ScrollView with children view, how to intercept scroll conditionally

Question

I have a container ViewGroup, lets call it screen inside a ScrollView. This container view hosts a number of other Views let's call them widgets, and some of them are interested in preventing the ScrollView from scrolling and using the MotionEvent theirselves (for example a pannable image)

I can't figure out the proper event intercept strategy to use. ScrollView always processes the event before the children, or the children process the event but scrollview is disabled.

I read about issuing getParent().requestDisableInterceptTouchEvent() in the child views if this view wants to capture the event, but their onTouchEvent is not called, I suppose because ScrollView has engulfed the event beforehand. I guess the fact that I have 2 levels of layers (container + widgets) prevents this from working, I suppose the container ViewGroup has to play an important part here, but I can't figure out which one...

Can I know, at the ScrollView's onInterceptTouchEvent level, which widget on the container viewGroup has been touched to decide if I should intercept or not?

or...

How can the 'widget' layers in the ViewGroup get the event before ScrollView so I can call getParent().onRequestDisableInterceptTouch() ... or is it getParent().getParent().onRequestDisableInterceptTouch()?

Thanks in advance

I've read related questions but no luck ... Handle touch events in ScrollView Android

Answer 1

Well after a night of coca cola & debugging I managed to get this to work. I share the solution just in case it is of interest to anyone, because it took me quite a lot of time to get it running.

I didn't manage to get it running with getParent().onRequestDisableInterceptTouch(), I was close, but couldn't find a way for the child widgets to get the MotionEvents they need for scrolling once I intercepted the touch on the parent, so even though the outer scroll was prevented correctly, the inner widgets didn't scroll.

So the solution is to interceptTouchEvents in the children ONLY, and if the children is scrollable (known property), and the touch is ACTION_DOWN, then disable the scrollview two levels above. If the touch is ACTION_UP, we enable the scrollview.

To enable/disable the scrollview I just intercept the touch event and with a flag filter the event or not.

I did three auxiliary classes, one for the ScrollView, one for the Container, One for the widgets:

This class wraps every widget and, if I call setNeedsScroll(true) , then touches will be intercepted, and when it is touched, it will (tell the container to) tell the scrollview to disable itself. When the touch is released, it will re-enable the scrollview.

class WidgetWrapperLayout extends FrameLayout {

    private boolean mNeedsScroll=false;

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

  /** Called anytime, ie, during construction, to indicate that this 
    * widget uses vertical scroll, so we need to disable its container scroll 
    */

    public void setNeedsScroll(boolean needsScroll) { 
        mNeedsScroll=needsScroll; 
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        if (mNeedsScroll)  {
            switch (ev.getAction()) {  
            case MotionEvent.ACTION_DOWN:
                ((SlideLayout)getParent()).setEnableScroll(false);
                break;
            case MotionEvent.ACTION_UP:
                ((SlideLayout)getParent()).setEnableScroll(true);
                break;
            }
            return false;
        }
        return super.onInterceptTouchEvent(ev);
    }
}

This is the container, only child of the scrollview, and holds the different widgets. It just provides methods for the children so they can enable/disable the scroll:

public class ContainerLayout extends FrameLayout {

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

    public void setEnableScroll(boolean status) {
        if (Conf.LOG_ON) Log.d(TAG, "Request enable scroll: "+status);
        ((StoppableScrollView)getParent()).setScrollEnabled(status);
    }
}

and finally a scrollview capable of deactivation. It disables the scroll 'old-skool', intercepting and blocking events.

public class StoppableScrollView extends ScrollView {

    private String TAG="StoppableScrollView";

    private boolean mDisableScrolling=false;

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

    /** Enables or disables ScrollView scroll */
    public void setScrollEnabled (boolean status) { 
        if (Conf.LOG_ON) Log.d(TAG, "Scroll Enabled "+status);
        mDisableScrolling=!status; 
    }


    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        if (mDisableScrolling) return false;
        return super.onInterceptTouchEvent(ev);
    }
}

Answer 2

Implement View.OnTouchListener in your Activity and add that listener to the ScrollView. Then return true in onTouchEvent(...). Before returning call the onTouch of the children you want to handle that event.