     }
     
     private func _setupTitleItemsContainerView(_ itemsContainerView: UIView) {
-        addSubview(itemsContainerView)
+        _containerView.addSubview(itemsContainerView)
         
         _navigationBackItem._view.trailingAnchor.constraint(equalTo: itemsContainerView.leadingAnchor).isActive = true
-        self.topAnchor.constraint(equalTo: itemsContainerView.topAnchor).isActive = true
-        self.bottomAnchor.constraint(equalTo: itemsContainerView.bottomAnchor).isActive = true
+        _containerView.topAnchor.constraint(equalTo: itemsContainerView.topAnchor).isActive = true
+        _containerView.bottomAnchor.constraint(equalTo: itemsContainerView.bottomAnchor).isActive = true
     }
     
     private func _setupNavigationItemsContainerView(_ itemsContainerView: UIView, transition: Bool = false) {
-        addSubview(itemsContainerView)
+        _containerView.addSubview(itemsContainerView)
         
         let _constraint = _titleItemsContainerView.trailingAnchor.constraint(equalTo: itemsContainerView.leadingAnchor, constant: -DefaultTabNavigationItemEdgeMargin)
         _constraint.isActive = true

            }
        }
    }
    // MARK: - Private Properties.
    fileprivate var _normalBackgroundColor: UIColor?
    fileprivate lazy var __titleAlignmentLabel: UILabel = _createGeneralAlignmentLabel()
    fileprivate lazy var _titleItemsContainerView: UIView = _createGeneralContainerView()
    fileprivate lazy var _itemsContainerView: UIView = _createGeneralContainerView()

    
    internal func _toggleShowingOfNavigationBackItem(shows: Bool, duration: TimeInterval = 0.5, animated: Bool) {
        if let _horizontal = _horizontalConstraintOfBackItem {
            _containerView.removeConstraint(_horizontal)
        }
        var _horizontal: NSLayoutConstraint
        if shows {
            _horizontal = _navigationBackItem._view.leadingAnchor.constraint(equalTo: _containerView.leadingAnchor)
        } else {
            _horizontal = _navigationBackItem._view.trailingAnchor.constraint(equalTo: _containerView.leadingAnchor)
        }
        _horizontal.isActive = true
        _horizontalConstraintOfBackItem = _horizontal
