自定义ScrollView实现

用于监听ScrollView的滚动，来改变某一布局的透明度

引用
(http://blog.csdn.net/sinat_21283073/article/details/51315926)

1 自定义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);  
    }  
} 

}
`

2 使用

`
public class MainActivity extends Activity implements ScrollViewListener{

private ObservableScrollView scrollView;  
  
private ListView listView;  
  
private ImageView imageView;  
  
private TextView textView;  
  
private int imageHeight;  

@Override  
protected void onCreate(Bundle savedInstanceState) {  
    super.onCreate(savedInstanceState);  
    setContentView(R.layout.activity_main);  
    scrollView = (ObservableScrollView) findViewById(R.id.scrollview);  
    listView = (ListView) findViewById(R.id.listview);  
    imageView = (ImageView) findViewById(R.id.imageview);  
    textView = (TextView) findViewById(R.id.textview);  
    initListeners();  
    initData();  
}  
  
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);  
        }  
    });  
}  



private void initData() {  
    ArrayAdapter<String> adapter = new ArrayAdapter<String>(MainActivity.this, android.R.layout.simple_list_item_1, getResources().getStringArray(R.array.data));  
    listView.setAdapter(adapter);  
}  



@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));//AGB由相关工具获得，或者美工提供  
    } else if (y > 0 && y <= imageHeight) {  
        float scale = (float) y / imageHeight;  
        float alpha = (255 * scale);  
        // 只是layout背景透明(仿知乎滑动效果)  
        textView.setBackgroundColor(Color.argb((int) alpha, 227, 29, 26));  
    } else {  
        textView.setBackgroundColor(Color.argb((int) 255, 227, 29, 26));  
    }  
}  

}
`