int x, int y, MotionEvent ev)方法

#RecyclerView.java

boolean scrollByInternal(int x, int y, MotionEvent ev) {
     if (mAdapter != null) {
            ........................................
               ........只展示代码思路,细节请自行查看........
               ........................................
            if (x != 0) {
                // 关键代码2 去到 LinearLayoutManager 执行fill方法
                consumedX = mLayout.scrollHorizontallyBy(x, mRecycler, mState);
                unconsumedX = x - consumedX;
            }
            if (y != 0) {
                // 关键代码2 去到LinearLayoutManager 执行fill方法
                consumedY = mLayout.scrollVerticallyBy(y, mRecycler, mState);
                unconsumedY = y - consumedY;
            }
        }
        ....
}

现在走到了mLayout.scrollHorizontallyBy(x, mRecycler, mState);

接着去LinearLayoutManager() 中去找scrollHorizontallyBy() 方法

#LinearLayoutManager.java

    @Override
    public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler,
                                  RecyclerView.State state) {
        if (mOrientation == HORIZONTAL) {
            return 0;
        }
        // 关键代码3
        return scrollBy(dy, recycler, state);
    }

scrollBy()->

#LinearLayoutManager.java

 int scrollBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
       ........................................
     ........只展示代码思路,细节请自行查看........
     ........................................
     final int consumed = mLayoutState.mScrollingOffset
                // 关键代码4
                + fill(recycler, mLayoutState, state, false);
}

接着找到fill()方法

#LinearLayoutManager.java

int fill(RecyclerView.Recycler recycler, LayoutState layoutState,
             RecyclerView.State state, boolean stopOnFocusable) {

        if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {

            // 关键代码19 缓存ViewHolder
            recycleByLayoutState(recycler, layoutState);
        }

        // 循环调用
        while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {

           // 关键代码5 [用来4级复用]
            layoutChunk(recycler, state, layoutState, layoutChunkResult);

                  ........................................
                    ........只展示代码思路,细节请自行查看........
                  ........................................
        }

    }

看到这里只需要记住以下两点即可:

recycleByLayoutState(recycler, layoutState); 缓存ViewHolder
layoutChunk(recycler, state, layoutState, layoutChunkResult); 四级复用

有人可能会问,这里为什么是四级?不是说的三级嘛?

其实三级和四级都无所谓,知识点是不会变的,只是层级越多,理解就越深刻,越细罢了

直接进入到缓存的代码:

#LinearLayoutManager.java

 private void recycleByLayoutState(RecyclerView.Recycler recycler, LayoutState layoutState) {
        if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {
            // 关键代码21 缓存底部
            recycleViewsFromEnd(recycler, layoutState.mScrollingOffset);
        } else {
            // 关键代码20 缓存头部
            recycleViewsFromStart(recycler, layoutState.mScrollingOffset);
        }
    }

这里如果是向下滑动,就会缓存头部那么就会执行到 recycleViewsFromStart()

如果是向上滑动,就会缓存尾部那么就会执行到recycleViewsFromEnd()

recycleViewsFromStart() 和 recycleViewsFromEnd() 随便点开一个看看,里面代码都差不多一样.

#LinearLayoutManager.java

 private void recycleViewsFromStart(RecyclerView.Recycler recycler, int dt) {

        if (mShouldReverseLayout) {
            for (int i = childCount - 1; i >= 0; i--) {
            ...
                    // 关键代码22
                    recycleChildren(recycler, childCount - 1, i);
                    return;
            }
        } else {
            for (int i = 0; i < childCount; i++) {
            ...
                    // 关键代码23
                    recycleChildren(recycler, 0, i);
                    return;
            }
        }
    }

这里无论走哪一个if() 都会走到recycleChildren()方法

#LinearLayoutManager.java

private void recycleChildren(RecyclerView.Recycler recycler, int startIndex, int endIndex) {
        if (startIndex == endIndex) {
            return;
        }
        if (endIndex > startIndex) {
            for (int i = endIndex - 1; i >= startIndex; i--) {
                // 移除View  关键代码23 [执行到RecyclerView.removeAndRecycleViewAt()]
                removeAndRecycleViewAt(i, recycler);
            }
        } else {
            for (int i = startIndex; i > endIndex; i--) {
                removeAndRecycleViewAt(i, recycler);
            }
        }
    }

