android - 在 ScrollView 中显示 View 时的通知方式？

Question

我有一个简单的问题:

假设我对 scrollView(或 horizo​​ntalScrollView)有一些看法。有什么方法可以添加一个监听器，当这样的 View 进入可见区域内外时，它会告诉我？

我见过的唯一类似的问题是: Android: how to check if a View inside of ScrollView is visible?但我想在发生此类事件(变得隐藏/可见)时得到通知。

Answer 1

子类化您正在使用的 View 类(我为 ImageView 做了这个，因为我只是将它们添加到我的 ScrollView 中):

public class PeekImageView extends ImageView implements ViewTreeObserver.OnScrollChangedListener {
    private static final String LOG_TAG = "PeekImageView";
    private InViewportListener inViewportListener;
    private boolean isInViewport = false;

    public PeekImageView(Context context) {
        super(context);
    }

    public PeekImageView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public PeekImageView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    public interface InViewportListener {
        void onViewportEnter(PeekImageView view);
        void onViewportExit(PeekImageView view);
    }

    public void setInViewportListener(InViewportListener listener) {
        this.inViewportListener = listener;
    }

    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();
        ViewTreeObserver vto = getViewTreeObserver();
        if (vto != null) {
            vto.addOnScrollChangedListener(this);
        }
    }

    @Override
    protected void onDetachedFromWindow() {
        super.onDetachedFromWindow();
        ViewTreeObserver vto = getViewTreeObserver();
        if (vto != null) {
            vto.removeOnScrollChangedListener(this);
        }
    }

    @Override
    public void onScrollChanged() {
        Rect bounds = new Rect();
        boolean inViewport = getLocalVisibleRect(bounds);
        Log.d(LOG_TAG, "is in view " + bounds + " : " + inViewport + " ; " + bounds);
        if (inViewportListener != null && isInViewport != inViewport) {
            if (inViewport) {
                inViewportListener.onViewportEnter(this);
            } else {
                inViewportListener.onViewportExit(this);
            }
        }
        isInViewport = inViewport;
    }
}

将 InViewportListener 附加到此 PeekImageView 的实例将在 View 进入或离开窗口的可见部分(视口(viewport))时通知您。