Question

I am trying to access values from a user control MVVM in a parent MVVM class – so I can set the menu to enable/disable. I cannot visualize how to wire this up.

Any help is appreciated for the below since I have stuck (as I learn to implement MVVM)

Q1) As mentioned above – how do I access values from user control (child) MVVM onto the parent windowviewmodel

Q2) Once these values are accessible – I need to wire them with the CommandBinding of the menu (Edit) item (for enable/disable). How do I do that?

Q3) Minor question – currently in my loginviewmodel – only when I tab out does the Submit button get enabled – if I wanted to do it as soon as I type a character – how do I go about?

Code for my loginscreen.xaml.cs

public partial class Login_Screen_User_Control : UserControl
{
    public Login_Screen_User_Control()
    {
        InitializeComponent();
        var loginmodel = new myWindowViewModel();
        DataContext = loginmodel;
    }
}

public class myWindowViewModel
{
    private string _username;
    private string _password;
    public ICommand OkCommand { get; set; }

    public myWindowViewModel()
    {
        OkCommand = new myCommand(myOkExecute, myCanOkExecute);
    }

    public string userName
    { 
        get { return _username; }
        set { _username = value; }
    }

    public string passWord
    {
        get { return _password; }
        set { _password = value; }
    }

    private void myOkExecute(object parameter)
    {
        //Authetication Logic - lets assume this came back as true
    }

    private bool myCanOkExecute(object parameter)
    {
        return (string.IsNullOrEmpty(userName) ? false : (userName.Length > 0 ? true : false));
    }

}

Code for my ParentApp.xaml

<Window x:Class="Complete_Application.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:loc="clr-namespace:_05_User_Control_POC;assembly=05-User_Control_POC"
    Title="MainWindow" Height="350" Width="525">
<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition Height="*"/>
    </Grid.RowDefinitions>
    <Menu Grid.Row="0" Name="menu1" Height="25">
        <MenuItem Header="_File">
            <MenuItem Header="_New"/>
            <MenuItem Header="_Open"/>
            <MenuItem Header="_Close"/>
        </MenuItem>
        <MenuItem Header="_Edit">
            <MenuItem Header="_New"/>
            <MenuItem Header="_Open"/>
            <MenuItem Header="_Close"/>
        </MenuItem>
    </Menu>
    <Grid Grid.Row="1">
        <loc:Login_Screen_User_Control></loc:Login_Screen_User_Control>
    </Grid>
</Grid>

Code for myparent.xaml.cs

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        var mainwindowviewmodel = new parentWindowViewModel();
        DataContext = mainwindowviewmodel;
    }
}

public class parentWindowViewModel
{
    public parentWindowViewModel()
    {
        //??
    }
}

I have looked at the following article – the person is doing the exact similar thing but through singleton pattern which I dont want to use – WPF MVVM – Simple login to an application

Answer 1

A1)
Option A: use a Mediator/Messenger to transfer the values (mvvm light messenger)
Option B: (recommended) have LoginVM as a child property of your parent and use DataTemplate

parentWindowViewModel { 
   LoginVM LoginChild { get; }
}

<Window>...
   <Window.Resources>...
      <DataTemplate DataType={x:Type vm:LoginVM}>...

   <ContentControl Content={Binding LoginChild}/>

A2) I don’t completely understand via the wording, but {Binding LoginChild.VALUE}

A3) Binding.UpdateSourceTrigger