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WPF/MVVM: switching between views

I'm trying to learn WPF/MVVM, and I'm currently working on how to switch between views. I've started by finding some example to study in the interntet. The one I'm using is quite simple: two views (named "Home and "Account") that only display a label, to keep the xaml and VM simple, and a main window with two buttons to switch between the views.
The DataTemplates are declared in the App.xaml file (together with the namespaces), so they should be global to the whole project:
<Application.Resources>
<DataTemplate DataType="{x:Type viewmodels:HomeViewModel}">
<views:HomeView/>
</DataTemplate>
<DataTemplate DataType="{x:Type viewmodels:AccountViewModel}">
<views:AccountView/>
</DataTemplate>
</Application.Resources>
The way I've understood it, the trick is done by a third VM (called MainViewModel.cs) that implements a SelectedViewModel attribute that keeps track of the VM that must be displayed, plus and ICommand bound to the buttons:
private BaseViewModel _selectedViewModel;
public BaseViewModel SelectedViewModel
{
get { return _selectedViewModel; }
set
{
_selectedViewModel = value;
OnPropertyChanged(nameof(SelectedViewModel));
}
}
public ICommand UpdateViewCommand { get; set; }
MainWindow.xaml looks like this:
<ContentControl Grid.Row="0" Content="{Binding SelectedViewModel}"/>
<StackPanel Grid.Row="1" Orientation="Horizontal" HorizontalAlignment="Center">
<Button Margin="10" Width="200" Content="Home" Command="{Binding UpdateViewCommand}" CommandParameter="Home"/>
<Button Margin="10" Width="200" Content="Account" Command="{Binding UpdateViewCommand}" CommandParameter="Account"/>
</StackPanel>
In a separate class file (UpdateViewModel.cs) the UpdateViewModel class implements the Execute method as follows:
public void Execute (object parameter)
{
if (parameter.ToString() == "Home")
{
viewModel.SelectedViewModel = new HomeViewModel();
}
else if (parameter.ToString() == "Account")
{
viewModel.SelectedViewModel = new AccountViewModel();
}
}
I hope I've given the idea without boring you. It all works and lets me understand the basics. Now I wanted to try a variant, i.e. take one view (the "Account" one) and implement a button that would switch directly to the other view. I thought all I had to do was to bind the button to the UpdateViewModel class, and initially I modified the Account.xaml code as follows:
<Button Content="Button" Command="{Binding Path=UpdateViewCommand}" CommandParameter="Home"/>
The program runs, but when I click on the button in the Account view, nothing happens. So I changed this to something more complex:
<UserControl.DataContext>
<src:MainViewModel/>
</UserControl.DataContext>
...
<Button Content="Button" Command="{Binding Path=UpdateViewCommand}" CommandParameter="Home"/>
But the result is the same. I suspect it has to do with the binding, but can't see how to change it. Anybody can help?

Welcome to SO!
Your button bindings are done relative to the current DataContext, so binding to UpdateViewCommand in your AccountView (say) will try to bind to the UpdateViewCommand in your AccountViewModel, rather than your MainViewModel.
There are two ways to solve this. The first is to change your button bindings to bind to the parent's view model instead:
<Button Content="Button" Command="{Binding Path=DataContext.UpdateViewCommand, RelativeSource={RelativeSource AncestorType={x:Type local:MainWindow}}}" CommandParameter="Home"/>
A major disadvantage here, of course, is that the visual hierarchy your child views reside in may not always be as predictable as this.
The second (better) way is to add UpdateViewCommand handlers to each of your child view models, and then have them pass control on to whatever you actually want to handle it. In practice you would typically create a base class for all your children, to reduce code duplication, and you would create a service (e.g. INavigationService) for them to call. Your MainViewModel (say) would then implement this interface, and you would use dependency injection to inject that reference into the child view models at their moment of creation (or just have the parent pass itself directly into their constructors, if you don't want to use a full DI framework).

How can I bind a CommandParameter with a child UserControl's child element?