接着这里会执行到RecyclerView的removeAndRecycleViewAt()方法

#RecyclerView.java

        // 关键代码24
        public void removeAndRecycleViewAt(int index, Recycler recycler) {
            final View view = getChildAt(index);
            removeViewAt(index);
            // 关键代码25
            recycler.recycleView(view);
        }

继续往下执行

#RecyclerView.java

 public void recycleView(View view) {
            .......
            ViewHolder holder = getChildViewHolderInt(view);
            // 缓存
            recycleViewHolderInternal(holder);
        }

接着继续执行recycleViewHolderInternal()

#RecyclerView.java

void recycleViewHolderInternal(ViewHolder holder) {
            ........................................
            ........只展示代码思路,细节请自行查看........
            ........................................
             boolean cached = false;

            if (forceRecycle || holder.isRecyclable()) {
                // mViewCacheMax = 缓存的最大值 
                // mViewCacheMax = 2
                // 如果viewHolder是无效、未被移除、未被标记的
                if (mViewCacheMax > 0
                        && !holder.hasAnyOfTheFlags(ViewHolder.FLAG_INVALID
                        | ViewHolder.FLAG_REMOVED
                        | ViewHolder.FLAG_UPDATE
                        | ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN)) {
                    int cachedViewSize = mCachedViews.size();

                    // 关键代码24
                    // mViewCacheMax = 2
                    if (cachedViewSize >= mViewCacheMax && cachedViewSize > 0) {
                        // 如果viewholder存满2个则移除第0个位置 
                        // 保证mCachedViews 最多能缓存2个ViewHolder
                        recycleCachedViewAt(0);
                        cachedViewSize--;
                    }
                    ....
                    // 保存ViewHolder数据 [mCachedViews数据不会超过2个]
                    mCachedViews.add(targetCacheIndex, holder);
                    cached = true;
                }
                 if (!cached) {
                    // 当ViewHolder不改变时候(只有一个ViewHolder) 就会直接存到缓存池中
                    addViewHolderToRecycledViewPool(holder, true);
                    recycled = true;
                }
                ........................................
                   ........只展示代码思路,细节请自行查看........
                ........................................
        }

通过 ****关键代码24**** 可知,mCachedViews 最多能保存2个ViewHolder

如果第三个ViewHolder来临的时候,就会先删除掉第0个,然后在 mCachedViews.add(targetCacheIndex, holder);

然后再来看看 recycleCachedViewAt(0)的细节!

#RecyclerView.java

    void recycleCachedViewAt(int cachedViewIndex) {
            ...
            ViewHolder viewHolder = mCachedViews.get(cachedViewIndex);

            // 关键代码25
            // 添加到ViewPool到缓存里面取
            addViewHolderToRecycledViewPool(viewHolder, true);

            // 将第0个ViewHolder移除
            mCachedViews.remove(cachedViewIndex);
        }

继续执行到 addViewHolderToRecycledViewPool()方法

将mCachedViews.get(0)中的ViewHolder获取出来,添加到缓存池中,并删除

#RecyclerView.java

void addViewHolderToRecycledViewPool(ViewHolder holder, boolean dispatchRecycled) {
            .....
            // 向缓存池中 保存ViewHolder 关键代码28
            getRecycledViewPool().putRecycledView(holder);
        }

点进来看看putRecycledView()方法

#RecyclerView.java

// SparseArray 类似与 HashMap<int,ScrapData>
// 特点: key相同会保留最后一个,
//      会根据key的int值排序(从小到大)
SparseArray<ScrapData> mScrap = new SparseArray<>();

 public void putRecycledView(ViewHolder scrap) {
       // 获取ViewHolder布局类型
      final int viewType = scrap.getItemViewType();

      // 根据布局类型来获取ViewHolder
       final ArrayList scrapHeap = getScrapDataForType(viewType).mScrapHeap;

       // 判断缓存池的大小
       // mScrap.get(viewType).mMaxScrap 默认为 5
       // 同一种ViewType 只保存5个ViewHolder
        if (mScrap.get(viewType).mMaxScrap <= scrapHeap.size()) {
           return;
        }

       // 清空ViewHolder记录
        scrap.resetInternal();

