Android 滑动改变top颜色

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/**
* 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);
    }
}

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));
    }
}