Paging Library without Room

Question

All examples of the new paging library have been with Room library and Room creates a Data Source for us. In my own case, I need to create my custom data source.

Here is a method in my view model class that ought to return the live data. My livedata always returns null.

 LiveData<PagedList<ApiResult>> getData(){

    LivePagedListProvider<Integer,ApiResult> p = new LivePagedListProvider<Integer, ApiResult>() {
        @Override
        protected DataSource<Integer, ApiResult> createDataSource() {
            return new DataClass();
        }

    };

    listLiveData = p.create(0,new PagedList.Config.Builder()
            .setPageSize(5) //number of items loaded at once
            .setPrefetchDistance(0)// the distance to the end of already loaded list before new data is loaded
            .build());
    return listLiveData;
}

And here is the Data class

public class DataClass extends TiledDataSource<ApiResult> {

    private List<ApiResult> result = new ArrayList<>();

    @Override
    public int countItems() {
        return result.size();
    }

    @Override
    public List<ApiResult> loadRange(int startPosition, int count) {

        Call<String> call = NetworkModule.providesWebService().makeRequest();
        call.enqueue(new Callback<String>() {
            @Override
            public void onResponse(@NonNull Call<String> call, @NonNull Response<String> response) {
                Log.i(DataClass.this.getClass().getSimpleName() + " - onResponse", String.valueOf(response));
                result = parseJson(response.body());
            }

            @Override
            public void onFailure(@NonNull Call<String> call, @NonNull Throwable t) {
                Log.i(DataClass.this.getClass().getSimpleName() + " - onFailure", t.getMessage());
            }

        });

        return result;
    }

}

Answer 1

You can create a Custom Data Source, usually we built backend API to fetch data that takes pagenumber as a parameter to return the specified page.

For this situation you can use PageKeyedDataSource. Here is a sample code of PageKeyedDataSource using the StackOverflow API. The below code is using Retrofit to get the data from the StackOverflow API.

public class ItemDataSource extends PageKeyedDataSource<Integer, Item> {

    //the size of a page that we want
    public static final int PAGE_SIZE = 50;

    //we will start from the first page which is 1
    private static final int FIRST_PAGE = 1;

    //we need to fetch from stackoverflow
    private static final String SITE_NAME = "stackoverflow";


    //this will be called once to load the initial data
    @Override
    public void loadInitial(@NonNull LoadInitialParams<Integer> params, @NonNull final LoadInitialCallback<Integer, Item> callback) {
        RetrofitClient.getInstance()
                .getApi().getAnswers(FIRST_PAGE, PAGE_SIZE, SITE_NAME)
                .enqueue(new Callback<StackApiResponse>() {
                    @Override
                    public void onResponse(Call<StackApiResponse> call, Response<StackApiResponse> response) {
                        if (response.body() != null) {
                            callback.onResult(response.body().items, null, FIRST_PAGE + 1);
                        }
                    }

                    @Override
                    public void onFailure(Call<StackApiResponse> call, Throwable t) {

                    }
                });
    }

    //this will load the previous page
    @Override
    public void loadBefore(@NonNull final LoadParams<Integer> params, @NonNull final LoadCallback<Integer, Item> callback) {
        RetrofitClient.getInstance()
                .getApi().getAnswers(params.key, PAGE_SIZE, SITE_NAME)
                .enqueue(new Callback<StackApiResponse>() {
                    @Override
                    public void onResponse(Call<StackApiResponse> call, Response<StackApiResponse> response) {

                        //if the current page is greater than one
                        //we are decrementing the page number
                        //else there is no previous page
                        Integer adjacentKey = (params.key > 1) ? params.key - 1 : null;
                        if (response.body() != null) {

                            //passing the loaded data
                            //and the previous page key 
                            callback.onResult(response.body().items, adjacentKey);
                        }
                    }

                    @Override
                    public void onFailure(Call<StackApiResponse> call, Throwable t) {

                    }
                });
    }

    //this will load the next page
    @Override
    public void loadAfter(@NonNull final LoadParams<Integer> params, @NonNull final LoadCallback<Integer, Item> callback) {
        RetrofitClient.getInstance()
                .getApi()
                .getAnswers(params.key, PAGE_SIZE, SITE_NAME)
                .enqueue(new Callback<StackApiResponse>() {
                    @Override
                    public void onResponse(Call<StackApiResponse> call, Response<StackApiResponse> response) {

                        if (response.body() != null) {
                            //if the response has next page
                            //incrementing the next page number
                            Integer key = response.body().has_more ? params.key + 1 : null; 

                            //passing the loaded data and next page value 
                            callback.onResult(response.body().items, key);
                        }
                    }

                    @Override
                    public void onFailure(Call<StackApiResponse> call, Throwable t) {

                    }
                });
    }
}

Here you can see we are getting the result by making a Retrofit call and then we are pushing the result to callback.

For a detailed, step by step explanation go through this Android Paging Library Tutorial.

Answer 2

I think this can help:
1. countItems() should return DataSource.COUNT_UNDEFINED
2. loadRange(int startPosition, int count): You can directly execute your query.
3. So now you can delete result global variable

Also, turnoff placeholders:

listLiveData = p.create(0,new PagedList.Config.Builder()
        .setPageSize(5) //number of items loaded at once
        .setPrefetchDistance(10) //Must be >0 since placeholders are off
        .setEnablePlaceholders(false)
        .build());

Here is the updated code for Data Class:

public class DataClass extends TiledDataSource<ApiResult> {

@Override
public int countItems() {
 return DataSource.COUNT_UNDEFINED;
}

@Override
public List<ApiResult> loadRange(int startPosition, int count) {

 Call<String> call = NetworkModule.providesWebService().makeRequest();
 Response<String> response = call.execute();
 return parseJson(response.body());
}

You can check an example project here: https://github.com/brainail/.samples/tree/master/ArchPagingLibraryWithNetwork