Binding ViewModel to multiple windows

Question

I'm re-writing my windows forms project that does scoring for sheep shearing events (don't ask, its a huge sport in New Zealand) from vbnet to wpf c# and have struck a problem that I can't seem to overcome.

I have two windows. One is the source window where you type things in (like the current event name), and the other window will display this information in a flash way for projection onto a screen (so will be on a second monitor) along with some other data coming in via XML over the network. I have set it up as MVVM with a ViewModel and a Model as separate projects.

On my Main window, I can bind controls fine and if I type in one text box it immediately appears in another text box if it is bound to the same thing. However, on a second window, I have bound a control to the same thing and it is not updating.

I've been going around in circles on this for a week, every example on the net shows how to do it on One window which I have got working fine, but there is a lack of two window examples.

Here is what I have...

This is in my ViewModel project

namespace SheepViewModel
{
public class SheepViewModel : INotifyPropertyChanged


{
    private string _CurrentEventName;
    static SheepViewModel _details;

    public string CurrentEventName
    {
        get { return _CurrentEventName; }
        set
        {
            _CurrentEventName = value;
            OnPropertyChanged("CurrentEventName");
        }
    }

    public static SheepViewModel GetDetails()
    {
        if (_details == null)
            _details = new SheepViewModel();
        return _details;
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void OnPropertyChanged(string prop)
    {
        if (PropertyChanged != null)
             PropertyChanged(this, new PropertyChangedEventArgs(prop));
            Console.WriteLine("Test");
            }     
    }
}

Then I have a main window, there is no real code behind other than a line to open a second window which we will get to...

 public MainWindow()
    {
        ScoreScreen SW = new ScoreScreen();
        SW.Show();
        InitializeComponent();
    }

Then the XAML

<Window x:Class="Sheep_Score_3._1.MainWindow"
    xmlns:xctk="http://schemas.xceed.com/wpf/xaml/toolkit"
    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:vm="clr-namespace:SheepViewModel;assembly=SheepViewModel"
    mc:Ignorable="d"
    Title="MainWindow" Height="433.689" Width="941.194">
<Window.DataContext>
    <vm:SheepViewModel/>
</Window.DataContext>
<Window.Resources>
<Grid Margin="0,0,0,0">
<TextBox x:Name="CurrentEventName" Height="23" Margin="131.01,163.013,0,0" TextWrapping="Wrap" VerticalAlignment="Top" HorizontalAlignment="Left" Width="327.151" Text="{Binding CurrentEventName, Mode=TwoWay}"/>
    <TextBox Text="{Binding CurrentEventName, Mode=TwoWay}" Margin="39.605,0,0,108.567" Height="49.111" VerticalAlignment="Bottom" HorizontalAlignment="Left" Width="399" />
</Grid>

The above code all works fine, if I type text in the first textbox it appears in the second text box. If I put a console.writeline in the notify part then I can see it hitting it and updating.

Now I add a second window, setup exactly the same way...

<Window x:Class="Sheep_Score_3._1.ScoreScreen"
    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:vm="clr-namespace:SheepViewModel;assembly=SheepViewModel"
    mc:Ignorable="d"
    Title="ScoreScreen" Height="300" Width="300">
<Window.DataContext>
    <vm:SheepViewModel/>
</Window.DataContext>
<Grid>
    <TextBox x:Name="textBlock" HorizontalAlignment="Left" Margin="79.374,116.672,0,0" TextWrapping="Wrap" Text="{Binding CurrentEventName, Mode=TwoWay}" VerticalAlignment="Top"/>
</Grid>

Again, no real code behind in this.

The strange thing, is if I make this control two way and type in it, I can see it hitting the same notify section, but it is not updating the other window.

I'm not sure what I am missing here so any help in pointing me in the right direction would be much appreciated.

Answer 1

I would make the ViewModel as an application resource

<Application.Resources>
    <VM:ViewModel x:Key="SharedViewModel" />
    ....
</Application.Resources>

then in the each window you call it like this

DataContext="{StaticResource MainViewModel}"

I'm still new to this concept, and I'm not sure if this is optimal. It works though!