FirestoreRecyclerAdapter - 我如何知道何时检索到数据？

Question

我正在使用FirestoreRecyclerAdapter 检索数据，完成后，我需要检查是否已检索到任何项目。我不知道该怎么做。

我从一个名为FragmentChartsList 的类中调用它，如下所示。这应该最初设置适配器，将“名称”作为mOrder 的值。稍后，包含此 Fragment 的 Activity 可以调用 setOrderField()，并使用用户从 Spinner 中选择的不同值 mOrder。

每次调用setOrderField()，都会创建一个新的适配器实例并将其附加到recyclerView。此时我需要检查新版本的适配器是否包含任何数据，或者显示“未找到图表”消息，或者显示检索到的图表（显然，如果列表只是被排序，那么items 保持不变，但我将对其进行扩展，以允许用户按不同条件过滤图表，因此返回的图表数量会改变）。

目前，setOrderField() 调用refreshViewOnNewData()，它应该找出显示了多少个图表；如果为 0，则应显示“未找到图表”消息，如果为 >0，则应显示包含图表的 RecyclerView。

目前，当我尝试计算图表时，我总是得到 0 的值。我怀疑这是因为适配器尚未完成从数据库中检索它们，但我找不到任何可以让我添加某种“onComplete”侦听器的东西，所以我知道它已经完成了。

谁能建议我如何做到这一点？

public abstract class FragmentChartsList extends Fragment {

    private FirebaseFirestore mDatabaseRef;
    private ChartListAdapter mAdapter;
    private Query mChartsQuery;
    private RecyclerView mRecycler;
    private String mOrder = "name";

    private TextView mLoadingList, mEmptyList;

    public FragmentChartsList() {}

    @Override
    public View onCreateView(@NonNull LayoutInflater inflater,
                             ViewGroup container, Bundle savedInstanceState) {
        super.onCreateView(inflater, container, savedInstanceState);

        View rootView = inflater.inflate(
                R.layout.fragment_charts_list, container, false);

        mRecycler = rootView.findViewById(R.id.charts_list);
        mRecycler.setHasFixedSize(true);

        mLoadingList = rootView.findViewById(R.id.loading_list);
        mEmptyList = rootView.findViewById(R.id.empty_list);

        // Set up Layout Manager, and set Recycler View to use it
        LinearLayoutManager mManager = new LinearLayoutManager(getActivity());
        mManager.setReverseLayout(true);
        mManager.setStackFromEnd(true);
        mRecycler.setLayoutManager(mManager);

        // Connect to the database
        mDatabaseRef = FirebaseFirestore.getInstance();

        setOrderField(mOrder); // Initialised to "name"

        return rootView;
    }

    @Override
    public void onStart() {
        super.onStart();
        mAdapter.startListening();
    }

    @Override
    public void onStop() {
        super.onStop();
        mAdapter.stopListening();
    }

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

        mAdapter.stopListening();
    }


    // HELPER FUNCTIONS

    public void setOrderField(String order) {

        mOrder = order;
        mChartsQuery = getQuery(mDatabaseRef, mOrder);

        // Update recycler options
        FirestoreRecyclerOptions<Chart> recyclerOptions = new FirestoreRecyclerOptions.Builder<Chart>()
                .setQuery(mChartsQuery, Chart.class)
                .build();

        mAdapter = new ChartListAdapter(recyclerOptions, getActivity());
        mAdapter.startListening();

        mRecycler.swapAdapter(mAdapter, true);

        refreshViewOnNewData();

    }

    private void refreshViewOnNewData() {

        // Hide "loading" text
        mLoadingList.setVisibility(View.GONE);

        // Check number of charts being shown
        //if (mAdapter != null && (mAdapter.getCount() > 0)) {
            // If > 0, show Charts
            mEmptyList.setVisibility(View.GONE);
            mRecycler.setVisibility(View.VISIBLE);

        } else {
            // If number of Charts = 0
            //    show "no charts"
            mEmptyList.setVisibility(View.VISIBLE);
            mRecycler.setVisibility(View.GONE);
        }

    }

}

适配器类如下所示：

public class ChartListAdapter extends FirestoreRecyclerAdapter<Chart, ChartViewHolder> {

    private Activity mActivity;
    private int mCount;

    public ChartListAdapter(FirestoreRecyclerOptions<Chart> recyclerOptions, Activity activity) {
        super(recyclerOptions);

        mActivity = activity;

    }

    @Override
    protected void onBindViewHolder(@NonNull ChartViewHolder holder, int position, @NonNull Chart model) {

        final String chartKey = this.getSnapshots().getSnapshot(position).getId();

        model.setKey(chartKey);

        // Set click listener for the chart
        // On click, the user can view the chart
        holder.itemView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(mActivity, ActivityViewChart.class);
                intent.putExtra("ChartKey", chartKey);
                mActivity.startActivity(intent);
            }
        });

        // Implement long-click menu
        mActivity.registerForContextMenu(holder.itemView);

        // Bind Chart to ViewHolder
        holder.bindToChart(model);
    }

    @NonNull
    @Override
    public ChartViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext())
                .inflate(R.layout.item_chart, parent, false);

        return new ChartViewHolder(view);
    }

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

        mCount = getItemCount();

    }

    public int getCount() {
        return mCount;
    }

}

Answer 1

想通了...我需要在查询上设置一个侦听器。

所以，我现在有：

    mChartsQuery.addSnapshotListener(new EventListener<QuerySnapshot>() {
        @Override
        public void onEvent(@Nullable QuerySnapshot queryDocumentSnapshots, @Nullable FirebaseFirestoreException e) {


            if (queryDocumentSnapshots != null) {

                mLoadingList.setVisibility(View.GONE);

                if(queryDocumentSnapshots.size() > 0) {
                    mEmptyList.setVisibility(View.GONE);
                    mRecycler.setVisibility(View.VISIBLE);
                }else {
                    mEmptyList.setVisibility(View.VISIBLE);
                    mRecycler.setVisibility(View.GONE);
                }
            }

        }
    });

}

还从适配器类中删除了mCount，以及getCount 和onDataChanged