package com.example.test;
import android.content.Context;
import android.os.Build;
import android.util.AttributeSet;
import android.widget.ScrollView;
public class ObservableScrollView extends ScrollView implements
    ObservableScrollable {
  private boolean mDisableEdgeEffects = true;
  private OnScrollChangedCallback mOnScrollChangedListener;
  public ObservableScrollView(Context context) {
    super(context);
  }
  public ObservableScrollView(Context context, AttributeSet attrs) {
    super(context, attrs);
  }
  public ObservableScrollView(Context context, AttributeSet attrs,
      int defStyle) {
    super(context, attrs, defStyle);
  }
  @Override
  protected void onScrollChanged(int l, int t, int oldl, int oldt) {
    super.onScrollChanged(l, t, oldl, oldt);
    if (mOnScrollChangedListener != null) {
      mOnScrollChangedListener.onScroll(l, t);
    }
  }
  @Override
  protected float getTopFadingEdgeStrength() {
    if (mDisableEdgeEffects
        && Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB) {
      return 0.0f;
    }
    return super.getTopFadingEdgeStrength();
  }
  @Override
  protected float getBottomFadingEdgeStrength() {
    if (mDisableEdgeEffects
        && Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB) {
      return 0.0f;
    }
    return super.getBottomFadingEdgeStrength();
  }
  @Override
  public void setOnScrollChangedCallback(OnScrollChangedCallback callback) {
    mOnScrollChangedListener = callback;
  }
}

package com.example.test;
public interface ObservableScrollable {
  void setOnScrollChangedCallback(OnScrollChangedCallback callback);
}

package com.example.test;
public interface OnScrollChangedCallback {
  void onScroll(int l, int t);
}

package com.example.test;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.Window;
import android.widget.RelativeLayout;
import android.widget.ScrollView;
public class MainActivity extends Activity implements OnScrollChangedCallback {
  ObservableScrollable sv;
  RelativeLayout rl;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    setContentView(R.layout.activity_main);
    rl = (RelativeLayout) findViewById(R.id.rl);
    sv = (ObservableScrollable) findViewById(R.id.sv);
    sv.setOnScrollChangedCallback(this);
  }
  @Override
  public void onScroll(int l, int t) {
    float newAlpha = (float)t/500;
    rl.setAlpha(newAlpha);
  }
}