Closing-EventTrigger on a WPF Window - Problems with DataContext - wpf

Question

I have a view, which initializes a viewmodel inside the windows resources. Further more I give my grid the DataContext.
My question is, how I can add a command to my windows closing event keeping mvvm in memory? I tried the version of this post:
Handling the window closing event with WPF / MVVM Light Toolkit
... but its not working using an event-trigger, because I can't access the viewmodel from outside my grid, so I can't access my command.
Any solution for my problem?
Greetings
Jannik
Edit: Here's my xaml:
<Window x:Class="WpfApplication1.Views.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:viewModels="clr-namespace:WpfApplication1.ViewModels"
xmlns:converter="clr-namespace:WpfApplication1.Converter"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<viewModels:MainWindowViewModel x:Key="ViewModel"/>
</Window.Resources>
<Grid DataContext="{StaticResource ViewModel}">...</Grid>
</Window>

Answer 1

You can reference to members of a static resource this way:
Command="{Binding Path=CloseCommand, Source={StaticResource ViewModel}}"
Here's the complete test project. I used a text box with a binding to ensure data is saved.
<Window x:Class="WpfApplication1.Views.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:viewModels="clr-namespace:WpfApplication1.ViewModels"
xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<viewModels:MainWindowViewModel x:Key="ViewModel"/>
</Window.Resources>
<i:Interaction.Triggers>
<i:EventTrigger EventName="Closing">
<i:InvokeCommandAction Command="{Binding Path=CloseCommand, Source={StaticResource ViewModel}}"/>
</i:EventTrigger>
</i:Interaction.Triggers>
<Grid DataContext="{StaticResource ViewModel}">
<TextBox Text="{Binding Txt, UpdateSourceTrigger=PropertyChanged}"/>
</Grid>
</Window>
In ViewModel code, I used a static reference to store data (LastInstance). you can replace it with your own method.
Also I used Command which is a custom implementation of ICommand. If you want I can add the complete implementation here.
public MainWindowViewModel()
{
//save or load here...
if (LastInstance != null) Txt = LastInstance.Txt;
CloseCommand = new Command(o => LastInstance = this);
//...
}
public static ViewModel LastInstance;
//Txt Dependency Property
public string Txt
{
get { return (string)GetValue(TxtProperty); }
set { SetValue(TxtProperty, value); }
}
public static readonly DependencyProperty TxtProperty =
DependencyProperty.Register("Txt", typeof(string), typeof(ViewModel), new UIPropertyMetadata(null));
//CloseCommand Dependency Property
public Command CloseCommand
{
get { return (Command)GetValue(CloseCommandProperty); }
set { SetValue(CloseCommandProperty, value); }
}
public static readonly DependencyProperty CloseCommandProperty =
DependencyProperty.Register("CloseCommand", typeof(Command), typeof(ViewModel), new UIPropertyMetadata(null));

Answer 2

The typical approach to this problem is to have a MainViewModel and set the DataContext of you Window to it. Then define other viewModels in the MainViewModel.
<Window>
<Grid DataContext="{Binding MyGridViewModel}">
</Grid>
<DockPanel DataContext="{Binding AnotherViewModel}">
</DockPanel>
</Window>
in MainWindow constructor:
this.DataContext = new MainViewModel();
in MainViewModel constructor:
this.MyGridViewModel = new OtherViewModel();
This way you have many options to find the desired object through viewModel references.