Does this code break MVVM? - wpf

Question

Building my first project with MVVM, I would like to know if the following code breaks the model.
The aim of the command is to switch between two UserControls (ucDrumStandard and ucDrumStandardList. Both share the same viewmodel.
This viewmodel inherits from a viewmodelbase containing the "ParentContext".
UserControls are 'stored' in ParentContext.listOfViews.
(Apologize for my VB Code ;-)
#Region "CmdSwitchDrumStandardView"
Public ReadOnly Property CmdSwitchDrumStandardView() As ICommand
Get
If _cmdSwitchDrumStandardView Is Nothing Then
_cmdSwitchDrumStandardView = New RelayCommand(AddressOf SwitchDrumStandardView)
End If
Return _cmdSwitchDrumStandardView
End Get
End Property
Private Sub SwitchDrumStandardView()
' There are two views(ucDrumStandard and ucDrumStandardList) for the same viewmodel
If ParentContext.CurrentView.Uid = "ucDrumStandard" Then
' switch to ucDrumStandardGrid
If ParentContext.ListOfViews.ContainsKey("ucDrumStandardGrid") Then
ParentContext.CurrentView = (From obj As KeyValuePair(Of String, UIElement) In ParentContext.ListOfViews
Where obj.Key = "ucDrumStandardGrid"
Select obj.Value).FirstOrDefault
Else
Dim m_ucDrumStandardGrid = New ucDrumStandardGrid
ParentContext.ListOfViews.Add("ucDrumStandardGrid", m_ucDrumStandardGrid)
ParentContext.CurrentView = m_ucDrumStandardGrid
End If
ElseIf ParentContext.CurrentView.Uid = "ucDrumStandardGrid" Then
' switch to ucDrumStandard
If ParentContext.ListOfViews.ContainsKey("ucDrumStandard") Then
ParentContext.CurrentView = (From obj As KeyValuePair(Of String, UIElement) In ParentContext.ListOfViews
Where obj.Key = "ucDrumStandard"
Select obj.Value).FirstOrDefault
Else
Dim m_ucDrumStandard = New ucDrumStandard
ParentContext.ListOfViews.Add("ucDrumStandard", m_ucDrumStandard)
ParentContext.CurrentView = m_ucDrumStandard
End If
End If
End Sub
#End Region

Answer 1

In the MVVM pattern the communication between the ViewModel & View layers should be done only by Binding & Commands. ViewModel code shouldn't use FrameworkElements so yes, your code "break" the MVVM pattern.
Your ParentContext which I assume is a ViewModel class should have a CurrentView property that's of type of the ViewModel layer of the "Page" objects.
The render of those ViewModel objects in the View layer should be done using DataTemplate & Bindings. I'll add a little sample illustrating all this.
//ViewModel
public class AppVM : INotifyPropertyChanged {
//your code...
private PageVM _currentView;
public PageVM CurrentView {
get {return _currentView;}
set {
_currentView = value;
OnPropertyChanged("CurrentView");
}
}
}
public class PageVM : INotifyPropertyChanged {
//your "view" data visible in the UI
}
//XAML
<ContentControl Content="{Binding CurrentView, Mode=OneWay}">
<ContentControl.Resources>
<DataTemplate DataType="{x:Type ucDrumStandardVM}">
<ucDrumStandard/>
</DataTemplate>
<DataTemplate DataType="{x:Type ucDrumStandardListVM}">
<ucDrumStandardList/>
</DataTemplate>
</ContentControl.Resources>
</ContentControl>
If you really want to keep the same ViewModel object for your two "pages" then you have to use a Trigger
<ContentControl Content="{Binding CurrentView, Mode=OneWay}">
<ContentControl.Style>
<Style TargetType="{x:Type ContentControl}">
<Style.Triggers>
<DataTrigger Binding="{Binding CurrentView.Uid, Mode=OneWay}" Value="ucDrumStandard">
<Setter Property="ContentTemplate">
<DataTemplate>
<ucDrumStandard/>
</DataTemplate>
</Setter>
</DataTrigger>
<DataTrigger Binding="{Binding CurrentView.Uid, Mode=OneWay}" Value="ucDrumStandardGrid">
<Setter Property="ContentTemplate">
<DataTemplate>
<ucDrumStandardList/>
</DataTemplate>
</Setter>
</DataTrigger>
</Style.Triggers>
</Style>
</ContentControl.Resources>
</ContentControl>