FadingActionBar is great library, but it lacks ability to bind listeners to track how much ActionBarDrawable has been faded in/out.

NotifyingFadingActionBar.java

import android.annotation.SuppressLint;
import android.app.Activity;
import android.content.Context;
import android.graphics.drawable.Drawable;
import android.os.Build;
import android.support.v7.app.ActionBarActivity;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewTreeObserver;
import android.widget.AbsListView;
import android.widget.ListView;

import com.manuelpeinado.fadingactionbar.FadingActionBarHelper;

import java.lang.reflect.Field;

/**
 * Created by Nikola on 10/15/2014.
 */
public class NotifyingFadingActionBar extends FadingActionBarHelper {
    private final Context mContext;
    private AbsListView.OnScrollListener mInternalScrollListener;
    private ActionBarDrawableAlphaCallback mAlphaCallback;
    private ListView mListView;
    private int mInitialBottom;
    private float mPreviousRatio;
    private boolean hasDispatchedCompleted = false;
    private View mHeaderView;
    private boolean isActionBarCompat = false;
    private android.support.v7.app.ActionBar mSupportActionBar;
    private List<AbsListView.OnScrollListener> mListeners = new ArrayList<AbsListView.OnScrollListener>();

    public enum ScrollDirection{
        DOWN, UP;
    }
    public interface ActionBarDrawableAlphaCallback{
        void onAlphaComplete();
        void onAlphaChanged(ScrollDirection direction, float fadingFactor);
    }

    public NotifyingFadingActionBar(Context c) {
        mContext = c;
        mPreviousRatio = 0.0f;
        try {
            Field scrollField = getClass().getSuperclass().getSuperclass().getDeclaredField("mOnScrollListener");
            scrollField.setAccessible(true);
            mInternalScrollListener = (AbsListView.OnScrollListener)scrollField.get(this);
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }
    public NotifyingFadingActionBar headerLayoutResId(int resId){
        mHeaderView = LayoutInflater.from(mContext).inflate(resId, null);
        super.headerView(mHeaderView);
        return this;
    }
    @Override
    public void initActionBar(Activity activity){
        checkForCompat(activity);
        super.initActionBar(activity);
        final View contentView = activity.findViewById(android.R.id.content);
        ViewTreeObserver vto = contentView.getViewTreeObserver();
        if(vto != null && vto.isAlive()){
            vto.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
                @Override
                public void onGlobalLayout() {
                    removeGlobalLayoutListener(contentView, this);
                    findListView(contentView);
                }
            });
        }else{
            findListView(contentView);
        }

    }
    
      public NotifyingFadingActionBar addOnScrollListener(AbsListView.OnScrollListener listener){
        addPartnerScrollListener(listener);
        return this;
    }

    private void addPartnerScrollListener(AbsListView.OnScrollListener listener){
        if(!mListeners.contains(listener)){
            mListeners.add(listener);
        }
    }
    private void notifyAllOnScrollListeners(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount){
        for(AbsListView.OnScrollListener listener : mListeners){
            listener.onScroll(view, firstVisibleItem, visibleItemCount, totalItemCount);
        }
    }
    private void notifyAllOnScrollStateChangedListeners(AbsListView view, int scrollState){
        for(AbsListView.OnScrollListener listener : mListeners){
            listener.onScrollStateChanged(view, scrollState);
        }
    }

    private void checkForCompat(Activity activity) {
        isActionBarCompat = activity instanceof ActionBarActivity && Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB;
        if(isActionBarCompat) {
            mSupportActionBar = ((ActionBarActivity)activity).getSupportActionBar();
        }
    }

    private void findListView(View contentView){
        mListView = (ListView)contentView.findViewById(android.R.id.list);
        mListView.setOnScrollListener(new NotifyingScrollListener());
        mInitialBottom = mHeaderView.getBottom();
        mHeaderView = (View)mHeaderView.getParent();

    }
    public class NotifyingScrollListener implements AbsListView.OnScrollListener{

        @Override
        public void onScrollStateChanged(AbsListView view, int scrollState) {
            Log.i("Fader", "OnScrollStateChanged()");
            if(mInternalScrollListener != null){
               mInternalScrollListener.onScrollStateChanged(view, scrollState);
            }
            notifyAllOnScrollStateChangedListeners(view, scrollState);
        }

        @Override
        public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
            if(mInternalScrollListener != null){
               mInternalScrollListener.onScroll(view, firstVisibleItem,visibleItemCount,totalItemCount);
            }
            if( mHeaderView.getBottom() == mHeaderView.getBottom()/2 && !hasDispatchedCompleted){
                hasDispatchedCompleted = true;
                if( mAlphaCallback != null)
                    mAlphaCallback.onAlphaComplete();
            }else if(mHeaderView.getBottom() > mHeaderView.getBottom()/2 ){
                 hasDispatchedCompleted = false;
                 int scrolled =  mInitialBottom - mHeaderView.getBottom();
                 float ratio = (float) scrolled/ mInitialBottom;
                 ratio*=2;
                 dispatchRatioChanged(ratio);
            }
             notifyAllOnScrollListeners(view, firstVisibleItem, visibleItemCount, totalItemCount);
        }
    }

    private void dispatchRatioChanged(float ratio) {
        Log.i(TAG, "dispatchRatio() "+ratio + " old: "+mPreviousRatio);
        if(ratio != mPreviousRatio) {
            if( mAlphaCallback != null)
                mAlphaCallback.onAlphaChanged(mPreviousRatio > ratio ? ScrollDirection.DOWN : ScrollDirection.UP, ratio);
            mPreviousRatio = ratio;
        }
    }

    public NotifyingFadingActionBar withCallback(ActionBarDrawableAlphaCallback callback){
        mAlphaCallback = callback;
        return this;
    }
    @Override
    protected void setActionBarBackgroundDrawable(Drawable drawable) {
        if(!isActionBarCompat)
            super.setActionBarBackgroundDrawable(drawable);
        else
            mSupportActionBar.setBackgroundDrawable(drawable);

    }

    @SuppressLint("NewApi")
    @Override
    protected int getActionBarHeight() {
        return isActionBarCompat ? mSupportActionBar.getHeight() : super.getActionBarHeight();
    }

    @Override
    protected boolean isActionBarNull() {
        return isActionBarCompat? mSupportActionBar == null : super.isActionBarNull();
    }


    @SuppressWarnings("deprecation")
    @SuppressLint("NewApi")
    public static void removeGlobalLayoutListener(View v, ViewTreeObserver.OnGlobalLayoutListener listener){
        if (Build.VERSION.SDK_INT < 16) {
            v.getViewTreeObserver().removeGlobalOnLayoutListener(listener);
        } else {
            v.getViewTreeObserver().removeOnGlobalLayoutListener(listener);
        }
    }
}