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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