Develop Reference

Wire multiple UserControls with separate ViewModels in MainWindow

I’m tyring to create an WPF MVVM application to edit music albums (cue sheets) with a waveform display. The MainWindow should have the follwing design:
At the top of the MainWindow will be a MenuBar.
Below a display of the audio waveform
Below a display of the audio tracks of the album.
To make the MainWindow.xaml not so big and have some kind of functional separation I decided to create separate view models and views for the waveform display and the album/tracks display. The separate views I’ve created as WPF UserControls each.
In app.xaml the views and view models are connected together:
<Application.Resources>
<DataTemplate DataType="{x:Type ViewModels:MainViewModel}">
<Views:Main/>
</DataTemplate>
<DataTemplate DataType="{x:Type ViewModels:CueAlbumViewModel}">
<Views:CueAlbum/>
</DataTemplate>
<DataTemplate DataType="{x:Type ViewModels:WaveFormViewModel}">
<Views:Waveform/>
</DataTemplate>
</Application.Resources>
The MainWindow has set the MainViewModel as DataContext:
<Window.DataContext>
<ViewModels:MainViewModel/>
</Window.DataContext>
The bindings of MainWindow <-> MainViewModel are working so far.
But what's not working is the binding of the UserControls to their custom DataSources in MainWindow. E.g. for the view CueAlbum I tried it this way:
<Views:CueAlbum Grid.Row="1"
DataContext="{Binding CueAlbumViewModel}"/>
But the connection isn't done properly. The constructor of CueAlbumViewModel isn't called at all.
So, can this be achieved properly in WPF / MVVM without code-behind in the view?
Or is using user controls with own DataContexts in MainWindow bad design at all?
(Another question that might arise after this is solved is how to set the Buttons of the MainWindows ToolbarTray to the dedicated UserContols..)

WPF - MVVM DataTemplate load to memory for reuse

I have a window which has several different DataTemplate that are load to a ContentControl based on a RadioButton (The RadioButton sends a command to the ModelView which sets the Content property of the ContentControl.
It works well, but now several views contain a "heavy" object (Eyeshot CAD viewer).
Switching to any of these view causes a delay (at this moment there's absolutely zero logic in the whole software other than the view/view model)
Is there a way to load the view and the heavy control to memory once and then reuse it when switching to its view? (The ViewModel of that view is currently a singleton but that doesn't help)
<DockPanel>
<StackPanel Orientation="Horizontal" DockPanel.Dock="Top" Height="160" Margin="0,0,0,12">
... Removed for clarity
</StackPanel>
<ContentControl x:Name="Tabs" Content="{Binding SelectedTabViewModel}" Margin="0,12,0,12"/>
</DockPanel>

On your DataTemplate, you can set the attribute x:Shared="True", this will allow the framework to reuse the visual control (inside the datatemplate) for another ContentPresenter.
This doesn't load the component at starting, but, this reuse it once instantiated.

How to set the DataContext of a KeyBinding to a specific ViewModel?

This seems like such a basic question but after hours of searching around and not figuring out what I'm doing wrong I decided it's time to ask for help!
I'm new to WPF and the MVVM pattern, but am trying to create an application that has several windows you can navigate through by clicking buttons. This is accomplished by having the app window display UserControls using DataTemplates, so there's no content currently shared between pages (though there will be once I create the navigation area). Here's what the XAML looks like for the main window, with there currently only being one page in the application:
<Window x:Class="WPF_Application.ApplicationView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WPF_Application"
Title="ApplicationView" Height="300" Width="300" WindowStartupLocation="CenterScreen" WindowState="Maximized">
<Window.Resources>
<DataTemplate DataType="{x:Type local:LoginMenuViewModel}">
<local:LoginMenuView />
</DataTemplate>
</Window.Resources>
<DockPanel>
<ContentControl
Content="{Binding CurrentPageViewModel}" />
</DockPanel>
Now what I'd like to do is add a KeyBinding that reacts to the escape button being pressed. When this is done "LogoutCommand" should fire in the LoginMenuViewModel. I'm stuck getting the keybinding to trigger any commands within LoginMenuViewModel, and I've figured it's probably because the DataContext needs to be set to reference LoginMenuViewModel. For the life of me I can't get this to work.
Am I going about application-wide commands completely the wrong way? Is there some super simple fix that will make me smack my forehead in shame? Any insight is appreciated!

I do not know your ViewModel code, so it is not easy to give you details hints.
First of all, if your are using MVVM, you should have your own implementation of ICommand interface. You can find here the most common one (and a simple MVVM tutorial too).
After you have your own command class, your ViewModel should expose your LogoutCommand:
public class ViewModel
{
/* ... */
public ICommand LogoutCommand
{
get
{
return /* your command */
}
}
/* ... */
}
In your code behind you will set: DataContext = new ViewModel(); and at this point you can declare the KeyBindings that you need:
<Window.InputBindings>
<KeyBinding Key="Escape" Command="{Binding Path=LogoutCommand, Mode=OneWay}" />
</Window.InputBindings>
In this way when the Window is active and the user press the "Esc" key, your LogoutCommand is executed.
This is a brief summary that I hope will guide you in deepening the MVVM and its command system.