I have the following xaml view:
<UserControl x:Class="MyViews.PersonView"
xmlns:views="clr-namespace:MyViews"
[...]
>
[...]
<dxb:BarManager x:Name="MainBarManager">
<dxb:BarManager.Items>
<dxb:BarButtonItem x:Name="bbiPrint"
Content="{Binding Print, Source={StaticResource CommonResources}}"
Command="{Binding PrintPersonsCommand}"
CommandParameter="{Binding PersonsCardView, ElementName=CardUserControl}"
/>
</dxb:BarManager.Items>
<Grid>
<Grid.RowDefinitions>
[...]
</Grid.RowDefinitions>
<views:CardView x:Name="CardUserControl" Grid.Row="2"/>
</Grid>
[...]
</UserControl>
The CardView is defined as follows:
<UserControl x:Class="MyViews.CardView"
[...]>
[...]
<dxg:GridControl ItemsSource="{Binding Persons}" SelectedItems="{Binding SelectedPersons}" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" SelectionMode="MultipleRow">
[...]
<dxg:GridControl.View>
<dxg:CardView x:Name="PersonsCardView"
[...]
CardTemplate="{StaticResource DisplayCardTemplate}"
PrintCardViewItemTemplate="{StaticResource PrintCardTemplate}"/>
</dxg:GridControl.View>
[...]
</dxg:GridControl>
</UserControl>
The PrintPersonsCommand is defined as follows in my ViewModel:
public class PersonViewModel
{
public PersonViewModel(...)
{
[...]
PrintPersonsCommand = new Prism.Commands.DelegateCommand<DataViewBase>(PrintPersons, CanPrintPersons);
}
public Prism.Commands.DelegateCommand<DataViewBase> PrintPersonsCommand { get; private set; }
private void PrintPersons(DataViewBase view)
{
_printService.ShowGridViewPrintPreview(view);
}
private bool CanPrintPersons(DataViewBase view)
{
return true;
}
}
Now, when I click the Print button, the above PrintPersons method is always fed with null. How do I pass CardUserControl.PersonsCardView in my MyViews.PersonView xaml above, how do I pass that PersonCardView to my command? In other words, how do I fix
CommandParameter="{Binding PersonsCardView, ElementName=CardUserControl}"
to make it work?
Currently, the only solution I've found to this problem is to replace the Command and CommandParameter with
ItemClick="OnPrintBtnClick"
and then in the PersonView's code-behind file to do:
private void OnPrintBtnClick(object sender, ItemClickEventArgs e)
{
var ctxt = DataContext as PersonViewModel;
ctxt.PrintPersonsCommand.Execute(CardUserControl.PersonsCardView);
}
That works but I can't believe there is no other way. I'm not happy with that solution because I don't have the benefits of using the Command any more, like e.g. the automatic evaluation of the Command's CanExecute method. I could also put the CardView's xaml code in the PersonView.xaml but I like my controls to be in separate files because I have the feeling it's more structured and each user control has its own responsibilities which can nicely be split into separate files. Also, that solution binds my view to my view model too tightly.
Can someone help me out please?

Without changing your existing view and viewmodel hierarchy, I was able to pass the GridControl.View to the PersonViewModel using the Tag property
You can assign the CardView to the Tag property at the bottom of your CardView UserControl, and then access this Tag as CommandParameter.
CardView UserControl
<UserControl x:Class="MyViews.CardView"
[...]>
[...]
<dxg:GridControl ItemsSource="{Binding Persons}" SelectedItems="{Binding SelectedPersons}" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" SelectionMode="MultipleRow">
[...]
<dxg:GridControl.View>
<dxg:CardView x:Name="PersonsCardView"
[...]
CardTemplate="{StaticResource DisplayCardTemplate}"
PrintCardViewItemTemplate="{StaticResource PrintCardTemplate}"/>
</dxg:GridControl.View>
[...]
</dxg:GridControl>
<UserControl.Tag>
<Binding ElementName="PersonsCardView"/>
</UserControl.Tag>
</UserControl>
Print Button Xaml:
<dxb:BarButtonItem x:Name="bbiPrint"
Content="{Binding Print, Source={StaticResource CommonResources}}"
Command="{Binding PrintPersonsCommand}"
CommandParameter="{Binding ElementName=CardUserControl, Path=Tag}"
/>