        //add
        scrapHeap.add(scrap);
}
 // 清空ViewHolder记录
 void resetInternal() {
            mFlags = 0;
            mPosition = NO_POSITION;
            mOldPosition = NO_POSITION;
            mItemId = NO_ID;
            mPreLayoutPosition = NO_POSITION;
            mIsRecyclableCount = 0;
            mShadowedHolder = null;
            mShadowingHolder = null;
            clearPayload();
            mWasImportantForAccessibilityBeforeHidden = View.IMPORTANT_FOR_ACCESSIBILITY_AUTO;
            mPendingAccessibilityState = PENDING_ACCESSIBILITY_STATE_NOT_SET;
            clearNestedRecyclerViewIfNotNested(this);
        }

// 根据不同viewType 获取ViewHolder
 private ScrapData getScrapDataForType(int viewType) {
            ScrapData scrapData = mScrap.get(viewType);
            if (scrapData == null) {
                scrapData = new ScrapData();
                mScrap.put(viewType, scrapData);
            }
            return scrapData;
        }

可以看出,缓存池,中最多保存5个同一类型的ViewHolder,并且ViewHolder是空的ViewHolder,

而且缓存池中保存的都是mCachedViews移除的数据!!

小结

mCachedViews 保存即将离开屏幕外的2个ViewHolder

mRecyclerPool 缓存池中:同一种ItemViewType类型能够默认最多保存5个空数据的ViewHolder.

带入实战看看效果:

这里以单布局(ItemViewType = 0)为例

我的layoutManger为GridLayoutManager(content,7),所以每次划出屏幕的时候,就直接会划走7个ViewHolder

可以看出,划出去的一刹那,前5个不会执行onCreateViewHolder(),后2个会执行onCreateViewHolder()

⚠️:onCreateViewHolder() 是用来创建ViewHolder的,后面复用的时候会说!

走到这里,只是分析了RecyclerView从onTouchEvent()–>MOVE事件滑动事件

最终会把ViewHolder保存mCachedViews, mCachedViews只能保存2个ViewHolder

如果第三个ViewHolder来临的时候,就保存到缓存池(mRecyclerPool)中

缓存池(mRecyclerPool)最多保存5个空的ViewHolder…

这只是一种缓存的入口,缓存还有另一种入口,在RecyclerView 的 onLayout()的时候

mAttachedScrap和mChangedScrap 会缓存屏幕内可见的ViewHolder

onLayout()入口

#RecyclerView.java
    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        // 入口
        dispatchLayout();
    }

接着执行dispatchLayout()

#RecyclerView.java

void dispatchLayout() {
       .....
       dispatchLayoutStep2();
       ......
}

接着执行dispatchLayoutStep2()

#RecyclerView.java

private void dispatchLayoutStep2() {
          ......

        // 在这里先缓存
        mLayout.onLayoutChildren(mRecycler, mState);
        .....
}

接着走到LinearLayoutManager.onLayoutChildren()方法

#LinearLayoutManager.java

 @Override
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
        ....
        //会执行到: RecyclerView.detachAndScrapAttachedViews()
         detachAndScrapAttachedViews(recycler);
        ......
}

这里会走到RecyclerView.detachAndScrapAttachedViews(),这行代码非常关键,可以说是缓存屏幕内的ViewHolder的起点,后面完成”探探“效果也需要用到!!

#RecyclerView.java

public void detachAndScrapAttachedViews(Recycler recycler) {
            final int childCount = getChildCount();
            for (int i = childCount - 1; i >= 0; i--) {
                final View v = getChildAt(i);
                // 回收机制关键代码1
                scrapOrRecycleView(recycler, i, v);
            }
}

继续走scrapOrRecycleView()

#RecyclerView.java

private void scrapOrRecycleView(Recycler recycler, int index, View view) {
            final ViewHolder viewHolder = getChildViewHolderInt(view);
           ...
            if (viewHolder.isInvalid() && !viewHolder.isRemoved()
                    && !mRecyclerView.mAdapter.hasStableIds()) {
                removeViewAt(index);
                // 缓存机制关键代码2 主要用来处理 cacheView ,RecyclerViewPool的缓存
                recycler.recycleViewHolderInternal(viewHolder);
            } else {
                detachViewAt(index);
                // 缓存机制关键代码3
                recycler.scrapView(view);
            }
}

这里有两个非常关键的点

缓存机制关键代码2 主要用来处理 cacheView ,RecyclerViewPool的缓存recycler.recycleViewHolderInternal(viewHolder); // 这个关键点上面已经分析过了!!,忘记的ctrl+F搜索看看看一看
recycler.scrapView(view); // 缓存屏幕内的ViewHolder

