Detect end of fling on ScrollView

Question

I've overridden ScrollView to pass MotionEvents to a GestureDetector to detect fling events on the ScrollView. I need to be able to detect when the scrolling stops. This doesn't coincide with the MotionEvent.ACTION_UP event because this usually happens at the start of a fling gesture, which is followed by a flurry of onScrollChanged() calls on the ScrollView.

So basically what we are dealing with here is the following events:

1. onFling
2. onScrollChanged, onScrollChanged, onScrollChanged, ... , onScrollChanged

There's no callback for when the onScrollChanged events are done firing. I was thinking of posting a message to the event queue using a Handler during onFling and waiting for the Runnable to execute to signal the end of the fling, unfortunately it fires after the first onScrollChanged call.

Any other ideas?

Sebiferous

Yes, as I mentioned in my question that approach doesn't work. – Sebiferous

Have you seen/tried my answer? – Coracoid

Answer 1

I've combined a few of the answers from here to construct a working listener that resembles the way AbsListView does it. It's essentially what you describe, and it works well in my testing.

Note: you can simply override ScrollView.fling(int velocityY) rather than use your own GestureDetector.

import android.content.Context;
import android.util.AttributeSet;
import android.widget.ScrollView;

public class CustomScrollView extends ScrollView {

    private static final int DELAY_MILLIS = 100;

    public interface OnFlingListener {
        public void onFlingStarted();
        public void onFlingStopped();
    }

    private OnFlingListener mFlingListener;
    private Runnable mScrollChecker;
    private int mPreviousPosition;

    public CustomScrollView(Context context) {
        this(context, null, 0);
    }

    public CustomScrollView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

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

        mScrollChecker = new Runnable() {
            @Override
            public void run() {
                int position = getScrollY();
                if (mPreviousPosition - position == 0) {
                    mFlingListener.onFlingStopped();
                    removeCallbacks(mScrollChecker);
                } else {
                    mPreviousPosition = getScrollY();
                    postDelayed(mScrollChecker, DELAY_MILLIS);
                }
            }
        };
    }

    @Override
    public void fling(int velocityY) {
        super.fling(velocityY);

        if (mFlingListener != null) {
            mFlingListener.onFlingStarted();
            post(mScrollChecker);
        }
    }

    public OnFlingListener getOnFlingListener() {
        return mFlingListener;
    }

    public void setOnFlingListener(OnFlingListener mOnFlingListener) {
        this.mFlingListener = mOnFlingListener;
    }

}

Coracoid

I haven't tried this, but the constant messages on the UI thread might be a performance issue no? – Sebiferous

Considering that this is similar to the way its done with the Scroller in AbsListView, I think you'll be fine. They actually update every 40ms. Check out AbsListView.FlingRunnable – Coracoid

Just don't get confused with android.support.v7.widget.RecyclerView.onFlingListener – Counterpoise

Answer 2

Thank you @PaulBurke +1

Xamarin Solution

using Android.Content;
using Android.Runtime;
using Android.Util;
using Android.Widget;
using System;

public class CustomScrollView : ScrollView
{
    public event EventHandler FlingEnded;
    public event EventHandler FlingStarted;

    private Action ScrollChecker;
    private int PreviousPosition;
    private const int DELAY_MILLIS = 100;

    public CustomScrollView(Context context) : base(context) => Init();
    public CustomScrollView(Context context, IAttributeSet attrs) : base(context, attrs) => Init();
    public CustomScrollView(Context context, IAttributeSet attrs, int defStyleAttr) : base(context, attrs, defStyleAttr) => Init();
    public CustomScrollView(Context context, IAttributeSet attrs, int defStyleAttr, int defStyleRes) : base(context, attrs, defStyleAttr, defStyleRes) => Init();
    public CustomScrollView(IntPtr javaReference, JniHandleOwnership transfer) : base(javaReference, transfer) { }

    private void Init()
    {
        ScrollChecker = () =>
        {
            int position = ScrollY;
            if (PreviousPosition - position == 0)
            {
                FlingEnded?.Invoke(this, new EventArgs());
                RemoveCallbacks(ScrollChecker);
            }
            else
            {
                PreviousPosition = ScrollY;
                PostDelayed(ScrollChecker, DELAY_MILLIS);
            }
        };
    }

    public override void Fling(int velocityY)
    {
        base.Fling(velocityY);

        FlingStarted?.Invoke(this, new EventArgs());
        Post(ScrollChecker);
    }
}

Usage:

myCustomScrollView.FlingEnded += myCustomScrollView_FlingEnded;

protected void myCustomScrollView_FlingEnded(object sender, EventArgs e) =>
{
    //Do onFlingEnded code here
};

Counterpoise