无法使用 android Paging 库加载下一个数据

Question

我正在尝试使用 Room-Paging-LiveData-ViewModel 显示通话记录列表。没有分页我的代码工作得很好。我也想使用分页。

在我的数据库中，我总共有 25 条通话记录。前 9 个通话记录显示在列表中。

通过调试，我发现在通过 读取视图模型中的数据时Dao，它返回大小为 25 的列表。但其中只有前 9 个是非空的。列表中的所有其他条目都为空。

我期待空数据会很快刷新，因为这是一个分页列表。但问题是 null 永远不会被有效数据刷新。

并且视图模型的observe 方法只被调用一次，只有第一次。

我想我做错了什么。

这是下面的代码

片段

public class CallLogListFragment extends Fragment {
    private static final String TAG = "RecentCallsFragment";

    public static String getTAG() {
        return TAG;
    }

    public static Fragment newInstance() {
        return new CallLogListFragment();
    }

    public CallLogListFragment() {
    }

    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        FragmentCallLogListBinding binding = DataBindingUtil.inflate(inflater, R.layout.fragment_call_log_list, container, false);
        CallLogListAdapter adapter = new CallLogListAdapter();
        binding.list.setAdapter(adapter);
        CallLogListViewModel model = ViewModelProviders.of(this).get(CallLogListViewModel.class);
        model.getCallLogList().observe(this, adapter::refreshData);
        return binding.getRoot();
    }

}

适配器

public class CallLogListAdapter extends PagedListAdapter<CallLogItem, CallLogListAdapter.ViewHolder> {
    CallLogListAdapter() {
        super(DIFF_CALLBACK);
    }

    void refreshData(List<CallLogItem> data) {
        DiffUtil.DiffResult calculatedDiff = DiffUtil.calculateDiff(new CallLogListDiffUtilCallBack(this.data, data));
        this.data.clear();
        this.data.addAll(data);
        calculatedDiff.dispatchUpdatesTo(this);
    }

    private List<CallLogItem> data = new ArrayList<>();

    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        return new ViewHolder(DataBindingUtil.inflate(
                LayoutInflater.from(parent.getContext()),
                R.layout.call_log_list_single_item,
                parent, false
        ));
    }

    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
        CallLogItem item = data.get(position);
        holder.binding.setCallLog(item);
    }

    @Override
    public int getItemCount() {
        return data.size();
    }

    class ViewHolder extends RecyclerView.ViewHolder {
        public CallLogListSingleItemBinding binding;
        public ViewHolder(@NonNull CallLogListSingleItemBinding binding) {
            super(binding.getRoot());
            this.binding = binding;
        }
    }

     private static DiffUtil.ItemCallback<CallLogItem> DIFF_CALLBACK =
        new DiffUtil.ItemCallback<CallLogItem>() {
            @Override
            public boolean areItemsTheSame(CallLogItem oldItem, CallLogItem newItem) {
                return oldItem.getHeaderDateVisibility() == newItem.getHeaderDateVisibility()
                        && oldItem.getCallId().equals(newItem.getCallId());
            }

            @Override
            public boolean areContentsTheSame(@NonNull CallLogItem oldItem, @NonNull CallLogItem newItem) {
                return areItemsTheSame(oldItem, newItem);
            }
        };
}

道

@Dao
public interface CallLogDao extends BaseDao<CallLog>{
    @Query("SELECT * FROM log")
    List<CallLog> getAll();

    @Query("SELECT * FROM log WHERE number=:number")
    CallLog findByName(String number);

    @Query("SELECT * FROM log order by date desc")
    LiveData<List<CallLog>> getAllLive();

    @Query("SELECT * FROM log order by date desc")
    DataSource.Factory<Integer, CallLog> getAllLivePaged();
}

视图模型

public class CallLogListViewModel extends ViewModel {
    private LiveData<List<CallLogItem>> callLogList;

    public CallLogListViewModel() {
        callLogList = Transformations.map(new LivePagedListBuilder<>(AppDatabase.get().callLogDao().getAllLivePaged(), 3).build(), input -> {
            List<CallLogItem> list = new ArrayList<>();
            for (int i = 0; i < input.size(); i++) {
                boolean isHeader = true;
                CallLog callLog = input.get(i);
                if(callLog!=null) {
                    if (i > 0) {
                        CallLog previousCallLog = input.get(i - 1);
                        if(previousCallLog!=null) {
                            isHeader = TimeFormat.isDifferentDate(callLog.date, previousCallLog.date);
                        }
                    }
                    list.add(CallLogItem.Companion.from(callLog, isHeader));
                }
            }
            return list;
        });
    }

    LiveData<List<CallLogItem>> getCallLogList() {
        return callLogList;
    }
}

后来我试着做

private LiveData<List<CallLogItem>> callLogList; 

到分页列表，如

private LiveData<PagedList<CallLogItem>> callLogList; 

但我没有找到合适的方法来转换成那样。

Answer 1

对于分页列表适配器有两件事需要注意。1. 数据将在内部处理，无需声明任何数据结构来手动处理数据。2. 中有一个默认方法被submitList调用PagedListAdapter。有必要通过该方法向适配器提交分页列表。

改装适配器

public class CallLogListAdapter extends PagedListAdapter<CallLogItem, CallLogListAdapter.ViewHolder> {
    private Context context;
    CallLogListAdapter(Context context) {
        super(DIFF_CALLBACK);
        this.context = context;
    }

    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        return new ViewHolder(DataBindingUtil.inflate(
                LayoutInflater.from(parent.getContext()),
                R.layout.call_log_list_single_item,
                parent, false
        ));
    }

    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
        CallLogItem item = getItem(position);
        if (item != null) {
            holder.binding.setCallLog(item);
            ImageUtil.setImage(holder.binding.ivProfileImage, item.getImageUrl(), item.getName());
        } else {
            holder.binding.invalidateAll();
        }
    }


    class ViewHolder extends RecyclerView.ViewHolder {
        public CallLogListSingleItemBinding binding;

        public ViewHolder(@NonNull CallLogListSingleItemBinding binding) {
            super(binding.getRoot());
            this.binding = binding;
        }
    }

    private static DiffUtil.ItemCallback<CallLogItem> DIFF_CALLBACK =
            new DiffUtil.ItemCallback<CallLogItem>() {
                @Override
                public boolean areItemsTheSame(CallLogItem oldItem, CallLogItem newItem) {
                    return oldItem.getHeaderDateVisibility() == newItem.getHeaderDateVisibility()
                            && oldItem.getCallId()!=null &&  oldItem.getCallId().equals(newItem.getCallId());
                }

                @Override
                public boolean areContentsTheSame(@NonNull CallLogItem oldItem, @NonNull CallLogItem newItem) {
                    return areItemsTheSame(oldItem, newItem);
                }
            };
}

修改后的数据传递到适配器

CallLogListViewModel model = ViewModelProviders.of(this).get(CallLogListViewModel.class);
model.getCallLogList().observe(this, adapter::submitList);