这里直接看看recycler.scrapView(view);的细节

void scrapView(View view) {
            final ViewHolder holder = getChildViewHolderInt(view);
            // 如果标记没有移除,或者失效等清空 就会缓存
            if (holder.hasAnyOfTheFlags(ViewHolder.FLAG_REMOVED | ViewHolder.FLAG_INVALID)
                    || !holder.isUpdated() || canReuseUpdatedViewHolder(holder)) {
                holder.setScrapContainer(this, false);
                // 一级缓存位置点1
                mAttachedScrap.add(holder);
            } else {
                if (mChangedScrap == null) {
                    mChangedScrap = new ArrayList<ViewHolder>();
                }
                holder.setScrapContainer(this, true);
                // 一级缓存位置点2
                mChangedScrap.add(holder);
            }
}

走到这里4级缓存就结束了

总结一下:

参考深入理解Android RecyclerView的缓存机制： https://segmentfault.com/a/1190000040421118

/ 复用机制 /

回到fill()方法。ctrl + F搜索一下,上边说过

#LinearLayoutManager.java

int fill(RecyclerView.Recycler recycler, LayoutState layoutState,
             RecyclerView.State state, boolean stopOnFocusable) {

        final int start = layoutState.mAvailable;
        if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {
            .....
            // 关键代码19 [用来4级缓存]
            recycleByLayoutState(recycler, layoutState);
        }
         ....

        // 循环调用
        while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {

           // 关键代码5 [用来4级复用]
            layoutChunk(recycler, state, layoutState, layoutChunkResult);

                  ........................................
                    ........只展示代码思路,细节请自行查看........
                  ........................................
        }
}

缓存是进入的recycleByLayoutState(recycler, layoutState);方法

复用是进入的layoutChunk()方法

执行到layoutState.next(recycler);方法

#LinearLayoutManager.java

void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,
                     LayoutState layoutState, LayoutChunkResult result) {
        // 获取当前view
        // 关键代码6
        View view = layoutState.next(recycler);

        // 测量View
        measureChildWithMargins(view, 0, 0);
        .....
}

接着执行到recycler.getViewForPosition(mCurrentPosition);

#LinearLayoutManager.java

View next(RecyclerView.Recycler recycler) {
           .....
            // 关键代码7 [复用机制入口]
            final View view = recycler.getViewForPosition(mCurrentPosition);
            return view;
}

然后继续执行到getViewForPosition()–> getViewForPosition()

#RecyclerView.java

 public View getViewForPosition(int position) {
            // 关键代码8
            return getViewForPosition(position, false);
}

View getViewForPosition(int position, boolean dryRun) {
     // 关键代码10 所有的复用都在这里

      return tryGetViewHolderForPositionByDeadline(position, dryRun, FOREVER_NS).itemView;
}

最终会执行到tryGetViewHolderForPositionByDeadline(),所有的复用代码都在这里了!

#RecyclerView.java

  ViewHolder tryGetViewHolderForPositionByDeadline(int position,
                                                         boolean dryRun, long deadlineNs) {
             ViewHolder holder = null;

            // 一级别复用 [mChangedScrap]
            if (mState.isPreLayout()) {
                // 关键代码11
                holder = getChangedScrapViewForPosition(position);
                fromScrapOrHiddenOrCache = holder != null;
            }

            // 一级复用 [mAttachedScrap]
            if (holder == null) {
                // 通过位置
                // 关键代码12
                holder = getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);

            }

            // 二级复用 [mCachedViews]
             if (holder == null) {
                    // 获取布局类型
                    final int type = mAdapter.getItemViewType(offsetPosition);

                    // 2) Find from scrap/cache via stable ids, if exists
                    // 2) 通过稳定ID从废料/缓存中查找（如果存在）
                    if (mAdapter.hasStableIds()) {
                    // 关键代码13 根据Id来复用
                    holder = getScrapOrCachedViewForId(mAdapter.getItemId(offsetPosition),
                            type, dryRun);
                    }
             }

             // 三级复用 【自定义复用】
                if (holder == null && mViewCacheExtension != null) {
                    // 关键代码14
                    // 自定义复用
                    final View view = mViewCacheExtension
                            .getViewForPositionAndType(this, position, type);
                    if (view != null) {
                        holder = getChildViewHolder(view);
                     }
                }

