@Override
  protected void onPostCreate(@Nullable Bundle savedInstanceState) {
      super.onPostCreate(savedInstanceState);
      setRootViewDefaultFocusHighlight();
  }
   //添加一个1*1像素的 view
   private void setRootViewDefaultFocusHighlight () {
      //此方法只能在Android O上使用
      if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
          ViewGroup root = ((ViewGroup)findViewById(android.R.id.content));
          if (root != null) {
              View view = new View(getContext());
              view.setFocusable(true);
              view.setFocusableInTouchMode(true);
              view.requestFocus();
              view.requestFocusFromTouch();
              view.setDefaultFocusHighlightEnabled(false);
              root.addView(view, new ViewGroup.LayoutParams(1, 1));
          }
      }
  }
  //或者直接使用 第一个 view
private void setRootViewDefaultFocusHighlight2 () {
      //此方法只能在Android O上使用
      if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
          View root = ((ViewGroup)findViewById(android.R.id.content)).getChildAt(0);
          if (root != null) {
              root.setFocusable(true);
              root.setFocusableInTouchMode(true);
              root.requestFocus();
              root.requestFocusFromTouch();
              root.setDefaultFocusHighlightEnabled(false);
          }
      }
  }

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int newHeight = MeasureSpec.getSize(heightMeasureSpec) - mStickyViewMarginTop;
    int newHeightSpec = MeasureSpec.makeMeasureSpec(newHeight, MeasureSpec.getMode(heightMeasureSpec));
    super.onMeasure(widthMeasureSpec, newHeightSpec);
    if (mTopView != null) {
        //不限制顶部 view 的高度
        mTopView.measure(widthMeasureSpec, MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED));
        ViewGroup.LayoutParams params = mViewPager.getLayoutParams();
        mMaxViewPagerHeight = Math.max(mMaxViewPagerHeight, (getMeasuredHeight() - mStickyView.getMeasuredHeight()));
        params.height = mMaxViewPagerHeight;
        setMeasuredDimension(getMeasuredWidth(), mTopView.getMeasuredHeight() + mStickyView.getMeasuredHeight() + mViewPager.getMeasuredHeight());
    }
}

@Override
public boolean onStartNestedScroll(View child, View target, int nestedScrollAxes) {
    NTLog.d(TAG, "onStartNestedScroll--" + "ViewCompat.SCROLL_AXIS_VERTICAL = " + ViewCompat.SCROLL_AXIS_VERTICAL + "; nestedScrollAxes= " + nestedScrollAxes);
    //拦截垂直滚动
    return  (nestedScrollAxes & getNestedScrollAxes()) != 0;
}    
@Override
public int getNestedScrollAxes() {
    return ViewCompat.SCROLL_AXIS_VERTICAL;
}

@Override
public void onNestedPreScroll(View target, int dx, int dy, int[] consumed) {
    NTLog.d(TAG, "onNestedPreScroll");
    //上滑
    boolean hiddenTop = dy > 0 && getScrollY() < mMaxScrollY;
    //下滑动，且子 view 不可滑动了
    boolean showTop = dy < 0 && getScrollY() >= 0 && !ViewCompat.canScrollVertically(target, -1);
    if (hiddenTop || showTop) {
        // 滚动自己
        scrollBy(0, dy);
        //消费掉 dy
        consumed[1] = dy;
    }
}

@Override
public boolean onNestedPreFling(View target, float velocityX, float velocityY) {
    NTLog.d(TAG, "onNestedPreFling");
    if (velocityY == 0) {
        return false;
    }
    mCurDirection = velocityY < 0 ? SCROLL_DOWN : SCROLL_UP;
    if (mScrollY > 0) {
        mScroller.fling(0, getScrollY(), (int)velocityX, (int)velocityY, 0, 0,
                -Integer.MAX_VALUE, Integer.MAX_VALUE);
        invalidate(); //执行此代码是为了能够 fling 后执行一下computeScroll()方法
        return true;
    }
    return false;
}

