android?scrollview顶部渐渐消失实现实例详解

public class TopFadeScrollView extends ScrollView {
    private static final int MAX_ALPHA = 255; // 最大透明度
    private static final int SCROLL_THRESHOLD = 200; // 滑动阈值
    private View mTopView; // 顶部视图
    public TopFadeScrollView(Context context) {
        super(context);
        init();
    }
    public TopFadeScrollView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }
    public TopFadeScrollView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }
    private void init() {
        // 设置滚动监听器
        setOnScrollChangeListener(new OnScrollChangeListener() {
            @Override
            public void onScrollChange(View v, int scrollX, int scrollY, int oldScrollX, int oldScrollY) {
                updateTopViewAlpha(scrollY);
            }
        });
    }
    public void setTopView(View topView) {
        mTopView = topView;
    }
    private void updateTopViewAlpha(int scrollY) {
        // 计算透明度的值
        int alpha = (int) (MAX_ALPHA * (scrollY * 1.0f / SCROLL_THRESHOLD));
        if (alpha > MAX_ALPHA) {
            alpha = MAX_ALPHA;
        } else if (alpha < 0) {
            alpha = 0;
        }
        // 设置顶部视图的透明度
        mTopView.setAlpha(alpha);
    }
}

<LinearLayout xmlns:android="
    xmlns:app="
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">
    <!-- 顶部视图 -->
    <ImageView
        android:id="@+id/topView"
        android:layout_width="match_parent"
        android:layout_height="200dp"
        android:scaleType="centerCrop"
        android:src="http://9i0i.com/pic.php?p=@drawable/top_image" />
    <!-- 自定义的 ScrollView -->
    <com.example.app.TopFadeScrollView
        android:layout_width="match_parent"
        android:layout_height="match_parent">
        <!-- 页面内容 -->
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical">
            <!-- 其他视图 -->
        </LinearLayout>
    </com.example.app.TopFade