Control Focus in dialog box with Prism 7.x and 8.x? - wpf

Question

I need to set a TextBox being focused when the dialog box was opened. It was working with Prism 6.x; but is no longer working since 7.x and 8.x. The framework difference is that Prism 6.x uses InteractionRequest for dialog box; while Prism 7/8 uses the dialog service. Does Prism 7 and 8 also introduce some new approaches related to dialog box? Following is the code snippet of XAML setting FocusManager:
<UserControl x:Class="FeatureModule.Views.MyDialogView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:prism="http://prismlibrary.com/"
prism:ViewModelLocator.AutoWireViewModel="True"
FocusManager.FocusedElement="{Binding ElementName=edit}"
mc:Ignorable="d" d:DesignHeight="140" d:DesignWidth="400">
It does not function any more. Does it related to the dialog service?

Answer 1

The dialog service starting from Prism 7.2.x is completely different from the legacy interaction requests in prior versions, so it is hard to tell what exactly was the breaking change. Setting the focus manager property on the root element of the UserControl instead of on the UserControl could solve this the issue, but it might not work under other circumstances, read below why.
<UserControl x:Class="FeatureModule.Views.MyDialogView" ...>
<Grid FocusManager.FocusedElement="{Binding ElementName=edit}">
<!-- ...your markup -->
</Grid>
</UserControl>
I think the issue is more general. WPF has two different focus concepts, logical and keyboard focus. What you want to set on the TextBox is keyboard focus, but the FocusManager only sets logical focus.
Logical focus pertains to the FocusManager.FocusedElement within a specific focus scope.
The important part is the relationship between logical and keyboard focus.
An element with logical focus does not necessarily have keyboard focus, but an element with keyboard focus will have logical focus.
This means, even if you set the logical focus on a different element it might not work.
Keyboard focus pertains to the element that is currently receiving keyboard input. There can be only one element with keyboard focus.
Unfortunately, the Keyboard.FocusedElement property has only a getter and is not a dependency property, so you cannot bind to it, but you can call the Keyboard.Focus method instead. In order to avoid code-behind, you can either create a small behavior or simply an attached property like this:
public static class KeyboardFocus
{
public static readonly DependencyProperty ElementProperty = DependencyProperty.RegisterAttached(
"Element", typeof(FrameworkElement), typeof(KeyboardFocus), new PropertyMetadata(null, OnElementChanged));
public static FrameworkElement GetElement(DependencyObject dependencyObject)
{
return (FrameworkElement)dependencyObject.GetValue(ElementProperty);
}
public static void SetElement(DependencyObject dependencyObject, FrameworkElement value)
{
dependencyObject.SetValue(ElementProperty, value);
}
private static void OnElementChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
if (e.NewValue is FrameworkElement frameworkElement)
frameworkElement.Loaded += OnLoaded;
}
private static void OnLoaded(object sender, RoutedEventArgs e)
{
var frameworkElement = (FrameworkElement)sender;
frameworkElement.Loaded -= OnLoaded;
Keyboard.Focus(frameworkElement);
}
}
This attached property can be used on any FrameworkElement, not only TextBox. You can customize it to fit your needs. In XAML apply it to the root element of your UserControl, not the UserControl itself, otherwise the binding element might not be found e.g.:
<UserControl x:Class="FeatureModule.Views.MyDialogView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:prism="http://prismlibrary.com/"
xmlns:local="clr-namespace:FeatureModule.Views"
prism:ViewModelLocator.AutoWireViewModel="True"
FocusManager.FocusedElement="{Binding ElementName=edit}"
mc:Ignorable="d" d:DesignHeight="140" d:DesignWidth="400">
<Grid local:KeyboardFocus.Element="{Binding ElementName=edit}">
<!-- ...your markup. -->
</Grid>
</UserControl>
Alternatively, you can set the property on the TextBox itself.
<TextBox local:KeyboardFocus.Element="{Binding RelativeSource={RelativeSource Self}}"/>