Paging library returns empty list initially

问题

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;
}

Fragment:

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

回答1:

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()

回答2:

I also had this problem once and I still can't figure it out why it does not work for some fragments. The solution I found, aldo being more like a fast sketchy fix is to load the fragment twice.

回答3:

Yassin Ajdi is right. loadinitial() calls immediately on the same thread where PagedList is created on. As your API is async, the method runs empty for the first time

回答4:

If, like me, anyone is using an asynchronous RxJava/Kotlin call in loadInitial. I finally figured out a solution after many painful hours.

I tried using a delayed Handler (500ms) in the Observer method but it was fickle and didn't work in every scenario. No matter how much I tried to make it synchronous using setFetcher and Rx observeOn it wouldn't consistently work.

My solution was to use .blockingSubscribe in my Observable. My data fetcher was using a Socket library that had its own concurrency out of my scope, so I couldn't guarantee that I could make the process wholly synchronous as Paging requires. (A process which needs better documentation IMO). Anyway, here's my solution, hopefully it helps others with same issue:

    override fun loadInitial(
            params: LoadInitialParams<Int>,
            callback: LoadInitialCallback<Int, ResultItem>
    ) {
       mySocketClientRxRequest()
                .subscribe ({
                    callback.onResult(it.resultItems 1, 2)
                },{
                    it.printStackTrace()
                })
    }

to

    override fun loadInitial(
            params: LoadInitialParams<Int>,
            callback: LoadInitialCallback<Int, ResultItem>
    ) {
       mySocketClientRxRequest()
                .blockingSubscribe ({
                    callback.onResult(it.resultItems 1, 2)
                },{
                    it.printStackTrace()
                })
    }

来源：https://stackoverflow.com/questions/53633224/paging-library-returns-empty-list-initially