            // 四级复用 [mRecyclerPool(缓存池复用)]
              if (holder == null) {
                    // 关键代码15 从缓存池获取viewHolder
                    holder = getRecycledViewPool().getRecycledView(type);

                }

            // 最终,如果走到这里,holder == 0,表示没有缓存,那么则创建ViewHolder
            if (holder == null) {
                    // 如果四级缓存都是 null, 那么就由适配器创建 ViewHolder
                    holder = mAdapter.createViewHolder(RecyclerView.this, type);
            }

            // 走到这了的时候,ViewHolder != null
            // 绑定布局
            if (mState.isPreLayout() && holder.isBound()) {
               .....
            } else if (!holder.isBound() || holder.needsUpdate() || holder.isInvalid()) {
                 ......
                // 关键代码17
                // 在这里调 onBindViewHolder() 绑定数据
                bound = tryBindViewHolderByDeadline(holder, offsetPosition, position, deadlineNs);
                ......
            }
            ......
}

看一下tryBindViewHolderByDeadline(),绑定ViewHolder的具体绑定细节:

private boolean tryBindViewHolderByDeadline(ViewHolder holder, int offsetPosition,
                                                    int position, long deadlineNs) {
           ....
           // 最终绑定位置
            mAdapter.bindViewHolder(holder, offsetPosition);
           ...
}

复用机制比缓存机制简单很多,因为复用入口就一个。看看流程图一目了然!

/ 探探效果实战 /

⚠️:为了全局性考虑,实战采用java,底部附 java/kotlin 源码

要想实战,那就得先实现最普通的效果,这段代码没啥营养,直接看效果!

自定义LayoutManager

public class CardStack3LayoutManager extends RecyclerView.LayoutManager {
    @Override
    public RecyclerView.LayoutParams generateDefaultLayoutParams() {
         return new RecyclerView.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT,
                ViewGroup.LayoutParams.WRAP_CONTENT);
    }

     // 必须重写 在 RecyclerView->OnLayout()时候调用,用来摆放 Item位置
     @Override
    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
        super.onLayoutChildren(recycler, state);
    }
}

需要重写generateDefaultLayoutParams()方法,咋们是仿造着 LinearLayoutManager()来写,所以直接参考 LinearLayoutManager()就可以

注意:这里的 onLayoutChildren() 需要手动重写!

主要功能都在onLayoutChildren()中编写

#CardStack2LayoutManager.java

 // 最开始显示个数
 public static final int MAX_SHOW_COUNT = 4;

    @Override
    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
        super.onLayoutChildren(recycler, state);

         // 调用RecyclerView的缓存机制 缓存 ViewHolder
        detachAndScrapAttachedViews(recycler);

        // 最下面图片下标
        int bottomPosition = 0;
        // 获取所有图片
        int itemCount = getItemCount();

        if (itemCount > MAX_SHOW_COUNT) {
            // 获取到从第几张开始
            bottomPosition = itemCount - MAX_SHOW_COUNT;
        }

         for (int i = bottomPosition; i < itemCount; i++) {
            // 获取当前view宽高
            View view = recycler.getViewForPosition(i);

            addView(view);

            // 测量
            measureChildWithMargins(view, 0, 0);

//            getWidth() RecyclerView 宽
//            getDecoratedMeasuredWidth() View的宽
            int widthSpace = getWidth() - getDecoratedMeasuredWidth(view);
            int heightSpace = getHeight() - getDecoratedMeasuredHeight(view);

            // LinearLayoutManager#layoutChunk#layoutDecoratedWithMargins
            // 绘制布局
            layoutDecoratedWithMargins(view, widthSpace / 2,
                    heightSpace / 2,
                    widthSpace / 2 + getDecoratedMeasuredWidth(view),
                    heightSpace / 2 + getDecoratedMeasuredHeight(view));
            }
}

这段代码就是获取所有的 ItemView,然后全部布局到屏幕中心

先来看看当前的效果:

detachAndScrapAttachedViews()上面提到过,是缓存的入口,会直接调用到RecyclerView.detachAndScrapAttachedViews()方法

测量布局,摆放的代码参考自 LinearLayoutManager(),思路就是吧当前View添加到RecyclerView中,然后在测量View,最后在摆放(布局)View

最后让View摆放时候有缩放层级:

#CardStack2LayoutManager.java

 // 最开始显示个数
    public static final int MAX_SHOW_COUNT = 4;

