How can I specify a ViewModel to a XAML window? - xaml

Question

I am trying to share a ViewModel between XAML windows. This is necessary to allow multiple views of the object instance to receive events from the ViewModel.
Specifying the ViewModel as a resource in the XAML, then overwriting it in an alternate constructor does not work. The binding will still be to the default instance created in the default constructor and will not receive events from or update the proper instance.
This does not work:
MyWindow.xaml:
<Window x:Class="MyNamespace.MyWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:MyNamespace"
Title="My Window"
Width="700" Height="550">
<Window.Resources>
<local:MyViewModel x:Key="MyModel"/>
</Window.Resources>
<ContentPresenter Content="{StaticResource MyModel}"/>
</Window>
MyWindow.xaml.cs
imports ...;
namespace MyNamespace {
public partial class MyWindow {
public MyWindow() {
InitializeComponent();
}
public MyWindow(MyViewModel model)
: this() {
Resources["MyModel"] = model;
}
}
}
Nor will this:
MyWindow.xaml.cs
imports ...;
namespace MyNamespace {
public partial class MyWindow {
public MyWindow()
: this(new MyViewModel()) { }
public MyWindow(MyViewModel model) {
Resources["MyModel"] = model; // Resources not yet initialized!
InitializeComponent();
}
}
}

Answer 1

if you are using Microsoft.Practices.Unity you can use TransientLifetimeManager.It will make sure that only one object of your viewmodel is created.

Answer 2

MyThis can be done by using properties on the code-behind and using the Binding tag in the XAML rather than StaticResource as follows:
MyWindow.xaml:
<Window x:Class="MyNamespace.MyWindow"
x:Name="this"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:MyNamespace"
Title="My Window"
Width="700" Height="550">
<ContentPresenter Content="{Binding MyModel, ElementName=this}"/>
</Window>
MyWindow.xaml.cs
imports ...;
namespace MyNamespace {
public partial class MyWindow {
public MyViewModel MyModel { get; private set; }
public MyWindow()
: this(new MyViewModel()) { }
public MyWindow(MyViewModel model) {
MyModel = model;
InitializeComponent();
}
}
}
Multiple windows (or other components) can use the same model instance.
Edit 06-Dec-12:
The XAML was not correct and the binding would not work. Added the x:Name attribute to the root element (Window), and added the ElementName argument to the Content attribute of the bound element (ContentPresenter).