图像源未在MVVM中更新

Question

当我修改状态时，我想要一个按钮来更新其内容的图像源。

XAML：

    <ItemsControl ItemsSource="{Binding PlayerStartups}" Grid.Column="0" Grid.Row="1" Grid.RowSpan="3"  Grid.ColumnSpan="2">
        <ItemsControl.ItemsPanel>
            <ItemsPanelTemplate>
                <UniformGrid Rows="{Binding PlayerStartups.Count}" Margin="10"/>
            </ItemsPanelTemplate>
        </ItemsControl.ItemsPanel>
        <ItemsControl.ItemTemplate>
            <DataTemplate>
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="2*"/>
                        <ColumnDefinition Width="*" />
                    </Grid.ColumnDefinitions>
                    <ComboBox ItemsSource="{Binding Source={StaticResource PlayerStartupStatusProvider}}"
                              SelectedItem="{Binding Status}"
                              x:Name="DataTemplateComboBox"
                              Margin="5" Grid.Column="0">
                        <ComboBox.ItemTemplate ... />
                    </ComboBox>
                    <Button Margin="5" Grid.Column="1"
                            Width="{Binding RelativeSource={RelativeSource Self}, Path=ActualHeight}"
                            Command="{Binding DataContext.SetPlayerPropertiesCommand, 
                            RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=Window}}" 
                            CommandParameter="{Binding}">
                        <Image Source="{Binding TokenSource}" 
                               Stretch="Fill"/>
                    </Button>
                </Grid>
            </DataTemplate>
        </ItemsControl.ItemTemplate>
    </ItemsControl>

PlayerStartups是ObservableCollection。每当组合框被修改为特定值时，按钮内容图像的来源应该改变，但这不会发生。 我怎么能做到这一点？ 这是负责价值变化的事件处理程序的一部分：

                ....
                if (status is ok)
                {
                    ((PlayerStartup)s).TokenSource = "pack://siteoforigin:,,,/Images/pic.jpg";
                }
                .....

我想在视图代码后面没有任何代码的情况下这样做。

PlayerStartup的属性和字段：

    private StartMenuViewModel.Status _status;
    private string _character;
    private string _name;
    private string _tokenSource;

    public StartMenuViewModel.Status Status { get { return _status; } set { _status = value; OnStatusChanged(); } }
    public string Character { get { return _character; } set { _character = value; OnCharacterChanged(); } }
    public string TokenSource { get { return _tokenSource; } set { _tokenSource = value; } }