    // item 平移Y轴距
    public static final int TRANSLATION_Y = 20;

    // 缩放的大小
    public static final float SCALE = 0.05f;

@Override
    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
        super.onLayoutChildren(recycler, state);

        // 缓存 ViewHolder
        detachAndScrapAttachedViews(recycler);

        // 最下面图片下标
        int bottomPosition = 0;
        // 获取所有图片
        int itemCount = getItemCount();

        //如果所有图片 > 显示的图片
        if (itemCount > MAX_SHOW_COUNT) {
            // 获取到从第几张开始
            bottomPosition = itemCount - MAX_SHOW_COUNT;
        }

        for (int i = bottomPosition; i < itemCount; i++) {
            // 获取当前view宽高
            View view = recycler.getViewForPosition(i);

            addView(view);

            // 测量
            measureChildWithMargins(view, 0, 0);

//            getWidth() RecyclerView 宽
//            getDecoratedMeasuredWidth() View的宽
            int widthSpace = getWidth() - getDecoratedMeasuredWidth(view);
            int heightSpace = getHeight() - getDecoratedMeasuredHeight(view);

            // LinearLayoutManager#layoutChunk#layoutDecoratedWithMargins
            // 绘制布局
            layoutDecoratedWithMargins(view, widthSpace / 2,
                    heightSpace / 2,
                    widthSpace / 2 + getDecoratedMeasuredWidth(view),
                    heightSpace / 2 + getDecoratedMeasuredHeight(view));

            /*
             * 作者:android 超级兵
             * TODO itemCount - 1  = 最后一个元素
                    最后一个元素 - i = 倒数的元素
             */
            int level = itemCount - 1 - i;

            if (level > 0) {
                int value = toDip(view.getContext(), TRANSLATION_Y);

                // 如果不是最后一个才缩放
                if (level < MAX_SHOW_COUNT - 1) {

                    // 平移
                    view.setTranslationY(value * level);
                    // 缩放
                    view.setScaleX(1 - SCALE * level);
                    view.setScaleY(1 - SCALE * level);
                } else {
                    // 最下面的View 和前一个View布局一样(level - 1)
                    view.setTranslationY(value * (level - 1));
                    view.setScaleX(1 - SCALE * (level - 1));
                    view.setScaleY(1 - SCALE * (level - 1));
                }
            }
        }
    }

    private int toDip(Context context, float value) {
        return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, value, context.getResources().getDisplayMetrics());
    }

当前效果为:

到目前为止,完成了ItemView的叠加摆放,接下来只需要添加上滑动即可!

RecyclerView拖拽滑动需要使用到ItemTouchHelper.SimpleCallback

public class SlideCardStackCallBack2<T> extends ItemTouchHelper.SimpleCallback {
    private final CardStackAdapter<T> mAdapter;

    public SlideCardStackCallBack2(CardStackAdapter<T> mAdapter) {
        super(0, 15);
        this.mAdapter = mAdapter;
    }

    // 拖拽使用,不用管
    @Override
    public boolean onMove(@NonNull RecyclerView recyclerView, @NonNull RecyclerView.ViewHolder viewHolder, @NonNull RecyclerView.ViewHolder target) {
        return false;
    }

   // 滑动结束后的处理
    @Override
    public void onSwiped(@NonNull RecyclerView.ViewHolder viewHolder, int direction) {

    }
}

这里需要传递两个参数:

参数一:dragDirs 拖拽
参数二:swipeDirs 滑动

这里咋们不用拖拽,直接给0就行,主要说一下滑动swipeDirs

#ItemTouchHelper.java

/**
     * Up direction, used for swipe & drag control.
     */
    public static final int UP = 1;    //1

    /**
     * Down direction, used for swipe & drag control.
     */
    public static final int DOWN = 1 << 1; //2 

    /**
     * Left direction, used for swipe & drag control.
     */
    public static final int LEFT = 1 << 2; //4

    /**
     * Right direction, used for swipe & drag control.
     */
    public static final int RIGHT = 1 << 3; //8

滑动主要以这几个位运算组

如果需要上下滑动那么就是 UP+DOWN = 1+2 = 3
如果是上下左滑动就是 UP + DOWN + LEFT = 1 + 2 + 4 = 7
那么如果是上下左右滑动就是 UP + DOWN + LEFT + RIGHT = 15

所以这里直接填15就表示可以上下左右滑动

onSwiped()处理:

