安卓下滑渐显标题栏

import android.content.Context;  
import android.util.AttributeSet;  
import android.widget.ScrollView;  
/** 
 * 带滚动监听的scrollview 
 * 
 */  
public class ObservableScrollView extends ScrollView {  
  
    public interface ScrollViewListener {  
  
        void onScrollChanged(ObservableScrollView scrollView, int x, int y,  
                int oldx, int oldy);  
  
    }  
  
    private ScrollViewListener scrollViewListener = null;  
  
    public ObservableScrollView(Context context) {  
        super(context);  
    }  
  
    public ObservableScrollView(Context context, AttributeSet attrs,  
            int defStyle) {  
        super(context, attrs, defStyle);  
    }  
  
    public ObservableScrollView(Context context, AttributeSet attrs) {  
        super(context, attrs);  
    }  
  
    public void setScrollViewListener(ScrollViewListener scrollViewListener) {  
        this.scrollViewListener = scrollViewListener;  
    }  
  
    @Override  
    protected void onScrollChanged(int x, int y, int oldx, int oldy) {  
        super.onScrollChanged(x, y, oldx, oldy);  
        if (scrollViewListener != null) {  
            scrollViewListener.onScrollChanged(this, x, y, oldx, oldy);  
        }  
    }  
  
}  

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"  
    xmlns:tools="http://schemas.android.com/tools"  
    android:layout_width="match_parent"  
    android:layout_height="match_parent"   >  
  
    <cn.hwwwwh.cn.ObservableScrollView  
        android:id="@+id/scrollview"  
        android:layout_width="match_parent"  
        android:layout_height="match_parent"  
        android:scrollbars="none" >  
  
        <LinearLayout  
            android:layout_width="match_parent"  
            android:layout_height="wrap_content"  
            android:orientation="vertical" >  
  
            <ImageView  
                android:id="@+id/imageview"  
                android:layout_width="match_parent"  
                android:layout_height="200dp"  
                android:background="@drawable/zuqiu" />  
  
        </LinearLayout>  
    </cn.hwwwwh.cn.ObservableScrollView>  
  
    <TextView  
        android:id="@+id/textview"  
        android:layout_width="match_parent"  
        android:layout_height="48dp"  
        android:gravity="center"  
        android:text="标题"  
        android:textSize="18sp"  
        android:textColor="@android:color/white"  
        android:background="#00000000" />  
  
</RelativeLayout>  

 private void initListeners() {  
        // 获取顶部图片高度后，设置滚动监听  
        ViewTreeObserver vto = imageView.getViewTreeObserver();  
        vto.addOnGlobalLayoutListener(new OnGlobalLayoutListener() {  
            @Override  
            public void onGlobalLayout() {  
                imageView.getViewTreeObserver().removeGlobalOnLayoutListener(  
                        this);  
                imageHeight = imageView.getHeight();  
  
                scrollView.setScrollViewListener(MainActivity.this);  
            }  
        });  
    }  

  @Override  
    public void onScrollChanged(ObservableScrollView scrollView, int x, int y,  
            int oldx, int oldy) {  
        // TODO Auto-generated method stub  
        // Log.i("TAG", "y--->" + y + "    height-->" + height);  
        if (y <= 0) {  
            textView.setBackgroundColor(Color.argb((int) 0, 227, 29, 26));//第一个参数0表示透明度范围是0~255，后面为RGB参数
        } else if (y > 0 && y <= imageHeight) {  
            float scale = (float) y / imageHeight;  
            float alpha = (255 * scale);  
            textView.setBackgroundColor(Color.argb((int) alpha, 227, 29, 26));  
        } else {  
            textView.setBackgroundColor(Color.argb((int) 255, 227, 29, 26));  
        }  
    }  
}