How do I initialize viewmodel's datacontext when the application starts? - wpf

Question

I started to use MVVMLight framework recently.
When I use ContentPresenter to swtich between ViewModel, it seems like it initialize the datacontext when it first displays.
However, I want it to initialize it's datacontext so it can keep track of any change from the initial loading of the application, or at least share the data with other viewmodel(I assume maybe I can use dataservice to keep track of all the data, but I could not find a right example to use it with contentpresenter & MVVMLight).
Below is the sample code I made. When I click Review button, "usercontrolview" will display "Picture Saved", however "contentpresenterview" will display "No Picture".
Sample Image
MainView.xaml
<Grid>
<Button x:Name="CaptureButton" Content="Capture" HorizontalAlignment="Left" Margin="34,10,0,0" VerticalAlignment="Top" Width="75"
Command="{Binding CaptureCommand}"/>
<Button x:Name="ReviewButton" Content="Review" HorizontalAlignment="Left" Margin="146,10,0,0" VerticalAlignment="Top" Width="75"
Command="{Binding ShowReviewCommand}"/>
<TextBlock FontSize="36"
FontWeight="Bold"
Foreground="Purple"
Text="{Binding CaptureStatus}"
VerticalAlignment="Center"
HorizontalAlignment="Center"
TextWrapping="Wrap" />
<Grid x:Name="usercontrolview" Visibility="{Binding ReviewModeOn, Converter={StaticResource BooleanToVisibilityConverter}}" Margin="0,50,150,0">
<view:ReviewView/>
</Grid>
<ContentPresenter x:Name="contentpresenterview" Content="{Binding CurrentContent}" Margin="150,50,0,0"/>
</Grid>
MainViewModel.cs (Partial)
public MainViewModel()
{
CaptureStatus = "No Picture";
CaptureCommand = new RelayCommand(Capture);
ShowReviewCommand = new RelayCommand(ShowReview);
ReviewModeOn = false;
}
public RelayCommand CaptureCommand { get; private set; }
private void Capture()
{
CaptureStatus = "Pictures Saved";
Messenger.Default.Send<NotificationMessage>(new NotificationMessage("Pictures Saved"), "Captured");
}
public RelayCommand ShowReviewCommand { get; private set; }
private void ShowReview()
{
ReviewModeOn = !ReviewModeOn;
CurrentContent = ContentViewModel;
}
And my template for ReviewViewModel & ReviewContentPresenterViewModel
public ***ViewModel()
{
Messenger.Default.Register<NotificationMessage>(this, "Captured", Captured);
CaptureStatus = "No Picture";
}
private void Captured(NotificationMessage notificationMessage)
{
CaptureStatus = notificationMessage.Notification;
}
private string _captureStatus;
public string CaptureStatus
{
get { return _captureStatus; }
set { Set(ref _captureStatus, value); }
}
======================= Update =======================
My template for ReviewView & ReviewContentPresenterView.
It takes DataContext by locator.
<UserControl x:Class="***View"
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:ignore="http://www.galasoft.ch/ignore"
mc:Ignorable="d ignore"
DataContext="{Binding ***ViewModel, Source={StaticResource Locator}}">
<Grid Background="Gray">
<TextBlock FontSize="36"
FontWeight="Bold"
Foreground="Purple"
Text="{Binding CaptureStatus}"
VerticalAlignment="Center"
HorizontalAlignment="Center"
TextWrapping="Wrap" />
</Grid>
</UserControl>

Answer 1

It seems your DataContext in the root Grid and all it's children is the MainViewModel and you want other DataContext for ContentPresenter. Create the other ViewModel as a resource e.g.
<Grid>
<Grid.Resources>
<local:TheViewModelIWant x:Key=ContentViewModel/>
</Grid.Resources>
...
...
<ContentPresenter DataContext={StaticResource ContentViewModel} ...
(By the way, if you are already creating an instance of TheViewModelIWant somewhere e.g. in other ViewModels or view code behind then you can stop using that instance and access this instance in C# code as Resources["ContentViewModel"])