         let complete = TabNavigationItem(title: "完成", target: self, selector: #selector(_handleCompleteAction(_:)))
         setTabNavigationItems([complete, cancel])
         
-        _setupBackToTopButton()
         // Load data of table view.
         tableView.reloadData()
     }

        
        return (lightColor, darkColor)
    }
    
    fileprivate func _setupBackToTopButton() {
        guard let tabNavigationBar = tabNavigationController?.tabNavigationBar else {
            return
        }
        _backToTopButton = UIButton(type: .system)
        _backToTopButton.translatesAutoresizingMaskIntoConstraints = false
        _backToTopButton.setImage(#imageLiteral(resourceName: "back_to_top"), for: .normal)
        _backToTopButton.tintColor = UIColor.application.titleColor
        _backToTopButton.addTarget(self, action: #selector(_handleBackToTop(_:)), for: .touchUpInside)
        tabNavigationBar.addSubview(_backToTopButton)
        _backToTopButton.centerXAnchor.constraint(equalTo: tabNavigationBar.centerXAnchor).isActive = true
        _backToTopButton.widthAnchor.constraint(equalToConstant: 44.0).isActive = true
        _backToTopButton.heightAnchor.constraint(equalToConstant: 44.0).isActive = true
        _backToTopButton.lastBaselineAnchor.constraint(equalTo: tabNavigationBar.lastBaselineAnchor).isActive = true
    }
}

// MARK: - Storyboard Supporting.

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}

// MARK: - Back to top.

extension CategoryAddingViewController {
    override func makeViewScrollToTopIfNecessary(at location: CGPoint) {
        tableView.scrollRectToVisible(CGRect(origin: .zero, size: CGSize(width: tableView.bounds.width, height: 1.0)), animated: true)
    }
}

