<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
  <com.lanmai.ObservableScrollView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/scrollview"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    >
    <RelativeLayout
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:orientation="vertical">
      <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">
          <!-- 中间就是填充的view就不写了-->
          <!--指定要置顶的view-->
        <TextView
          android:id="@+id/specific_text_view"
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          android:background="@android:color/holo_red_dark"
          android:gravity="center"
          android:text="text"
          android:textSize="40sp"/>
        <TextView
          android:layout_width="match_parent"
          android:layout_height="200dp"
          android:background="@android:color/darker_gray"
          android:gravity="center"
          android:text="text"
          android:textSize="40sp"/>
      </LinearLayout>
    </RelativeLayout>
  </com.lanmai.ObservableScrollView>
  <!--指定要置顶的相同的view visibility设置为gone -->
  <TextView
    android:id="@+id/specific_text_view_gone"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="@android:color/holo_red_dark"
    android:gravity="center"
    android:text="text"
    android:textSize="40sp"
    android:visibility="gone"/>
</RelativeLayout>

public class ObservableScrollView extends ScrollView {
  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 interface ScrollViewListener {
    void onScrollChanged(ScrollView scrollView, int x, int y, int oldx, int oldy);
  }
}

@Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_scroll_view);
    mTextView = ((TextView) findViewById(R.id.specific_text_view));
    mScrollView = ((ObservableScrollView) findViewById(R.id.scrollview));
    mVisibleTextView = ((TextView) findViewById(R.id.specific_text_view_gone));
    mTextView.setOnClickListener(this);
    mScrollView.setScrollViewListener(this);
  }

 @Override
  public void onScrollChanged(ScrollView scrollView, int x, int y, int oldx, int oldy) {
    int[] location = new int[2];
    mTextView.getLocationOnScreen(location);
    int xPosition = location[0];
    int yPosition = location[1];
    Log.d("ScrollViewActivity", "yPosition:" + yPosition);
    int statusBarHeight = getStatusBarHeight();
    Log.d("ScrollViewActivity", "statusBarHeight:" + statusBarHeight);
    if (yPosition <= statusBarHeight) {
      mVisibleTextView.setVisibility(View.VISIBLE);
    } else {
      mVisibleTextView.setVisibility(View.GONE);
    }
  }

int[] location = new int[2];
    mTextView.getLocationOnScreen(location);
    int xPosition = location[0];
    int yPosition = location[1];
   /* mTextView就是要悬浮的view，getLocationOnScreen（location）这个方法就是拿到view在屏幕中的位置 ，传入一个数组，最后得到的yPosition就是view在屏幕中的高度，这里面调用了native层的实现方式，所以数组能直接附上值*/
    // 值得注意的是，拿到的这个高度还包括状态栏的高度。只要减掉就可以了，状态栏的高度获取获取附上代码:
public int getStatusBarHeight() {
  int result = 0;
  int resourceId = getResources().getIdentifier("status_bar_height", "dimen", "android");
  if (resourceId > 0) {
    result = getResources().getDimensionPixelSize(resourceId);
  }
  return result;
}
    int statusBarHeight = getStatusBarHeight();
    Log.d("ScrollViewActivity", "statusBarHeight:" + statusBarHeight);
      通过获取到的状态栏高度，如果小于状态栏的高度就表示已经滑出屏幕了，将要置顶的view设置为visibvle否则设置为gone 
    if (yPosition <= statusBarHeight) {
      mVisibleTextView.setVisibility(View.VISIBLE);
    } else {
      mVisibleTextView.setVisibility(View.GONE);
    }

@Override
  public void onClick(View v) {
    int[] location = new int[2];
    v.getLocationOnScreen(location);
    int x = location[0];
    int y = location[1];
    mScrollView.scrollBy(0, location[1] - getStatusBarHeight());
  }