Question

In my android application, I have a scroll view which has a lot of children view.
How can I listen for an event when a view is switching from visible to ‘non visible to user’ due to user’s scroll action?

Answer 1

So the default ScrollView does not provide any sort of scroll listener, but it does call onScrollChanged() when the user scrolls, so you can implement your own:

public class YourScrollView extends ScrollView {
  private ScrollListener scrollListener;

  ... // constructors etc

  public void setScrollListener(ScrollListener scrollViewListener) {
    this.scrollListener = scrollViewListener;
  }

  @Override
  protected void onScrollChanged(int x, int y, int oldx, int oldy) {
    super.onScrollChanged(x, y, oldx, oldy);

    if (scrollListener != null) {  //I don't care about the direction, you might.
      scrollListener.onScrollChanged(); 
    }
  }

  public static interface ScrollListener {
    public void onScrollChanged();
  }

}

Now in your onScrollChanged you can go through your Views and check if they are visible:

int[] location = {0,0};
view.getLocationOnScreen(location);

//container height is the height of the scrollview
if (location[1] + view.getBottom() < 0 || location[1]  > containerHeight) {
  //your view is not visible
}