Android：使用PagedList时如何添加项目和刷新列表？

Question

我使用 PagedList 加载项目页面并在 RecyclerView 中显示它们，它成功显示数字列表（0,1,2,3....,99），列表始终可以向上滚动和下来没有任何问题。 当我试图单击一个按钮来调用 appendItems() 来追加新数据时，它仍然可以。但是，当我滚动列表时，应用程序崩溃了。

java.util.ConcurrentModificationException
    at java.util.AbstractList$SubAbstractList.size(AbstractList.java:360)
    at androidx.paging.PagedStorage.get(PagedStorage.java:153)
    at androidx.paging.PagedList.get(PagedList.java:384)
    at androidx.paging.AsyncPagedListDiffer.getItem(AsyncPagedListDiffer.java:206)
    at androidx.paging.PagedListAdapter.getItem(PagedListAdapter.java:156)
    at com.mysample.pagedlist.MyPagedListAdapter.onBindViewHolder(MyPagedListAdapter.java:38)
    at com.mysample.pagedlist.MyPagedListAdapter.onBindViewHolder(MyPagedListAdapter.java:19)

似乎我不知道附加更多数据和刷新显示的正确方法。谁能告诉我怎么解决？

api作为数据库和dao

public class Api {
    private static List<String> list = null;

    private static void initListIfNeeded() {
        if (list == null) {
            list = new ArrayList<>();
            for (int i = 0; i < 100; i++) {
                list.add("" + i);
            }
        }
    }

    public static List<String> pageItems(int page, int pageSize) {
        initListIfNeeded();
        int start = page * pageSize;
        int end = (page + 1) * pageSize;
        if (end > list.size()) {
            end = list.size();
        }
        if (page < 0 || start >= list.size()) {
            return new ArrayList<>();
        } else {
            return list.subList(start, end);
        }
    }

    public static void appendItems() {
        initListIfNeeded();
        int size = list.size();
        for (int i = 0; i < 10; i++) {
            list.add("" + (size + i));
        }
    }
}

用于处理数据分页的DataSource

public class MyDataSource extends PageKeyedDataSource<Integer, String> {

    public MyDataSource() {
    }

    @Override
    public void loadInitial(@NonNull LoadInitialParams<Integer> params, @NonNull LoadInitialCallback<Integer, String> callback) {
        callback.onResult(Api.pageItems(0, params.requestedLoadSize), -1, 1);
    }

    @Override
    public void loadBefore(@NonNull LoadParams<Integer> params, @NonNull LoadCallback<Integer, String> callback) {
        callback.onResult(Api.pageItems(params.key, params.requestedLoadSize), params.key - 1);
    }

    @Override
    public void loadAfter(@NonNull LoadParams<Integer> params, @NonNull LoadCallback<Integer, String> callback) {
        callback.onResult(Api.pageItems(params.key, params.requestedLoadSize), params.key + 1);
    }

}

对应的ViewModel类

public class MyViewModel extends ViewModel {

    public LiveData<PagedList<String>> list;

    public MyViewModel() {
        list = new LivePagedListBuilder<>(new DataSource.Factory<Integer, String>() {
            @Override
            public DataSource<Integer, String> create() {
                return new MyDataSource();
            }
        },  new PagedList.Config.Builder().setPageSize(20).setInitialLoadSizeHint(20).build()).build();
    }

    public void appendItems() {
        Api.appendItems();
    }
}

活动

public class PagedListActivity extends AppCompatActivity {

    private MyPagedListAdapter adapter;
    private MyViewModel viewModel;
    private RecyclerView recyclerView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_paged_list);
        adapter = new MyPagedListAdapter(LayoutInflater.from(this));

        recyclerView = findViewById(R.id.recyclerView);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        recyclerView.setAdapter(adapter);
        viewModel = ViewModelProviders.of(this).get(MyViewModel.class);
        viewModel.list.observe(this, new Observer<PagedList<String>>() {
            @DebugLog
            @Override
            public void onChanged(PagedList<String> strings) {
                adapter.submitList(strings);
            }
        });
    }

    @DebugLog
    public void onClick(View view) {
        viewModel.appendItems();
    }
}

适配器

public class MyPagedListAdapter extends PagedListAdapter<String, MyPagedListAdapter.VH> {

    private LayoutInflater inflater;

    protected MyPagedListAdapter(LayoutInflater inflater) {
        super(DIFF_CALLBACK);
        this.inflater = inflater;
    }

    @NonNull
    @Override
    public VH onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = inflater.inflate(R.layout.item_text, parent, false);
        return new VH(view);
    }

    @Override
    public void onBindViewHolder(@NonNull VH holder, int position) {
        holder.setData(getItem(position));
    }

    public class VH extends RecyclerView.ViewHolder {

        private final TextView txt;

        public VH(@NonNull View itemView) {
            super(itemView);
            txt = itemView.findViewById(R.id.txt);
        }

        public void setData(String text) {
            txt.setText(text);
        }
    }

    private static DiffUtil.ItemCallback<String> DIFF_CALLBACK = new DiffUtil.ItemCallback<String>() {
        @Override
        public boolean areItemsTheSame(@NonNull String oldItem, @NonNull String newItem) {
            return oldItem.equalsIgnoreCase(newItem);
        }

        @Override
        public boolean areContentsTheSame(@NonNull String oldItem, @NonNull String newItem) {
            return oldItem.equalsIgnoreCase(newItem);
        }
    };

}

Answer 1

从此堆栈跟踪：

java.util.ConcurrentModificationException
    at java.util.AbstractList$SubAbstractList.size(AbstractList.java:360)

我们可以看出

a) 当有人在迭代列表的过程中尝试修改列表时，应用程序崩溃
b）这发生在 SubAbstractList 实例上（在您调用时创建 list.subList(start, end);

应用程序崩溃，因为在您修改（附加项目）原始列表之后，在原始列表修改之前创建的子列表不再有效

尝试从Api#pageItems返回list.subList(start, end);而不是尝试
返回new ArrayList<>(list.subList(start, end));

要让 RecyclerView 显示您的更改，您应该调用 Adapter#notifyItemRangeInserted 或 RW 适配器