Android滑动组件悬浮固定在顶部效果

要想实现的效果是如下：

场景：有些时候是内容中间的组件当滑动至顶部的时候固定显示在顶部。

实现的思路：

1.目标组件（button）有两套，放在顶部和内容中间；

2.当内容中间的组件滑动至顶部栏位置时控制显示/隐藏顶部和中间的组件（涉及到组件获取在屏幕的位置知识点）；

activity代码：

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); 
    } 
  } 
} 

要点解析：

1.在onWindowFocusChanged()方法中获取屏幕状态栏和标题栏的高度（在onCreate()方法中是获取是0）；

2.因为布局中的ScrollView的onScrollChangeListener()方法低版本API不支持——>所以activity实现了自定义ScrollView中的onScrollChanged()接口方法——>在此方法中实现组件的显示/隐藏；

自定义ScrollView的代码：

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); 
  } 
} 

然后是布局文件：

<?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> 

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