Android滑动组件悬浮固定在顶部效果

public class MainActivity extends AppCompatActivity implements ObservableScrollView.ScrollViewListener { 
 private ObservableScrollView scrollView; 
 private Button topBtn1, topBtn2, middleBtn1, middleBtn2; 
 private View topPanel, middlePanel; 
 private int topHeight; 
 
 @Override 
 protected void onCreate(Bundle savedInstanceState) { 
  super.onCreate(savedInstanceState); 
  setContentView(R.layout.activity_main); 
  initViews(); 
  initListeners(); 
 
 } 
 
 @Override 
 public void onWindowFocusChanged(boolean hasFocus) { 
  super.onWindowFocusChanged(hasFocus); 
 
  Rect frame = new Rect(); 
  getWindow().getDecorView().getWindowVisibleDisplayFrame(frame); 
  int statusBarHeight = frame.top;//状态栏高度 
 
  int titleBarHeight = getWindow().findViewById(Window.ID_ANDROID_CONTENT).getTop();//标题栏高度 
  topHeight = titleBarHeight + statusBarHeight; 
 } 
 
 
 private void initViews() { 
  scrollView = (ObservableScrollView) findViewById(R.id.scrollView); 
  topPanel = findViewById(R.id.topPanel); 
  topBtn1 = (Button) topPanel.findViewById(R.id.button1); 
  topBtn2 = (Button) topPanel.findViewById(R.id.button2); 
 
  middlePanel = findViewById(R.id.middlePanel); 
  middleBtn1 = (Button) middlePanel.findViewById(R.id.button1); 
  middleBtn2 = (Button) middlePanel.findViewById(R.id.button2); 
 
 } 
 
 private void initListeners() { 
  topBtn1.setOnClickListener(new View.OnClickListener() { 
   @Override 
   public void onClick(View view) { 
    middleBtn1.setBackgroundColor(Color.WHITE); 
    topBtn1.setBackgroundColor(Color.WHITE); 
   } 
  }); 
 
  middleBtn1.setOnClickListener(new View.OnClickListener() { 
   @Override 
   public void onClick(View view) { 
    middleBtn1.setBackgroundColor(Color.BLUE); 
    topBtn1.setBackgroundColor(Color.BLUE); 
   } 
  }); 
 
  scrollView.setScrollViewListener(this); 
 
 
 } 
 
 
 @Override 
 public void onScrollChanged(ObservableScrollView scrollView, int x, int y, int oldx, int oldy) { 
  int[] location = new int[2]; 
  middleBtn1.getLocationOnScreen(location); 
  int locationY = location[1]; 
  Log.e("locationY", locationY + " " + "topHeight的值是：" + topHeight); 
 
  if (locationY <= topHeight && (topPanel.getVisibility() == View.GONE || topPanel.getVisibility() == View.INVISIBLE)) { 
   topPanel.setVisibility(View.VISIBLE); 
  } 
 
  if (locationY > topHeight && topPanel.getVisibility() == View.VISIBLE) { 
   topPanel.setVisibility(View.GONE); 
  } 
 
 } 
} 

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(ObservableScrollView scrollView, int x, int y, int oldx, int oldy); 
 
 } 
} 

<?xml version="1.0" encoding="utf-8"?> 
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android" 
 xmlns:tools="http://schemas.android.com/tools" 
 android:id="@+id/activity_main" 
 android:layout_width="match_parent" 
 android:layout_height="match_parent" 
 tools:context="com.example.administrator.slideholdapp.MainActivity"> 
 
 <com.example.administrator.slideholdapp.ObservableScrollView 
  android:layout_width="match_parent" 
  android:layout_height="match_parent" 
  android:id="@+id/scrollView"> 
 
  <LinearLayout 
   android:layout_width="match_parent" 
   android:layout_height="wrap_content" 
   android:orientation="vertical"> 
 
   <TextView 
    android:layout_width="match_parent" 
    android:layout_height="wrap_content" 
    android:layout_marginBottom="30dp" 
    android:text="@string/content" /> 
 
   <include android:id="@+id/middlePanel" layout="@layout/middle_item_layout"></include> 
 
   <TextView 
    android:layout_width="match_parent" 
    android:layout_height="wrap_content" 
    android:layout_marginTop="30dp" 
    android:text="@string/content" /> 
 
 
  </LinearLayout> 
 
 </com.example.administrator.slideholdapp.ObservableScrollView> 
 
 <include android:id="@+id/topPanel" layout="@layout/middle_item_layout" android:visibility="gone"/> 
</FrameLayout>