Why would using Caliburn Action inside a Blend KeyTrigger break my View? - wpf

Question

Caliburn Micro 2.0.2 and WPF 4 and .NET 4.5.
Using a very simple view and a very simple viewmodel to demonstrate, I notice that if I have a Caliburn Action inside a KeyTrigger, my View is not activated properly. When I say 'properly' I mean that if I look at it in Snoop, the whole UserControl is disabled. If I change it to a Window and open it as Window rather than an embedded control, Snoop cannot even find that Window and the control remains disabled.
Here is my ViewModel:
using System.Windows.Input;
using Caliburn.Micro;
namespace PlainSailTech.ViewModels.Admin
{
public class SystemConfigurationViewModel : Screen
{
private bool canShowDevLinks;
public bool CanShowDevLinks
{
get { return canShowDevLinks; }
set
{
if (value.Equals(canShowDevLinks)) return;
canShowDevLinks = value;
}
}
public void ShowDevLinks(KeyEventArgs args)
{
if (args != null)
args.Handled = true;
CanShowDevLinks = !CanShowDevLinks;
}
}
}
Here is my XAML:
<Window x:Class="PlainSailTech.Bookkeeping.Ui.Views.Admin.SystemConfigurationView"
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:cal="http://www.caliburnproject.org"
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
xmlns:ei="http://schemas.microsoft.com/expression/2010/interactions"
mc:Ignorable="d"
Title="Caliburn Action test"
d:DesignHeight="300" d:DesignWidth="300">
<i:Interaction.Triggers>
<ei:KeyTrigger Modifiers="Ctrl" Key="D" FiredOn="KeyUp" ActiveOnFocus="True">
<cal:ActionMessage MethodName="ShowDevLinks">
<cal:Parameter Value="$eventArgs" />
</cal:ActionMessage>
</ei:KeyTrigger>
</i:Interaction.Triggers>
<Grid>
<Grid Width="Auto">
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<Grid Grid.Row="0" Background="#FFFBFBFB" Visibility="{Binding CanShowDevLinks, Converter={StaticResource BoolToVisible}}">
<TextBlock Text="Dev links here" />
</Grid>
<Grid Grid.Row="1">
<TextBox VerticalAlignment="Top" IsReadOnly="False" IsEnabled="True" Width="150" Height="30">
<TextBox.Style>
<Style TargetType="TextBox">
<Setter Property="Background" Value="Blue"/>
<Setter Property="Foreground" Value="White"/>
<Setter Property="Text" Value="Enabled"/>
<Style.Triggers>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Background" Value="Red"/>
<Setter Property="Foreground" Value="Black"/>
<Setter Property="Text" Value="Disabled"/>
</Trigger>
</Style.Triggers>
</Style>
</TextBox.Style>
</TextBox>
</Grid>
</Grid>
</Grid>
</Window>
Notice that the TextBox is set to enabled but the whole control is being disabled. You can see this from this screen shot:
However, if I simply remove the Caliburn Action, everything is good again:
<i:Interaction.Triggers>
<ei:KeyTrigger Modifiers="Ctrl" Key="D" FiredOn="KeyUp" ActiveOnFocus="True">
</ei:KeyTrigger>
</i:Interaction.Triggers>
A few possible clues. The problem goes away if:
If I replace the Caliburn Action with:
<ei:ChangePropertyAction TargetObject="{Binding Mode=OneWay}" PropertyName="CanShowDevLinks" Value="True">
Or my ViewModel does not inherit from Screen or PropertyChangedBase.
If I override OnViewAttached and wire up a handler for EnableChanged, I can see the IsEnabled being set to false after the View is attached.
So it seems like an interaction between whatever is happening in the Screen/PropertyChangedBase base class and the wiring up of the Action.

Answer 1

Okay - the answer to this question is the person who asked it is an idiot. I have a guard property that has the name of the action's method. Because the KeyTrigger is below the UserControl, it is disabling the whole thing. Arghhh...
I knew that a guard property applied when you named a button, for example, but I didn't realise that it applied to any UI element that has an action attached to it. You live and learn, occasionally.