Based on the valuable input of Insane, I came up with the following two cleaner fixes:
Code-behind solution
In the PersonView, use the ItemClick event handler on the Print button:
<dxb:BarButtonItem x:Name="bbiPrint"
Content="{Binding Print, Source={StaticResource CommonResources}}"
ItemClick="OnPrintBtnClick"/>
Adapt the corresponding code-behind file like this:
public partial class PersonView : UserControl
{
readonly IPrintService _printService;
public PersonView(IPrintService printService)
{
_printService = printService;
InitializeComponent();
}
private void OnPrintBtnClick(object sender, ItemClickEventArgs e)
{
_printService.ShowGridViewPrintPreview(CardUserControl.PersonsCardView);
}
}
Because I want to gray-out the Print button when there is no selection, I still need to add some code to make that happen. I can get it by
1. updating the button code to
<dxb:BarButtonItem x:Name="bbiPrint"
Content="{Binding Print, Source={StaticResource CommonResources}}"
ItemClick="OnPrintBtnClick" IsEnabled="{Binding CanPrintPersons}"/>
refreshing the CanPrintPersons property in the PersonViewModel upon Persons selection change
That's it.
CardViewModel solution
In that solution, we have a PersonView with its underlying PersonViewModel and a CardView with its underlying CardViewModel. I will not describe that solution with all the details as it is overkill in my situation but for the sake of completeness, I'll give the main points. Upon clicking the Print button on the PersonView, the PersonViewModel's PrintCommand is called. That command emits a Print event to the CardViewModel which in turn calls its own PrintCommand. That latter command calls
_printService.ShowGridViewPrintPreview(View);
where the View is a CardViewModel's property that is set upon CardView loading with e.g.
<dxmvvm:Interaction.Behaviors>
<dxmvvm:EventToCommand EventName="Loaded" Command="{Binding ViewLoadedCommand}" CommandParameter="{Binding ElementName=PersonsCardView}" />
</dxmvvm:Interaction.Behaviors>
Because I have two child views I want to print, I'd need to add a view model for each one of those. In addition, those two view models plus the PersonViewModel need access to the list of Persons to be printed. In particular, they need a shared access to the same data, so that they are synchronized. A simple way to do that is explained here and is totally doable. But I think it is not worth the trouble for the simple use case I have as it adds more complexity than necessary.

UWP+Prism MVVM - How do I set the value of a view model property via XAML?

I have a property in a view model which I would like to be able to set via the XAML but I can't figure out how to do it.
I have a pretty basic user control (containing a list of items), two of which are to be placed on a page and I would like to be able to set one to be a 'Source' (defined by an enum) and one to be a 'Target'.
[The code below has been stripped down quite a bit so apologies if I've accidentally made some mistakes or missed something out.]
My enumeration is:
public enum ConversionSide
{
Source, // Convert something FROM whatever is here.
Target // Convert something TO whatever is here.
}
I have a page which looks like this:
<Page
x:Class="MyApp.Views.ConverterPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:models="using:MyApp.Models"
xmlns:my="using:MyApp.Controls"
xmlns:prismMvvm="using:Prism.Windows.Mvvm"
prismMvvm:ViewModelLocator.AutoWireViewModel="True"
Style="{StaticResource PageStyle}"
mc:Ignorable="d">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="1*" />
<ColumnDefinition Width="1*" />
</Grid.ColumnDefinitions>
<my:SelectorPage Name="SourceSelector" Grid.Column="0" />
<my:SelectorPage Name="TargetSelector" Grid.Column="1" />
</Grid>
</Page>
...where SelectorPage is a user control (I've called it a 'Page' to make the Prism AutoWire work but that's not the issue here) containing a list of items (all working fine) which looks like this...
<UserControl
x:Class="MyApp.Controls.SelectorPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:models="using:MyApp.Models"
xmlns:my="using:MyApp.Controls"
xmlns:prismMvvm="using:Prism.Windows.Mvvm"
prismMvvm:ViewModelLocator.AutoWireViewModel="True"
mc:Ignorable="d">
<ListView
Grid.Column="0"
ItemsSource="{x:Bind ViewModel.MyList, Mode=OneWay}"
SelectedItem="{x:Bind ViewModel.MySelectedItem, Mode=TwoWay}">
<ListView.Header>
<TextBlock Margin="0,8,0,8" HorizontalAlignment="Center" FontStyle="Italic" Text="Header Text" />
</ListView.Header>
<ListView.ItemContainerStyle>
<Style TargetType="ListViewItem">
<Setter Property="HorizontalContentAlignment" Value="Stretch" />
</Style>
</ListView.ItemContainerStyle>
<ListView.ItemTemplate>
<DataTemplate x:DataType="models:MyListItem">
<my:MyListItemTemplate />
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</UserControl>
..with code behind as...
public sealed partial class SelectorPage : UserControl
{
private SelectorViewModel ViewModel => DataContext as SelectorViewModel;
public SelectorPage()
{
this.InitializeComponent();
}
}
SelectorViewModel looks like this...
public class SelectorViewModel : ViewModelBase
{
private ConversionSide _side;
public ConversionSide Side
{
get { return _side; }
set { SetProperty(ref _side, value); }
}
// Many lines have been omitted for 'clarity'.
}
I would like to be able to set the Side property of SelectorViewModel in XAML like this...
<my:SelectorPage Name="SourceSelector" Grid.Column="0" Side="Source" />
<my:SelectorPage Name="TargetSelector" Grid.Column="1" Side="Target" />
(Once Side has been set, I do not expect it to ever change.)
How can I do this?
I've looked at using a dependency property but I can't get it to change the property in SelectorViewModel. When I add one in SelectorPage it's visible in the XAML and I can set it but it doesn't actually do anything so I'm probably not using it right. Putting a dependency property in the view model doesn't sound right to me but I could be wrong.
I've had a look around the web - Microsoft documentation, blogs, articles, stack overflow, etc. - but I can't find anything that explains things well enough for me to figure out what I'm supposed to do. The writings I've found seem to be exclusively about getting information from a bound property - which I'm okay with - but what I'm after is setting a property from the XAML.
Can anyone give my any clues please? I don't know if I'm just a tiny step away from getting what I want or if I'm miles away.

