Caliburn Micro Message.attach does not consider changes of datacontext - wpf

Question

This is a simplified example. I have a usercontrol that contains a "browse to folder" functionality, using a textbox and a button. Clicking the button would open up the browse-dialog, and would essentially fill in the textbox.
<UserControl x:Class="MyUserControl"
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"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="100"></ColumnDefinition>
<ColumnDefinition Width="*"></ColumnDefinition>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"></RowDefinition>
</Grid.RowDefinitions>
<!-- Folder -->
<TextBlock>Path</TextBlock>
<DockPanel LastChildFill="True" Grid.Column="1">
<Button DockPanel.Dock="Right" cal:Message.Attach="[Event Click] = [Action BrowseHotFolder()]" Content="..." HorizontalAlignment="Left" Width="25" Height="25" VerticalContentAlignment="Center" HorizontalContentAlignment="Center" Margin="0,0,5,0"/>
<TextBox Text="{Binding HotFolderPath, UpdateSourceTrigger=PropertyChanged, ValidatesOnNotifyDataErrors=True}" />
</DockPanel>
</Grid>
I have a listbox that contains a number of objects. The selected object will be fed into this usercontrol as datacontext.
<Window>
...
<Listbox ItemsSource="{Binding Items, Mode=OneWay}" SelectedItem="{Binding SelectedItem}">
...
<view:MyUserControl DataContext="{Binding SelectedItem}" />
</Window>
Now, let's say I have two items in my listbox and I have the first one selected. I fill in "foo" in the textbox of MyUserControl. Then I select the second item, and fill in "bar". The databinding works fine, and both items have the correct values set. If I then click the browse button on the first one and select a folder, it will change the textbox of the first item to the selected path. However, if I select the second item, and browse to a folder it will ALSO change the first item's textbox.
My guess is that the message attach syntax does not call the browse action on the correct Item. It disregards the datacontext (currently selected item) and just uses the first one.
What can I do about this?

Answer 1

I think your guess is correct; the target used for the Message.Attach is the first data context bound, and does not update when the context is changed after the user selection.
We saw a similar problem with user controls switched in a content control - the fix was to specify cal:Action.TargetWithoutContext="{Binding}" on the button.
The issue is mentioned here by Rob Eisenberg:
https://caliburnmicro.codeplex.com/discussions/257005

Answer 2

I have made a workaround by changing
cal:Message.Attach="[Event Click] = [Action BrowseHotFolder()]"
to
cal:Message.Attach="[Event Click] = [Action BrowseHotFolder($datacontext)]"
Now, the BrowseHotFolder-method still is called on the wrong ItemViewModel, but weirdly $datacontext passes the correct ItemViewModel. In the method itself I now do:
public void BrowsePath(ItemViewModel context)
{
context.Path = _folderBrowsingService.Browse();
}
This is a workaround, but solves the problem.