android - ScrollView and Gallery interfering

Question

I have a Gallery composed of many ScrollViews each of which occupies the whole screen. problem is the ScrollViews' onTouchEvent returns true and therefore prevent any other view in the DOM to handle the same event (which is swallowed after being processed at the ScrollView level). As a result the Gallery doesn't scroll anymore. On the other hand if I override onTouchEvent like this:

   @Override
   public boolean onTouchEvent(MotionEvent ev) {
      super.onTouchEvent(ev);
      return false; // <<<<<<<<<<<<<<<<<
   }   

then the Gallery receives its on event to process but the SrollView doesnt scroll anymore. Either way you lose! or do you?

problem sounds puzzling but I am sure if u stumbled upon it in the past u r gonna recognize it straight away as it a freaking damn one!

thanks

Answer 1

Here's my attempt at gallery that works with vertical ScrollViews.

It uses its own instance of GestureDetector and feeds it with MotionEvents from onInterceptTouchEvent.

When gesture detector recognizes a scroll, we determine whether it's horizontal or vertical and lock on the direction until the gesture is finished. This avoids diagonal scrolling.

If it's a horizontal scroll, onInterceptTouchEvent will return true so that future motion events go to inherited Gallery.onTouchEvent to do the actual scrolling.

Gallery's own gesture detector (mGestureDetector in Gallery.java) doesn't get all motion events and thus sometimes reports huge sudden scrolls that cause gallery to jump around. I've put in a a nasty hack that discards those.

The code:

import android.content.Context;
import android.util.AttributeSet;
import android.view.GestureDetector;
import android.view.MotionEvent;
import android.widget.Gallery;

public class BetterGallery extends Gallery {
    /* This gets set when we detect horizontal scrolling */
    private boolean scrollingHorizontally = false;

    /* This gets set during vertical scrolling. We use this to avoid detecting
     * horizontal scrolling when vertical scrolling is already in progress
     * and vice versa. */
    private boolean scrollingVertically = false;

    /* Our own gesture detector, Gallery's mGestureDetector is private.
     * We'll feed it with motion events from `onInterceptTouchEvent` method. */
    private GestureDetector mBetterGestureDetector;

    public BetterGallery(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        mBetterGestureDetector = new GestureDetector(new BetterGestureListener());
    }

    public BetterGallery(Context context, AttributeSet attrs) {
        super(context, attrs);
        mBetterGestureDetector = new GestureDetector(new BetterGestureListener());
    }

    public BetterGallery(Context context) {
        super(context);
        mBetterGestureDetector = new GestureDetector(new BetterGestureListener());
    }

    @Override
    public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX,
            float velocityY) {
        // Limit velocity so we don't fly over views
        return super.onFling(e1, e2, 0, velocityY);
    } 

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        // Documentation on this method's contract:
        // http://developer.android.com/reference/android/view/ViewGroup.html#onInterceptTouchEvent(android.view.MotionEvent)

        // Reset our scrolling flags if ACTION_UP or ACTION_CANCEL
        switch (ev.getAction()) {
        case MotionEvent.ACTION_UP:
        case MotionEvent.ACTION_CANCEL:
            scrollingHorizontally = false;
            scrollingVertically = false;
        }       

        // Feed our gesture detector
        mBetterGestureDetector.onTouchEvent(ev);

        // Intercept motion events if horizontal scrolling is detected
        return scrollingHorizontally;
    }


    @Override
    public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {
        // Hack: eat jerky scrolls caused by stale state in mGestureDetector
        // which we cannot directly access
        if (Math.abs(distanceX) > 100) return false;

        return super.onScroll(e1, e2, distanceX, distanceY);
    }


    @Override
    public boolean onTouchEvent(MotionEvent event) {
        // Reset our scrolling flags if ACTION_UP or ACTION_CANCEL
        switch(event.getAction()) {
        case MotionEvent.ACTION_UP:
        case MotionEvent.ACTION_CANCEL:
            scrollingHorizontally = false;
            scrollingVertically = false;
        }

        super.onTouchEvent(event);
        return scrollingHorizontally;
    }

    private class BetterGestureListener implements GestureDetector.OnGestureListener {

        @Override
        public boolean onDown(MotionEvent arg0) {
            return false;
        }

        @Override
        public boolean onFling(MotionEvent arg0, MotionEvent arg1, float arg2, float arg3) {
            return false;
        }

        @Override
        public void onLongPress(MotionEvent arg0) {
        }

        @Override
        public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {
            if (scrollingHorizontally || scrollingVertically) {
                // We already know we're scrolling, ignore this callback.
                // This avoids changing scrollingHorizontally / scrollingVertically
                // flags mid-scroll.
                return false;
            }

            scrollingHorizontally |= Math.abs(distanceX) > Math.abs(distanceY);
            // It's a scroll, and if it's not horizontal, then it has to be vertical
            scrollingVertically = !scrollingHorizontally;

            return false;
        }

        @Override
        public void onShowPress(MotionEvent arg0) {

        }

        @Override
        public boolean onSingleTapUp(MotionEvent arg0) {
            return false;
        }
    }
}

Warning: word "Better" in class name is likely misleading!

Update:

Forgot to mention, I've also set the activity to forward its onTouchEvent to gallery:

@Override
public boolean onTouchEvent(MotionEvent event) {
    return mGallery.onTouchEvent(event);
}

Update 2:

I've made some improvements to this code and put it up on bitbucket. There's also an example app. It demonstrates that this widget has issues with ListView as child :-/

Update 3:

Switched from Gallery to HorizontalScrollView as the base class for my custom widget. More on this here. Flings work, ListViews and ExpandableListViews as children work, tested on Android 1.6, 2.2, 2.3.4. Its behaviour is now quite close to that of Google apps, IMO.

Update 4:

Google has published ViewPager!