重新计算绑定表达式？

Question

我正在创建一个自定义控件，该控件具有-Command类型的CommandProperty，SelectedItems和object等其他属性，它们可能仅仅是对象或ObservableCollection<object>取决于其他属性IsMultiSelect。

这是android模拟器上的控件（它的ItemsSource属性中有4个项目）：

现在我需要将CommandParameter绑定到SelectedItems：

<controls:TogglesRow x:Name="TogRow"
                     ItemsSource="{Binding Items}" DisplayMemberPath="Name"
                     Command="{Binding ItemSelectedCommand}" 
                     CommandParameter="{Binding Source={x:Reference TogRow}, 
                     Path=SelectedItems}"/>

但是问题在于它需要前一个SelectedItems，这意味着在我第一次选择一个项目时，CommandParameter是null，下一次将选择第一个项目...等。

btn是轻击按钮（自定义按钮），在UI属性（propertyChanged，ItemsSourceProperty等）的DisplayMemberPathProperty委托中初始化。 ：

//In the foreach that process the ItemsSource collection:
btn.SelectionChanged += (s, e) =>
            {
                if (IsMultiSelect)
                {
                    if (btn.IsSelected)
                        (SelectedItems as ObservableCollection<object>).Add(item);
                    else
                        (SelectedItems as ObservableCollection<object>).Remove(item);
                }
                else
                {
                    var allToggleButtons = stackContainer.Children.Where(x => x is ToggleButton);
                    allToggleButtons?.ForEach(x => ((ToggleButton)x).IsSelected = false);
                    btn.IsSelected = true;
                    SelectedItems = item;
                }
                if (!IsMultiSelect && btn.IsSelected)
                    SelectedItemsChanged?.Invoke(this, new TogglesRowSelectionChangedEventArgs
                    {
                        SelectedItems = SelectedItems,
                        SelectedIndices = ItemsSource.IndexOf(item)
                    });
                else if (IsMultiSelect)
                {
                    SelectedItemsChanged?.Invoke(this, new TogglesRowSelectionChangedEventArgs
                    {
                        SelectedItems = SelectedItems,
                        SelectedIndices = (SelectedItems as ObservableCollection<object>).Select(x => ItemsSource.IndexOf(x))
                    });
                }
                Command?.Execute(CommandParameter);

            };

可绑定属性：

public static readonly BindableProperty CommandProperty =
            BindableProperty.Create(nameof(Command), typeof(ICommand), typeof(TogglesRow), null);

        public ICommand Command
        {
            get { return (ICommand)GetValue(CommandProperty); }
            set { SetValue(CommandProperty, value); }
        }

        public static readonly BindableProperty CommandParameterProperty =
            BindableProperty.Create(nameof(CommandParameter), typeof(object), typeof(TogglesRow), null);

        public object CommandParameter
        {
            get { return GetValue(CommandParameterProperty); }
            set { SetValue(CommandParameterProperty, value); }
        }

        public static readonly BindableProperty SelectedItemsProperty = BindableProperty.Create(nameof(SelectedItems), typeof(object), typeof(TogglesRow),
            defaultBindingMode: BindingMode.TwoWay);
        public object SelectedItems
        {
            get { return GetValue(SelectedItemsProperty); }
            set { SetValue(SelectedItemsProperty, value); }
        }

我知道，对CommandParameter属性进行 late-bind 绑定是合理的，但无论如何。例如，重新评估绑定表达式以获取最新的SelectedItems？