RecyclerView 不会去原来的起始位置

Question

最近我在回收站视图中添加了保存和恢复功能，因此当我在不同片段之间导航时，回收站视图在我回来时保持其位置，但现在当我再次单击片段时出现问题，我认为回收站视图应该转到原始起始位置的顶部，但没有它保持相同的位置，我必须通过向下滚动手动进入（转到顶部）

这是screen recording

当我从Home Fragment 导航到Profile Fragment 并再次返回Home fragment 时，您可以看到回收站视图的位置保持不变，但如果我再次单击Home 图标RecyclerView 应该会去回到顶部但它最终不会我必须手动执行它

这是我的代码

注意：- 如果有人想要我的代码的更多参考，请告诉我 将更新问题

Home_Fragment.java // 保存和恢复位置的实现在最后，如果你想跳到最后

@SuppressLint("SourceLockedOrientationActivity")
    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_home, container, false);
        requireActivity().setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
        MaterialToolbar materialToolbar = view.findViewById(R.id.toolbar);
        materialToolbar.setOnMenuItemClickListener(toolbarItemClickListener);
        postRecyclerView = view.findViewById(R.id.recyclerViewHome);
        shimmerFrameLayout = view.findViewById(R.id.shimmerEffect);
//        this is for one item per scroll
//        SnapHelper snapHelper = new PagerSnapHelper();
//        snapHelper.attachToRecyclerView(verticalRecyclerView);
        postRecyclerView.setAdapter(postsAdapter);
//        listState = savedInstanceState.getParcelable("ListState");

        StaggeredGridLayoutManager staggeredGridLayoutManager = new StaggeredGridLayoutManager(1, StaggeredGridLayoutManager.VERTICAL);
        postRecyclerView.setLayoutManager(
                staggeredGridLayoutManager
        );
        postRecyclerView.setItemAnimator(null);
        getData();


        postRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
            @Override
            public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {

                visibleItemCount = staggeredGridLayoutManager.getChildCount();
                totalItemCount = staggeredGridLayoutManager.getItemCount();
                int[] firstVisibleItems = null;
                firstVisibleItems = staggeredGridLayoutManager.findFirstVisibleItemPositions(firstVisibleItems);
                if (firstVisibleItems != null && firstVisibleItems.length > 0) {
                    pastVisibleItems = firstVisibleItems[0];
                }

                if (loading) {
                    if ((visibleItemCount + pastVisibleItems) >= totalItemCount) {
                        loading = false;
                        getData();
                        Log.d("tag", "LOAD NEXT ITEM");
                    }
                }
            }
        });
//        setupFirebaseAuth();
        shimmerFrameLayout.startShimmer();
        mUploads = new ArrayList<>();
        postsAdapter = new PostAdapter_Home(getContext(), mUploads);
        postRecyclerView.setAdapter(postsAdapter);
        postRecyclerView.scrollToPosition(Home_Fragment.saved_position);
        return view;
    }


    private void getData() {
        databaseReference.addValueEventListener(new ValueEventListener() {
            @SuppressLint("NotifyDataSetChanged")
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                if (snapshot.exists()) {
                    shimmerFrameLayout.stopShimmer();
                    shimmerFrameLayout.setVisibility(View.GONE);
                    postRecyclerView.setVisibility(View.VISIBLE);
                    mUploads.clear();
                    for (DataSnapshot dataSnapshot : snapshot.getChildren()) {
                        Upload upload = dataSnapshot.getValue(Upload.class);
                        assert upload != null;
                        upload.setmKey(dataSnapshot.getKey());
                        mUploads.add(upload);


                    }

                }
                postsAdapter.setUploads(mUploads);

                //notify the adapter
                postsAdapter.notifyDataSetChanged();
                loading = true;
            }


            @Override
            public void onCancelled(@NonNull DatabaseError error) {
                loading = true;
            }
        });
    }
 @Override
    public void onPause() {
        super.onPause();

        mBundleRecyclerViewState = new Bundle();

        mListState = postRecyclerView.getLayoutManager().onSaveInstanceState();

        mBundleRecyclerViewState.putParcelable(KEY_RECYCLER_STATE, mListState);
    }

    @Override
    public void onResume() {
        super.onResume();

        if (mBundleRecyclerViewState != null) {
            new Handler().postDelayed(new Runnable() {

                @Override
                public void run() {
                    mListState = mBundleRecyclerViewState.getParcelable(KEY_RECYCLER_STATE);
                    postRecyclerView.getLayoutManager().onRestoreInstanceState(mListState);

                }
            }, 50);
        }
    }


    public void onStop() {

        super.onStop();
//        if (mAuthListener != null) {
//            mAuth.removeAuthStateListener(mAuthListener);
//        }
    }
}

Answer 1

抱歉，我无法添加评论：在此处写消息。

我看到当您的方法“onScrolled”点击“加载”时，它会调用 getData() 方法，该方法会添加一个新的侦听器而不是简单的方法执行。这导致每次更改数据集时都会触发事件，然后累积的事件处理程序将执行相同的代码。滚动的次数越多，执行代码的次数就越多。由于此代码在 UI 线程上执行，因此会导致性能问题。可能是这种多次代码执行是您描述的问题的根源。

我不精通 Java，但这看起来是个问题。如果我错了，请纠正我。

Answer 2

您可以使用 StateRestorationPolicy 来保存和恢复回收站视图状态 https://medium.com/androiddevelopers/restore-recyclerview-scroll-position-a8fbdc9a9334

只需要在app.gradle中添加依赖

implementation "androidx.recyclerview:recyclerview:1.2.1" 

并在设置回收器视图适配器时添加代码

postsAdapter.stateRestorationPolicy = PREVENT_WHEN_EMPTY