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

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 {

    //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 params, @NonNull final LoadInitialCallback callback) {
        RetrofitClient.getInstance()
                .getApi().getAnswers(FIRST_PAGE, PAGE_SIZE, SITE_NAME)
                .enqueue(new Callback() {
                    @Override
                    public void onResponse(Call call, Response response) {
                        if (response.body() != null) {
                            callback.onResult(response.body().items, null, FIRST_PAGE + 1);
                        }
                    }

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

                    }
                });
    }

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

                    }
                });
    }

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