分页库最初返回空列表 [英] Paging library returns empty list initially

问题描述

我正在使用Paging库对从服务器中检索的项目列表进行分页。最初，在加载我的片段时，它将返回一个空列表。但是在更改了片段并返回到该片段之后，我可以看到列表已加载。调试后，我看到实际上是在获取数据，但是一个空列表传递给了我的片段。

I'm using Paging library to paginate a list of items I'm retrieving from my server. Initially, when my fragment is loaded, it returns an empty list. But after changing fragments and going back to that fragment, I can see the list loaded. After debugging I saw that data was actually being fetched, but an empty list was passed to my fragment.

ItemDataSource：

@Override
public void loadInitial(@NonNull LoadInitialParams<Integer> params, @NonNull LoadInitialCallback<Integer, Item> callback) {
    apiService.getItems(OFFSET)
    .enqueue(new Callback<ItemWrapper>() {
        @Override
        public void onResponse(@NonNull Call<ItemWrapper> call,@NonNull Response<ItemWrapper> response) {
            callback.onResult(response.body().getItems(), null, OFFSET + 25);
        }

        @Override
        public void onFailure(@NonNull Call<ItemWrapper> call,@NonNull Throwable t) {
            t.printStackTrace();
        }
    });
}

@Override
public void loadBefore(@NonNull LoadParams<Integer> params, @NonNull LoadCallback<Integer, Item> callback) {

}

@Override
public void loadAfter(@NonNull LoadParams<Integer> params, @NonNull LoadCallback<Integer, Item> callback) {
    apiService.getItems(params.key)
            .enqueue(new Callback<ItemWrapper>() {
                @Override
                public void onResponse(@NonNull Call<ItemWrapper> call,@NonNull Response<ItemWrapper> response) {
                    Integer key = response.body().getItems().isEmpty() ? null : params.key + 25;
                    callback.onResult(response.body().getItems(), key);
                }

                @Override
                public void onFailure(@NonNull Call<ItemWrapper> call,@NonNull Throwable t) {
                    t.printStackTrace();
                }
            });
}

ItemDataSourceFactory：

@Override
public DataSource create() {
    ItemDataSource itemDataSource = new ItemDataSource();
    itemLiveDataSource.postValue(itemDataSource);
    return itemDataSource;
}

public MutableLiveData<ItemDataSource> getItemLiveDataSource() {
    return itemLiveDataSource;
}

ItemViewModel：

private LiveData<ItemDataSource> liveDataSource;
private LiveData<PagedList<Item>> itemPagedList;

private ItemViewModel(Application application) {
    ItemDataSourceFactory factory = new ItemDataSourceFactory();
    liveDataSource = factory.getItemLiveDataSource();

    PagedList.Config config = (new PagedList.Config.Builder())
                .setEnablePlaceholders(false)
                .setPageSize(ItemDataSource.LIMIT).build();

    itemPagedList = (new LivePagedListBuilder(factory, config)).build();
}

public LiveData<PagedList<Item>> getItems() {
    return itemPagedList;
}

片段：

ItemViewModel itemViewModel = ViewModelProviders.of(this).get(ItemViewModel.class);
itemViewModel.getItems.observe(this, items -> {
    adapter.submitList(items);
})

推荐答案

不确定100％，但这是因为您正在运行异步请求。尝试将其更改为对于 loadInitial（）同步运行，例如 request.execute（）

Not 100% sure, but I think this is because you are running an asynchronous request. try to change it to run synchronously for loadInitial() like so request.execute()

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