导航栏背景透明度设置

• (void)setNeedsNavigationBackground:(CGFloat)alpha {
  // 导航栏背景透明度设置
  UIView *barBackgroundView = [[self.navigationController.navigationBar subviews] objectAtIndex:0];// _UIBarBackground
  UIImageView *backgroundImageView = [[barBackgroundView subviews] objectAtIndex:0];// UIImageView
  if (self.navigationController.navigationBar.isTranslucent) {
  if (backgroundImageView != nil && backgroundImageView.image != nil) {
  barBackgroundView.alpha = alpha;
  } else {
  UIView *backgroundEffectView = [[barBackgroundView subviews] objectAtIndex:1];// UIVisualEffectView
  if (backgroundEffectView != nil) {
  backgroundEffectView.alpha = alpha;
  }
  }
  } else {
  barBackgroundView.alpha = alpha;
  }

  // 对导航栏下面那条线做处理
  self.navigationController.navigationBar.clipsToBounds = alpha == 0.0;
  }