How to read TextBox focus from ViewModel - MVVM Light

I have two textBoxes and in my ViewModel I would like to be able to keep track of which box is currently in focus.

<TextBox x:Name="textBox1" Text="Text Box 1"/>
<TextBox x:Name="textBox2" Text="Text Box 2"/>

How can I read/identify which textBox is currently in focus from my ViewModel?

Solution

There are several ways how you can achieve this, some of them:

1) Use behavior:

You need System.Windows.Interactivity.dll

Behavior (setting IsFocused property will not make element focused, you need slightly extend behavior in order to achieve this)

public class FocusChangedBehavior : Behavior<UIElement>
{
    public static readonly DependencyProperty IsFocusedProperty = 
       DependencyProperty.Register(
       nameof(IsFocused),
       typeof(bool),
       typeof(FocusChangedBehavior),
       new FrameworkPropertyMetadata(default(bool), 
            FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));

    public bool IsFocused
    {
        get { return (bool)this.GetValue(IsFocusedProperty); }
        set { this.SetValue(IsFocusedProperty, value); }
    }

    /// <inheritdoc />
    protected override void OnAttached()
    {
        this.AssociatedObject.GotFocus += this.AssociatedObjectFocused;
        this.AssociatedObject.LostFocus += this.AssociatedObjectUnfocused;
    }

    /// <inheritdoc />
    protected override void OnDetaching()
    {
        this.AssociatedObject.GotFocus -= this.AssociatedObjectFocused;
        this.AssociatedObject.LostFocus -= this.AssociatedObjectUnfocused;
    }

    private void AssociatedObjectFocused(object sender, RoutedEventArgs e)
    {
        this.IsFocused = true;
    }

    private void AssociatedObjectUnfocused(object sender, RoutedEventArgs e)
    {
        this.IsFocused = false;
    }
}

In XAML you bind IsFocused to property in ViewModel.

xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"

<TextBox x:Name="textBox1" Text="Text Box 1">
    <i:Interaction.Behaviors>
        <local:FocusChangedBehavior IsFocused="{Binding IsFocusedTxt1}" />
    </i:Interaction.Behaviors>
</TextBox>

<TextBox x:Name="textBox2" Text="Text Box 2">
    <i:Interaction.Behaviors>
        <local:FocusChangedBehavior IsFocused="{Binding IsFocusedTxt2}" />
    </i:Interaction.Behaviors>
</TextBox>

Finally in View-Model create properties

public bool IsFocusedTxt1 { get; set; }

public bool IsFocusedTxt2 { get; set; }

2) Alternatively you could you use EventTrigger in the XAML

You need System.Windows.Interactivity.dll and MicrosoftExpressionInteractions (For the ActionCommand)

Event Triggers:

<TextBox x:Name="textBox1" Text="Text Box 1">
    <i:Interaction.Triggers>
        <i:EventTrigger  EventName="GotFocus">
            <i:InvokeCommandAction Command="{Binding NotifyFocusedReceivedTxt1Command}" />
        </i:EventTrigger>
    </i:Interaction.Triggers>
</TextBox>

In ViewModel create command NotifyFocusedReceivedTxt1Command

public ICommand NotifyFocusedReceivedTxt1Command { get; }

// in constructor
this.NotifyFocusedReceivedTxt1Command = new ActionCommand(this.FocusedReceivedTxt1);

// and method
private void FocusedReceivedTxt1()
{
    // Your logic
}

Also, if you don't want introduce many command/properties you could use same command and pass different textboxes by setting CommandParameter (slightly breaks MVVM, but not critically)

<TextBox x:Name="textBox1" Text="Text Box 1">
    <i:Interaction.Triggers>
        <i:EventTrigger  EventName="GotFocus">
            <i:InvokeCommandAction Command="{Binding NotifyFocusedReceivedCommand}" 
                                   CommandParameter="{Binding ., ElementName=textBox1}" />
        </i:EventTrigger>
    </i:Interaction.Triggers>
</TextBox>

<TextBox x:Name="textBox2" Text="Text Box 2">
    <i:Interaction.Triggers>
        <i:EventTrigger  EventName="GotFocus">
            <i:InvokeCommandAction Command="{Binding NotifyFocusedReceivedCommand}" 
                                   CommandParameter="{Binding ., ElementName=textBox2}" />
        </i:EventTrigger>
    </i:Interaction.Triggers>
</TextBox>

and

public ICommand NotifyFocusedReceivedCommand { get; }

// in constructor
this.NotifyFocusedReceivedCommand = new ActionCommand(this.FocusedReceived);

// and method
private void FocusedReceived(object control)
{
    var txt = (TextBox)control;
    bool isFocused = txt.IsFocused;
    string name = txt.Name;
}