记一Bug：StaggeredGridLayoutManager ：java.lang.IndexOutOfBoundsException: Index: 0, Size: 0

记一Bug：StaggeredGridLayoutManager ：java.lang.IndexOutOfBoundsException: Index: 0, Size: 0

崩溃

线上平台监控到一个bug，数量不多。一开始直找不到啥原因，先看报错日志：

咋一看，报错原因是数组越界了，但是你会发现具体报错位置是源码StaggeredGridLayoutManager$Span的calculateCachedStart报错了，具体如下：

 void calculateCachedStart() {
            final View startView = mViews.get(0);
            final LayoutParams lp = getLayoutParams(startView);
            mCachedStart = mPrimaryOrientation.getDecoratedStart(startView);
            if (lp.mFullSpan) {
                LazySpanLookup.FullSpanItem fsi = mLazySpanLookup
                        .getFullSpanItem(lp.getViewLayoutPosition());
                if (fsi != null && fsi.mGapDir == LayoutState.LAYOUT_START) {
                    mCachedStart -= fsi.getGapForSpan(mIndex);
                }
            }
        }

这里的mViews.get(0)中mViews的size为0，所以数组越界了。

复现

复现场景也找到了：使用viewpager2导致当前的fragment了，导致布局缓存的销毁，会导致调用了StaggeredGridLayoutManager#onDetachedFromWindow方法。由于我没有设置viewpager2的缓存数量（启动不多加载，启动快），在频繁切换的时候，会调用此方法。

  public void onDetachedFromWindow(RecyclerView view, RecyclerView.Recycler recycler) {
        super.onDetachedFromWindow(view, recycler);

        removeCallbacks(mCheckForGapsRunnable);
        for (int i = 0; i < mSpanCount; i++) {
        	//这里遍历清空了
            mSpans[i].clear();
        }
        // SGLM will require fresh layout call to recover state after detach
        view.requestLayout();
    }

所以在滑动瀑布流布局的时候会回调calculateCachedStart方法，导致崩溃了。

解决方案

设置viewpager2缓存数量

mViewPager.setOffscreenPageLimit(4);

这样就不会调用onDetachedFromWindow方法了，虽然解决问题了，但是启动的时候会多初始化这么多fragment，那么就影响了启动速度，这是万万不行的。不在乎启动速度，改这个就行了。

重写onDetachedFromWindow

 manager = new StaggeredGridLayoutManager(count,
                StaggeredGridLayoutManager.HORIZONTAL){
            @Override
            public void onDetachedFromWindow(RecyclerView view, RecyclerView.Recycler recycler) {
                //不处理了
            }
        };

处理SavedState

  public static class SavedState implements Parcelable {

        int mAnchorPosition;
        int mVisibleAnchorPosition; // Replacement for span info when spans are invalidated
        int mSpanOffsetsSize;
        int[] mSpanOffsets;
        int mSpanLookupSize;
        int[] mSpanLookup;
        List<LazySpanLookup.FullSpanItem> mFullSpanItems;
        boolean mReverseLayout;
        boolean mAnchorLayoutFromEnd;
        boolean mLastLayoutRTL;

        public SavedState() {
        }

        SavedState(Parcel in) {
            mAnchorPosition = in.readInt();
            mVisibleAnchorPosition = in.readInt();
            mSpanOffsetsSize = in.readInt();
            if (mSpanOffsetsSize > 0) {
                mSpanOffsets = new int[mSpanOffsetsSize];
                in.readIntArray(mSpanOffsets);
            }

            mSpanLookupSize = in.readInt();
            if (mSpanLookupSize > 0) {
                mSpanLookup = new int[mSpanLookupSize];
                in.readIntArray(mSpanLookup);
            }
            mReverseLayout = in.readInt() == 1;
            mAnchorLayoutFromEnd = in.readInt() == 1;
            mLastLayoutRTL = in.readInt() == 1;
            @SuppressWarnings("unchecked")
            List<LazySpanLookup.FullSpanItem> fullSpanItems =
                    in.readArrayList(LazySpanLookup.FullSpanItem.class.getClassLoader());
            mFullSpanItems = fullSpanItems;
        }

        public SavedState(SavedState other) {
            mSpanOffsetsSize = other.mSpanOffsetsSize;
            mAnchorPosition = other.mAnchorPosition;
            mVisibleAnchorPosition = other.mVisibleAnchorPosition;
            mSpanOffsets = other.mSpanOffsets;
            mSpanLookupSize = other.mSpanLookupSize;
            mSpanLookup = other.mSpanLookup;
            mReverseLayout = other.mReverseLayout;
            mAnchorLayoutFromEnd = other.mAnchorLayoutFromEnd;
            mLastLayoutRTL = other.mLastLayoutRTL;
            mFullSpanItems = other.mFullSpanItems;
        }

        void invalidateSpanInfo() {
            mSpanOffsets = null;
            mSpanOffsetsSize = 0;
            mSpanLookupSize = 0;
            mSpanLookup = null;
            mFullSpanItems = null;
        }

        void invalidateAnchorPositionInfo() {
            mSpanOffsets = null;
            mSpanOffsetsSize = 0;
            mAnchorPosition = RecyclerView.NO_POSITION;
            mVisibleAnchorPosition = RecyclerView.NO_POSITION;
        }

        @Override
        public int describeContents() {
            return 0;
        }

        @Override
        public void writeToParcel(Parcel dest, int flags) {
            dest.writeInt(mAnchorPosition);
            dest.writeInt(mVisibleAnchorPosition);
            dest.writeInt(mSpanOffsetsSize);
            if (mSpanOffsetsSize > 0) {
                dest.writeIntArray(mSpanOffsets);
            }
            dest.writeInt(mSpanLookupSize);
            if (mSpanLookupSize > 0) {
                dest.writeIntArray(mSpanLookup);
            }
            dest.writeInt(mReverseLayout ? 1 : 0);
            dest.writeInt(mAnchorLayoutFromEnd ? 1 : 0);
            dest.writeInt(mLastLayoutRTL ? 1 : 0);
            dest.writeList(mFullSpanItems);
        }

        public static final Parcelable.Creator<SavedState> CREATOR =
                new Parcelable.Creator<SavedState>() {
                    @Override
                    public SavedState createFromParcel(Parcel in) {
                        return new SavedState(in);
                    }

                    @Override
                    public SavedState[] newArray(int size) {
                        return new SavedState[size];
                    }
                };
    }

SavedState是布局的一些状态保持，对比activtiy的state是一样的。所以我重写onDetachedFromWindow方法保存状态，重写

改为GridLayoutManager

如果布局支持，可以改，GridLayoutManager滑动不会回调到calculateCachedStart方法。不建议，很Low。

总结

代码不规范，测试两行泪啊