title: Android Paging Library loadAfter() not called
body: <p>Let me just give a short overview on what I'm trying to do. So basically, I am new to this Paging Library in Android Jetpack and I'm trying to implement a paged list. The backend already did the part of paginating by request (<em>The backend returns 25 items per page as default</em>) so all I need to do is make a request and specify a page I want to display and then the items I needed will then be returned. I was able to display and make the <code>loadInitial()</code> work but after I scroll to the lowest item it won't fire the <code>loadAfter()</code>.</p>
  
  <p>My <code>PagedListAdapter</code> and <code>RecyclerView</code> is very straight forward so I don't think posting it here would help. The <code>onCreateViewHolder</code> returns a ViewHolder by Data Binding. The <code>onBindViewHolder</code> just calls a method inside my viewholder and set the items. I'm not overriding the <code>getItem()</code> method. That's about it for my adapter and the <code>DiffCallback</code>.</p>
  
  <p>However, below are my other code snippets. Note that I have modified the variable and method names for privacy.</p>
  
  <p><strong>My Config for the LivePagedListBuilder</strong>:</p>
  
  <pre><code>public MyViewModel() {
      PagedList.Config config =
          new PagedList.Config.Builder()
              .setPageSize(1)
              .setPrefetchDistance(1)
              .build();
  
      items = new LivePagedListBuilder&lt;&gt;(myDataSourceFactory, config).build();
  }
  
  public LiveData&lt;PagedList&lt;Item&gt;&gt; getItems() {
      return items;
  }
  </code></pre>
  
  <p><strong>The way I observe the LiveData in my View</strong>:</p>
  
  <pre><code>viewModel.getItems().observe(this, items -&gt; {
    adapter.submitList(items);
  });
  </code></pre>
  
  <p><strong>My Data Source</strong>: 
  ItemKeyedDataSource</p>
  
  <pre><code>private int defaultPage = 1;
  
  public void loadInitial(@NonNull LoadInitialParams&lt;Integer&gt; params, @NonNull 
   LoadInitialCallback&lt;Item&gt; callback) {
  
      remoteRepository.getItems(String.valueOf(defaultPage))
      ...
  }
  
  public void loadAfter(@NonNull LoadParams&lt;Integer&gt; params, @NonNull LoadCallback&lt;Item&gt; callback) {
  
      remoteRepository.getItems(String.valueOf(params.key))
      ...
  }
  
  public Integer getKey(@NonNull Item item) {
      return defaultPage++;
  }
  </code></pre>