@Override
    public void computeScroll() {
        if (mScroller.computeScrollOffset()) {
            final int currY = mScroller.getCurrY();
            if (mCurDirection == SCROLL_UP) {
                //手势向上
                if (isSticked()) {
                    //已经sticky,直接让子 view 滚动
                    int dy = mScroller.getFinalY() - currY;
                    if (dy > 0) {
                        int duration = mScroller.getDuration() - mScroller.timePassed();
                        mScrollable.smoothScrollBy((int) mScroller.getCurrVelocity(), dy, duration);
                        mScroller.abortAnimation();
                    }
                } else {
                    //滚动自己
                    int dy = mScroller.getCurrY() - mScrollY;
                    int toY = getScrollY() + dy;
                    scrollTo(0, toY);
                    invalidate();
                }
            } else {
                //手势向下
                if (mScrollable.isTop()) {
                    //子 view 滚动到了顶部
                    int dy = currY - mScrollY;
                    int toY = getScrollY() + dy;
                    scrollTo(0, toY);
                } else {
                    int dy = mScroller.getFinalY() - mScrollY;
                    int duration = mScroller.getDuration() - mScroller.timePassed();
                    mScrollable.smoothScrollBy(-(int)mScroller.getCurrVelocity(), dy, duration);
                }
                //刷新调用computeScroll() 方法,时时判断是否滚动 top 状态. 
                invalidate();
            }
        }
    }

public interface NestedScrollingChild {
    /**
     * 设置是否启用嵌套滚动
     */
    public void setNestedScrollingEnabled(boolean enabled);

    /**
     * 判断是否启用嵌套滚动
     */
    public boolean isNestedScrollingEnabled();

    /**
     * 开始嵌套滚动      
     * @param axes 标识方向，有 x, y 方形和默认值0
     */
    public boolean startNestedScroll(int axes);

    /**
     * 嵌套滚动结束
     */
    public void stopNestedScroll();

    /**        
     *  判断父 view 是否支持嵌套滚动
     */
    public boolean hasNestedScrollingParent();

    /**
     * 分发嵌套滚动，一般再 onTouch/onInterceptTouchEvent/dispatchTouchEvent 中调用
     * @param dxConsumed  x轴滚动的距离
     * @param dyConsumed  y 轴滚动的距离
     * @param dxUnconsumed  x 轴上未消费的距离
     * @param dyUnconsumed  y 轴上未消费的距离
     * @param offsetInWindow 子 View 的窗体偏移
      */
    public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed,
            int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow);

    /**
     * 滚动之前调用，进行分发预滚动事件
     */
    public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow);

    /**
     * 滑动时调用 ，分发滑动事件
     */
    public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed);

    /**
    *  滚动之前调用，分发预滚动事件
     */
    public boolean dispatchNestedPreFling(float velocityX, float velocityY);
}

public interface NestedScrollingParent {

    public boolean onStartNestedScroll(View child, View target, int nestedScrollAxes);

    public void onNestedScrollAccepted(View child, View target, int nestedScrollAxes);

    public void onStopNestedScroll(View target);

    public void onNestedScroll(View target, int dxConsumed, int dyConsumed,
            int dxUnconsumed, int dyUnconsumed);

    public void onNestedPreScroll(View target, int dx, int dy, int[] consumed);

    public boolean onNestedFling(View target, float velocityX, 
                                                      float velocityY,boolean consumed);
 
    public boolean onNestedPreFling(View target, float velocityX, float velocityY);

    public int getNestedScrollAxes();
}

public class Single {
    private static Single mInstance;
    private Single() {
    }
    public static Single getInstance() {
        if (mInstance == null) {
            synchronized (Single.class) {
                if (mInstance == null) {
                    mInstance = new Single();
                }
            }
        }
        return mInstance;
    }
}

public class Single {
    private static volatile Single mInstance;
    private Single() {
    }
    public static Single getInstance() {
        if (mInstance == null) {
            synchronized (Single.class) {
                if (mInstance == null) {
                    mInstance = new Single();
                }
            }
        }
        return mInstance;
    }
}

public class Single {
    private static final Single mInstance = new Single();
    private Single() {
    }
    public static Single getInstance() {
        return mInstance;
    }
}

public class Single {
    private Single() {}
    private static class SingleHolder {
        private static final Single instance = new Single();
    }

    public static Single getInstance(){
        return SingleHolder.instance;
    }
}