为什么 RecyclerView.OnScrollListener 不是接口而是抽象 class？

Question

要在 RecyclerView 中实施 Endless Scroll 模式，我想创建一个像

这样的 class

public class EndlessScrollAdapter<VH extends ViewHolder> 
        extends RecyclerView.Adapter<VH> implements RecyclerView.OnScrollListener {
}

因为EndlessScrollAdapter应该同时负责数据和滚动事件处理，这是最方便的实现方式。

但是，因为在recyclerview-v7-21.0.3中，OnScrollListener是这样声明的

/**
 * 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){}
}

我无法让 EndlessScrollAdapter 实现 OnScrollListener。

所以正如标题所说，OnScrollListener 是一个 class 而不是一个界面有什么好的理由吗？因为我觉得应该是接口。

Answer 1

I can't make EndlessScrollAdapter implement OnScrollListener.

这是真的，但是您可以有一个专用的 class extends RecyclerView.OnScrollListener（RecyclerView.OnScrollListener 的具体实例）。例如

private class MyScrollListener extends RecyclerView.OnScrollListener {
   // abstract methods implemenations
}

而你需要的只是

mRecyclerView.addOnScrollListener(new MySCrollListener());

Answer 2

我有同样的问题，它绝对是设计好的，正如这个错误报告中所回答的那样：

https://code.google.com/p/android/issues/detail?id=79283

Abstract classes allow frameworks to add new methods without breaking existing implementations.

也可以在这里找到介绍它的差异：

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.

不确定我个人是否同意更改，但是你去吧。

Answer 3

 yourRecyclerview.setOnScrollListener(new RecyclerView.OnScrollListener() {
        @Override
        public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
            super.onScrollStateChanged(recyclerView, newState);
        }

        @Override
        public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
            super.onScrolled(recyclerView, dx, dy);
        }
    });

Answer 4

避免在您的方法中设置侦听器的想法

...
recyclerView.addOnScrollListener(new ScrollListener());
...

并创建一个内部监听器class

private class ScrollListener extends RecyclerView.OnScrollListener {
    @Override
    public void onScrollStateChanged(RecyclerView recyclerView, int newState){
        // your code there
    }

    @Override
    public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
        // your code there
    }
}

为什么 RecyclerView.OnScrollListener 不是接口而是抽象 class？

Why RecyclerView.OnScrollListener is not an interface but an abstract class?

android

android-recyclerview

endlessscroll