android - PagerSnapHelper 中的 From-To Page Changed 监听器

Question

我在水平 RecyclerView 中使用 PagerSnapHelper 来实现类似 View 寻呼机的行为。

final PagerSnapHelper pagerSnapHelper = new PagerSnapHelper();
pagerSnapHelper.attachToRecyclerView(recyclerView);

效果很好，但我希望能够在用户向任一方向更改页面时获得回调。所以类似于 onSwipeLeft/onSwipeRight 回调。

我尝试在 PagerSnapHelper 中使用 findTargetSnapPosition，但这只给我 targetIndex 而不是当前索引。我尝试过类似的方法，但它并不总是有效。

@Override
public int findTargetSnapPosition(RecyclerView.LayoutManager layoutManager, int velocityX, int velocityY) {
    final int targetPos = super.findTargetSnapPosition(layoutManager, velocityX, velocityY);

    final View currentView = findSnapView(layoutManager);
    final int currentPos = layoutManager.getPosition(currentView);

    if (currentPos < targetPos) {
        callback.onSwipeRight();
    } else if (currentPos > targetPos) {
        callback.onSwipeLeft();
    }

    return targetPos;
}

有没有更好的方法来实现这个总是有效的？谢谢!

Answer 1

更新 2
!!重要提示!!
如果您以编程方式调用 scrollTo() 并将 SnapPagerScrollListener 与 ON_SETTLED 一起使用，则不会调用 onScrollStateChanged。所以旧的捕捉位置不会得到更新。 WIP，我会在修复后立即更新类(class)。

更新

原始类在第一个布局上通知时存在一些问题。现在它仅在项目位置第一次从 RecyclerView.NO_POSITION 更改为其他内容时触发。

为了进一步扩展为仅在用户手势时忽略/触发，因此对 scrollTo() 的非编程调用，请注意 onScrolled() 会被 dx 触发== 0 和 dy == 0 在编程调用的情况下。

public class SnapPagerScrollListener extends RecyclerView.OnScrollListener {

    // Constants
    public static final int ON_SCROLL = 0;
    public static final int ON_SETTLED = 1;

    @IntDef({ON_SCROLL, ON_SETTLED})
    public @interface Type {
    }

    public interface OnChangeListener {
        void onSnapped(int position);
    }

    // Properties
    private final PagerSnapHelper snapHelper;
    private final int type;
    private final boolean notifyOnInit;
    private final OnChangeListener listener;
    private int snapPosition;

    // Constructor
    public SnapPagerScrollListener(PagerSnapHelper snapHelper, @Type int type, boolean notifyOnInit, OnChangeListener listener) {
        this.snapHelper = snapHelper;
        this.type = type;
        this.notifyOnInit = notifyOnInit;
        this.listener = listener;
        this.snapPosition = RecyclerView.NO_POSITION;
    }

    // Methods
    @Override
    public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {
        super.onScrolled(recyclerView, dx, dy);
        if ((type == ON_SCROLL) || !hasItemPosition()) {
            notifyListenerIfNeeded(getSnapPosition(recyclerView));
        }
    }

    @Override
    public void onScrollStateChanged(@NonNull RecyclerView recyclerView, int newState) {
        super.onScrollStateChanged(recyclerView, newState);
        if (type == ON_SETTLED && newState == RecyclerView.SCROLL_STATE_IDLE) {
            notifyListenerIfNeeded(getSnapPosition(recyclerView));
        }
    }

    private int getSnapPosition(RecyclerView recyclerView) {
        RecyclerView.LayoutManager layoutManager = recyclerView.getLayoutManager();
        if (layoutManager == null) {
            return RecyclerView.NO_POSITION;
        }

        View snapView = snapHelper.findSnapView(layoutManager);
        if (snapView == null) {
            return RecyclerView.NO_POSITION;
        }

        return layoutManager.getPosition(snapView);
    }

    private void notifyListenerIfNeeded(int newSnapPosition) {
        if (snapPosition != newSnapPosition) {
            if (notifyOnInit && !hasItemPosition()) {
                listener.onSnapped(newSnapPosition);
            } else if (hasItemPosition()) {
                listener.onSnapped(newSnapPosition);
            }

            snapPosition = newSnapPosition;
        }
    }

    private boolean hasItemPosition() {
        return snapPosition != RecyclerView.NO_POSITION;
    }
}

用法:
只需将 SnapPagerScrollListener 实例添加到您的 RecyclerView

your_recycler_view.addOnScrollListener(new SnapPagerScrollListener(your_snap_helper, SnapPagerScrollListener.ON_SCROLL/ON_SETTLED, true/false, your_on_changed_listener));

Type 属性用于定义何时触发回调。

1. ON_SCROLL:用于在新 View /页面通过中间时立即通知回调
2. ON_SETTLED:用于通知RecyclerViews状态为SCROLL_STATE_IDLE后的回调。我使用该模式仅在滚动稳定后触发 API 调用。