This would set the Side property of the SelectorPage control to Source:
A view sets the property of a view model by two-way bind to it. For example, the following TextBox sets the string property of a view model called Test when you change the text in the TextBox:
<TextBox Text="{Binding Test, Mode=TwoWay}" />
So setting the property of a view model from the view typically applies to controls that handles some kind of input. Any default value of a source property should be defined in the view model:
private ConversionSide _side = ConversionSide.Source;
You shouldn't define the default values in the view.

Windows 10 Universal Compiled binding (x:Bind) conflict with ViewModel

I want to bind an element in a page to dependency property in code behind with compiled binding and same time bind another element to ViewModel with usual binding. But it gives a runtime error.
Here is my xaml code.
<Page
x:Class="XbindingProblem.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:XbindingProblem"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
DataContext="{Binding Main, Source={StaticResource Locator}}"
mc:Ignorable="d">
<Page.Resources>
<DataTemplate x:Key="UserDataTemplate" x:DataType="local:User">
<StackPanel>
<TextBlock Text="{x:Bind Name}" />
<TextBlock Text="{x:Bind Age}" />
</StackPanel>
</DataTemplate>
</Page.Resources>
<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
<StackPanel>
<TextBlock Text="{Binding Title}"/>
<ContentPresenter ContentTemplate="{StaticResource UserDataTemplate}" Content="{x:Bind CurrentUser, Mode=OneWay}"/>
</StackPanel>
</Grid>
Here CurrentUser is dependency property which is initially null and then change in run time. This gives following runtime error.
Incorrect type passed into template. Based on the x:DataType global::XbindingProblem.User was expected.
The problem is it passes the ViewModel into UserDataTemplate instead of CurrentUser dependency property when CurrentUser is null.
Can anyone have a good explanation on this problem?

If you remove DataContext="{Binding Main, Source={StaticResource Locator}}", it will work. Why? because {x:Bind CurrentUser} is looking for a property called CurrentUser sitting inside your MainPage.xaml.cs. Since the CurrentUser is indeed a dependency property of your page, it will just work.
However, by specifying the DataContext of your page, the x:Bind is now excepting a CurrentUser property inside your MainViewModel instance, and of course it's not going to find it, so a compile-time error will be thrown.
One possible fix is to set the this.CurrentUser really early, even before calling InitializeComponent.
this.CurrentUser = new User();
InitializeComponent();
But this is IMHO not the the right way of doing things, as it's basically a racing game - it tries to populate the ContentPresenter before the DataContext gets updated, and in the end you will end up having the TextBlock (of which Text binds to Title) and the ContentPresenter attached to different contexts!
So ask yourself why you need to create a dependency property for CurrentUser inside a Page object, instead of having a normal property (with INotifyPropertyChanged implementation) sitting inside your MainViewModel? I'd prefer the latter, 'cause it's more semantically correct.