WPF MVVM Navigation Technique

I know there are a lot of questions about WPF navigation, for application developed with MVVM pattern, and I have read tens and tens of answers but I'm missing probably something.
I started building an application following Rachel's article here. All works just fine, there's an ApplicationView Window with this XAML:
<Window x:Class="CashFlow.ApplicationView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:vm="clr-namespace:CashFlow.ViewModels"
xmlns:v="clr-namespace:CashFlow.Views"
Title="ApplicationView" Height="350" Width="600" WindowStartupLocation="CenterScreen">
<Window.Resources>
<!--Here the associations between ViewModels and Views-->
<DataTemplate DataType="{x:Type vm:HomeViewModel}">
<v:HomeView />
</DataTemplate>
</Window.Resources>
<!--Define here the application UI structure-->
<DockPanel>
<Border DockPanel.Dock="Left" BorderBrush="Black" BorderThickness="0,0,1,0">
<ItemsControl ItemsSource="{Binding PageViewModels}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<Button Content="{Binding Name}"
Command="{Binding DataContext.ChangePageCommand, RelativeSource={RelativeSource AncestorType={x:Type Window}}}"
CommandParameter="{Binding}"
Margin="2,5" />
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
</Border>
<ContentControl Content="{Binding CurrentPageViewModel}" />
</DockPanel>
The ApplicationViewModel, that is set as DataContext for this window when the application starts, maintains an ObservableCollection of my ViewModels. Thanks to data templates, it's possible to associate every view with its viewmodel, using a ContentControl to render the views. Navigation in this case is accomplished with a "side bar" of buttons, binded to ApplicationViewModel commands that perform the changes of CurrentPageViewModel object.
I'm wondering how I can perform navigation without the presence of that sidebar of Buttons. Having only the Content control, I should be able to change the CurrentPageViewModel from the others viewmodel? Probably the answer will be very trivial, but I can't see that right now.

Your top level homeviewmodel can orchestrate navigation via an eventbus pattern. To use eventbus, you would inject an object that tracks objects that want to be notified of events. Then when a view model raises an event, the homeviewmodel receives it and performs the currentpageviewmodel assignment that will navigate you to the next viewmodel.
Ex:
Messenger defines two methods - RegisterForEvent<IEvent>(ViewModel aViewModel), and RaiseEvent(IEvent event).
So you would define a function to subscribe to the events -
HomeViewModel.cs
...
void SubscribeForEvents() {
Messenger.RegisterForEvent<NavigationEvent>(this);
}
Then you inject the Messenger into your other view models, and from those view models, raise the event:
Messenger.RaiseEvent(new NavigationEvent { TargetViewModel = new TargetViewModel() });
Where the event is something like
public class NavigationEvent : IEvent {
ViewModel TargetViewModel { get;set;}
}

C Bauer is right with what you are missing. I found in order to switch the data context, you'll need a messenger service to flag your "applicationviewmodel" to switch its data context. A good discussion with the steps you need are spelled out in a discussion here.
Register the message to be received in your applicationviewmodel, then handle the data context switch in your receive message function.
Also, this might be true or not, but I had to use 1 window, with multiple user controls as opposed to multiple windows if I wanted to have 1 window showing at all times. Lastly, I followed Sheridan's example and defined my data templates in my app.xaml as opposed to the window itself.

Changing Main Viewing Area

I'm wondering how to go about creating different views in the main window when a button is pressed. I'm not sure of the correct terminology, so that has hampered my google fu.
I'm thinking that the main viewing area would be a content control, that I could change when a event happens. I made a small drawing to help illustrate my idea / thought.
Any input will be appreciated. Thanks!

It would be really easy to implement this senario using MVVM approach....
Make a ViewModel for you MainView. Then Define Properties of the ViewModels of your UserControls
For Example You have Two UserControl as FirstView and SecondView then make a properties in your viewmodels as ViewToLoadProperty of the type ViewModel (usually called as ViewModelBase)
Set bindings as
<!-- Panel For Hosting UserControls -->
<Border Grid.Column="2">
<ContentControl Name="userControlContentControl"
Content="{Binding Path=ViewToLoadProperty,
}">
<ContentControl.Resources>
<DataTemplate DataType="{x:Type ViewModelLayer:FirstViewModel}">
<ViewLayer:FirstView/>
</DataTemplate>
<DataTemplate DataType="{x:Type ViewModelLayer:SecondViewModel}">
<ViewLayer:SecondView />
</DataTemplate>
</ContentControl.Resources>
</ContentControl>
</Border>
<!-- Panel For Hosting UserControls -->
Then when you click the button Use a command to set the respective ViewModel Intance to this(ViewToLoadProperty) property...(Use RelayCommannds or something like it)
DataTempates would do the rest of the job by selecting the right View according to the right type of ViewModel
YOu can use MVVMLight toolkit if you are implementing MVVM Pattern.. :)

On the right you could have a frame. Then the button would bind a different page or user control to the content of that frame.