Why RecyclerView.OnScrollListener is not an interface but an abstract class?
android, android-recyclerview, endlessscroll
Solution
I had the same question, and it's definitely as designed, as answered in this bug report:
https://code.google.com/p/android/issues/detail?id=79283
Abstract classes allow frameworks to add new methods without breaking existing implementations.
Also the diff that introduced it can be found here:
https://android.googlesource.com/platform/frameworks/support/+/cef7b49%5E!/
This change adds RecyclerView as a first parameter to the scroll related callbacks.
It also fxies a bug where scroll callback was being called w/ the intended scroll amount instead of the real scroll amount.
I also changed it to be an abstract class instead of an interface to make future changes easier.
Not sure I agree with the change, personally, but there ya go.
Problem
To implement Endless Scroll pattern in to the `RecyclerView` I want to create a class like ``` public class EndlessScrollAdapter<VH extends ViewHolder> extends RecyclerView.Adapter<VH> implements RecyclerView.OnScrollListener { } ``` Because `EndlessScrollAdapter` should be responsible for both of the data and scroll event handling, this is the most convenient way to implement it. But, because in `recyclerview-v7-21.0.3`, `OnScrollListener` declared like this ``` /** * An OnScrollListener can be set on a RecyclerView to receive messages * when a scrolling event has occurred on that RecyclerView. * * @see RecyclerView#setOnScrollListener(OnScrollListener) */ abstract static public class OnScrollListener { /** * Callback method to be invoked when RecyclerView's scroll state changes. * * @param recyclerView The RecyclerView whose scroll state has changed. * @param newState The updated scroll state. One of {@link #SCROLL_STATE_IDLE}, * {@link #SCROLL_STATE_DRAGGING} or {@link #SCROLL_STATE_SETTLING}. */ public void onScrollStateChanged(RecyclerView recyclerView, int newState){} /** * Callback method to be invoked when the RecyclerView has been scrolled. This will be * called after the scroll has completed. * * @param recyclerView The RecyclerView which scrolled. * @param dx The amount of horizontal scroll. * @param dy The amount of vertical scroll. */ public void onScrolled(RecyclerView recyclerView, int dx, int dy){} } ``` I can't make `EndlessScrollAdapter` implement `OnScrollListener`. So as the title says, is there any good reason for `OnScrollListener` to be a class rather than an interface? Because I think it should be an interface.