The question is interesting, what I have done is just remove the datacontext and this is the code behind is similar to yours:
public sealed partial class BlankPage1 : Page
{
public User CurrentUser
{
get { return (User)GetValue(CurrentUserProperty); }
set { SetValue(CurrentUserProperty, value); }
}
// Using a DependencyProperty as the backing store for CurrentUser. This enables animation, styling, binding, etc...
public static readonly DependencyProperty CurrentUserProperty =
DependencyProperty.Register("CurrentUser", typeof(User), typeof(BlankPage1), new PropertyMetadata(null));
public BlankPage1()
{
this.InitializeComponent();
CurrentUser = new User() { Name = "Hello", Age = "20" };
}
}
public class User
{
public String Name { get; set; }
public String Age { get; set; }
}
might be you have the User class in another namespace or have another class in the typeof(...) of the dependency property. Because I tested that and works. The DataContext of the page can be whatever you want it won't affect.
Then I added the datacontext just to test:
<Page.DataContext>
<local:Main/>
</Page.DataContext>
and the code just for testing:
public class Main
{
public String Title { get; } = "title";
public User MainUser { get; set; }
}
And it does not throws any exception, appears the Main data and the CurrentUser data.
UPDATE. The error happens when the User is null so it is like the x:Bind is null it propagates to the Binding,To solve that (it was tough):
<Page x:Name="Root"
x:Class="Deletetb.BlankPage1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:Deletetb"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d" >
<Page.DataContext>
<local:Main/>
</Page.DataContext>
<Page.Resources>
<DataTemplate x:Key="UserDataTemplate" x:DataType="local:User">
<StackPanel>
<TextBlock Text="{x:Bind Name}" />
<TextBlock Text="{x:Bind Age}" />
</StackPanel>
</DataTemplate>
</Page.Resources>
<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
<StackPanel DataContext="{x:Null}">
<TextBlock Text="{Binding DataContext.Title, ElementName=Root}" />
<ContentPresenter ContentTemplate="{StaticResource UserDataTemplate}" Content="{x:Bind CurrentUser, Mode=OneWay}"/>
</StackPanel>
</Grid>
</Page>
Where is binding defined (TextBlock) I set the datacontext to null in the parent container (StackPanel) and bind by element name, and it does not crash, also I added a wait by code to test and set the Current User and it works. That was a challenge. hope it also works for you.

Although it breaks the idea of MVVM a bit, you can add a property to your page like this:
public MainViewModel viewModel => DataContext as MainViewModel;
And then in the XAML code reference the page property
<ContentPresenter Content="{x:Bind viewModel.CurrentUser, Mode=OneWay}" />

How do you wire up views in Silverlight and MVVM using a TreeView?

I am building a Silverlight app which comprises a TreeView of menu options in a lefthand column and a ContentView in a righthand column. The idea is that the SelectedItemChanged event of the TreeView will change the view in the content area.
What is the 'purest MVVM' way of achieving this?
My idea is to have a TreeMenuView and TreeMenuViewModel for managing the menu events, but after that I'm a bit lost. I could use an EventAggregator to send a message from the TreeMenuViewModel to a `ContentViewModel' that would then set its current ContentView based on the message args- but surely that breaks MVVM, in the sense that a ViewModel shouldn't know about UI constructs like a View?
Am I missing something simple here?
How does a ViewModel layer drive the View selection?

I would create a ShellViewModel which had:
ObservableCollection<ViewModelBase> AvailablePages
int SelectedPageIndex
ViewModelBase CurrentPage, which returns AvailablePages[SelectedPageIndex]
Your ShellView can be anything you want. If you want to display your AvailablePages in a TreeView, then go ahead. Just remember to bind SelectedIndex to `SelectedPageIndex
In your case, I would create a DockPanel with a TreeView on the Left bound to AvailablePages, and a ContentControl on the right with ContentControl.Content bound to CurrentPage
Edit
Here's an example
<DockPanel>
<TreeView DockPanel.Dock="Right"
ItemsSource="{Binding AvailablePages}"
SelectedItem="{Binding SelectedPageIndex}">
...
</TreeView>
<ContentControl Content="{Binding CurrentPage}" />
</DockPanel>
Then use DataTemplates to define how the ContentControl containing CurrentPage will look
<Window.Resources>
<DataTemplate DataType="{x:Type local:HomePageViewModel}" />
<local:HomePageView />
</DataTemplate>
<DataTemplate DataType="{x:Type local:CustomerViewModel}" />
<local:CustomerView />
</DataTemplate>
</Window.Resources>

Ok I give it a shot
in TreeMenuViewModel:
public string PropSelectedItem
{
get;
set;
}
in TreeMenuView:
<TreeView Context="{Binding TreeMenuViewModel}" Content="{Binding PropSelectedItem, Mode=OneWayToSource}"/>
in ContentViewModel:
public ViewModelBase PropSelectedItem
{
get
{
switch(TreeMenuViewModelStatic.PropSelectedItem)
{
case "Booo": return typeof(View1);
case "Foo": return typeof(View2);
}
}
private set;
}
in ContentView:
<ContentControl Context="{Binding TreeMenuViewModel}" Content="{Binding PropSelectedItem, Mode=OneWay}"/>
and you need a value convertor here