Pushing the ActionBar to the next level

HomeActivity.java

package com.cyrilmottier.android.translucentactionbar;

import android.app.Activity;
import android.graphics.drawable.Drawable;
import android.os.Build;
import android.os.Bundle;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.util.Log;
import android.view.Menu;
import android.widget.ScrollView;

public class HomeActivity extends Activity {

    private Drawable mActionBarBackgroundDrawable;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);

        mActionBarBackgroundDrawable = getResources().getDrawable(R.drawable.ab_background);
        mActionBarBackgroundDrawable.setAlpha(0);

        getActionBar().setBackgroundDrawable(mActionBarBackgroundDrawable);

        ((NotifyingScrollView) findViewById(R.id.scroll_view)).setOnScrollChangedListener(mOnScrollChangedListener);
        
        // Pre JELLY_BEAN_MR1 devices
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN_MR1) {
            mActionBarBackgroundDrawable.setCallback(mDrawableCallback);
        }
    }

    private NotifyingScrollView.OnScrollChangedListener mOnScrollChangedListener = new NotifyingScrollView.OnScrollChangedListener() {
        public void onScrollChanged(ScrollView who, int l, int t, int oldl, int oldt) {
            final int headerHeight = findViewById(R.id.image_header).getHeight() - getActionBar().getHeight();
            final float ratio = (float) Math.min(Math.max(t, 0), headerHeight) / headerHeight;
            final int newAlpha = (int) (ratio * 255);
            mActionBarBackgroundDrawable.setAlpha(newAlpha);
        }
    };
    
    // Pre JELLY_BEAN_MR1 devices
    private Drawable.Callback mDrawableCallback = new Drawable.Callback() {
    @Override
    public void invalidateDrawable(Drawable who) {
        getActionBar().setBackgroundDrawable(who);
    }

    @Override
    public void scheduleDrawable(Drawable who, Runnable what, long when) {
    }

    @Override
    public void unscheduleDrawable(Drawable who, Runnable what) {
    }
};
}