Question

I have a Gallery with an adapter which supplies it ScrollViews as its child views.
I need to make sure that the touch events are handled correctly and as expected:

1. When the user scrolls horizontally, the gallery scrolls horizontally.
2. When the user scrolls vertically, the scroll view scrolls vertically.
3. Both scrolls should never happen on the same gesture (the user must lift their finger to scroll the other view).
4. Everything must scroll smoothly.

Without overriding any methods, the scroll view is the only thing that scrolls – the gallery never scrolls.

So I understand I need to use onInterceptTouchEvent(…) in the gallery to decide to take over a certain series of MotionEvents but I am unsure how to check if the touch is horizontal or vertical in nature.

Answer 1

OK, after some major fiddling and logcat hacking, here’s the solution:

public class SwipeInterceptingGallery extends Gallery {

    private float mInitialX;
    private float mInitialY;
    private boolean mNeedToRebase;
    private boolean mIgnore;

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

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

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

    @Override
    public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX,
            float distanceY) {
        if (mNeedToRebase) {
            mNeedToRebase = false;
            distanceX = 0;
        }
        return super.onScroll(e1, e2, distanceX, distanceY);
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent e) {
        switch (e.getAction()) {
            case MotionEvent.ACTION_DOWN: {
                mIgnore = false;
                mNeedToRebase = true;
                mInitialX = e.getX();
                mInitialY = e.getY();
                return false;
            }

            case MotionEvent.ACTION_MOVE: {
                if (!mIgnore) {
                    float deltaX = Math.abs(e.getX() - mInitialX);
                    float deltaY = Math.abs(e.getY() - mInitialY);
                    mIgnore = deltaX < deltaY;
                    return !mIgnore;
                }
                return false;
            }
            default: {
                return super.onInterceptTouchEvent(e);
            }
        }
    }
}