#SlideCardStackCallBack2.java

@Override
    public void onSwiped(@NonNull RecyclerView.ViewHolder viewHolder, int direction) {
        // 当前滑动的View下标
        int layoutPosition = viewHolder.getLayoutPosition();
        // 删除当前滑动的元素
        CardStackBean<T> bean = mAdapter.getData().remove(layoutPosition);

        // 添加到集合第0个位置 造成循环滑动的效果
        mAdapter.addData(0, bean);
        mAdapter.notifyDataSetChanged();
}

这段代码很好理解,先删除当前滑动的View,然后在添加到最后一个,造成循环滑动的效果!来看看效果:

现在看来,还是有点生硬,添加一些滑动系数缩放:这里直接贴出完整代码:看图说话:

#SlideCardStackCallBack2.java

@Override
    public void onChildDraw(@NonNull Canvas c, @NonNull RecyclerView recyclerView, @NonNull RecyclerView.ViewHolder viewHolder, float dX, float dY, int actionState, boolean isCurrentlyActive) {
        super.onChildDraw(c, recyclerView, viewHolder, dX, dY, actionState, isCurrentlyActive);
        int maxDistance = recyclerView.getWidth() / 2;

            // dx = 当前滑动x位置
            // dy = 当前滑动y位置
        //sqrt 开根号
        double sqrt = Math.sqrt((dX * dX + dY * dY));

        // 放大系数
        double scaleRatio = sqrt / maxDistance;

        // 系数最大为1 
        if (scaleRatio > 1.0) {
            scaleRatio = 1.0;
        }

        int childCount = recyclerView.getChildCount();
        // 循环所有数据
        for (int i = 0; i < childCount; i++) {
            View view = recyclerView.getChildAt(i);

            int valueDip = toDip(view.getContext(), 20f);

            /*
             * 作者:android 超级兵
             * TODO
             *   childCount - 1 =  itemView总个数
             *    childCount - 1 - i = itemView总个数 - i = 从最后一个开始
             *
             * 假设 childCount - 1 = 7
             *     i累加
             *     那么level = childCount - 1 - 0 = 7
             *     那么level = childCount - 1 - 1 = 6
             *     那么level = childCount - 1 - 2 = 5
             *     那么level = childCount - 1 - 3 = 4
             *     那么level = childCount - 1 - 4 = 3
             *      。。。。
             */
            int level = childCount - 1 - i;
            if (level > 0) {
                // 最大显示叠加个数:CardStack2LayoutManager.MAX_SHOW_COUNT = 4
                if (level < CardStack2LayoutManager.MAX_SHOW_COUNT - 1) {
                    // 缩放比例: CardStack2LayoutManager.SCALE = 0.05
                    float scale = CardStack2LayoutManager.SCALE;

                    // valueDip * level  = 原始平移距离
                    // scaleRatio * valueDip = 平移系数
                    // valueDip * level - scaleRatio * valueDip = 手指滑动过程中的Y轴平移距离
                    // 因为是Y轴,所以向上平移是 - 号
                    view.setTranslationY((float) (valueDip * level - scaleRatio * valueDip));

                    // 1 - scale * level = 原始缩放大小
                    // scaleRatio * scale = 缩放系数
                    // 因为是需要放大,所以这里是 + 号
                    view.setScaleX((float) ((1 - scale * level) + scaleRatio * scale));
                    view.setScaleY((float) ((1 - scale * level) + scaleRatio * scale));
                }
            }
        }
    }

    private int toDip(Context context, float value) {
        return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, value, context.getResources().getDisplayMetrics());
    }

滑动系数图解:

⚠️:记得绑定 RecyclerView

// 创建拖拽
        val slideCardStackCallBack = SlideCardStackCallBack2(cardStackAdapter)
        val itemTouchHelper = ItemTouchHelper(slideCardStackCallBack)

        // 绑定拖拽
        itemTouchHelper.attachToRecyclerView(rootRecyclerView)

这里的注释比较清晰,来看看最终效果吧~

还有两个比较好玩的参数

// 设置回弹距离
    @Override
    public float getSwipeThreshold(@NonNull RecyclerView.ViewHolder viewHolder) {
        return 0.3f;
    }

    // 设置回弹时间
    @Override
    public long getAnimationDuration(@NonNull RecyclerView recyclerView, int animationType, float animateDx, float animateDy) {
        return 3000;
    }

很简单,直接看效果