A ListView that informs when and how much its content is scrolled.

ScrollTrackingListView.java

/*
 * Copyright 2015 Gerónimo Oñativia
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
import android.content.Context;
import android.os.Build;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;
import android.view.ViewTreeObserver;
import android.widget.AbsListView;
import android.widget.ListView;

/**
 * ListView that calls ScrollValueChangedListener.onScrollValueChanged
 * when its content is scrolled.
 * Informs as much as possible the amount scrolled.
 *
 * @author Gerónimo Oñativia / PixDart.com <[email protected]>
 */
public class ScrollTrackingListView extends ListView {
    private static final String TAG = ScrollTrackingListView.class.getName();

    private int firstChildInitialTop = Integer.MIN_VALUE;
    private boolean zeroScrollChanged = false;
    private ScrollValueChangedListener scrollValueChangedListener = null;

    public ScrollTrackingListView(Context context) {
        this(context, null);
    }

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

    public ScrollTrackingListView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP) init();
    }

    @TargetApi(Build.VERSION_CODES.LOLLIPOP)
    public ScrollTrackingListView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
        init();
    }

    private void init() {
        getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
            @Override
            public void onGlobalLayout() {
                View firstChild = getChildAt(0);
                if (firstChild != null) {
                    firstChildInitialTop = firstChild.getTop();
                    Log.d(TAG, "firstChildBaseTop obtained: " + firstChildInitialTop);
                    if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN)
                        getViewTreeObserver().removeGlobalOnLayoutListener(this);
                    else
                        getViewTreeObserver().removeOnGlobalLayoutListener(this);
                }
            }
        });
        getViewTreeObserver().addOnScrollChangedListener(new ViewTreeObserver.OnScrollChangedListener() {
            @Override
            public void onScrollChanged() {
                if (isReadyForComputation()) computeCurrentScroll();
            }
        });
        setOnScrollListener(new OnScrollListener() {
            @Override
            public void onScrollStateChanged(AbsListView view, int scrollState) {
            }

            @Override
            public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
                if (isReadyForComputation()) computeCurrentScroll();
            }
        });
    }

    public boolean isReadyForComputation() {
        return firstChildInitialTop != Integer.MIN_VALUE;
    }

    public void computeCurrentScroll() {
        if (scrollValueChangedListener == null) return;
        View firstChild = getChildAt(0);
        if (firstChild != null) {
            int firstChildNewTop = firstChild.getTop();
            int deltaY = firstChildNewTop - firstChildInitialTop;
            if (deltaY < 0 || (deltaY == 0 && zeroScrollChanged)) {
                zeroScrollChanged = deltaY != 0; //Do not call listener multiple times to inform ZERO scroll. Also skip first call by initializing as false on declaration.
                Log.d(TAG, "firstChildInitialTop:" + firstChildInitialTop + " firstChildNewTop:" + firstChildNewTop + " variation:" + deltaY);
                scrollValueChangedListener.onScrollValueChanged(deltaY);
            }
        }
    }

    public void setScrollValueChangedListener(ScrollValueChangedListener listener) {
        scrollValueChangedListener = listener;
    }

    public interface ScrollValueChangedListener {
        public void onScrollValueChanged(int currentScrollAmmount);
    }
}