Binding from multi object in to one value in XAML - xaml

Question

I have a ListView which have a DataTemplate like this:
<DataTemplate x:Key="FilterDataTemplate">
<StackPanel Orientation="Horizontal" Background="White" Height="50" HorizontalAlignment="Left">
<ComboBox x:Name="TermsItemComboBox" Width="160"
ItemsSource="{Binding ItemList}" Tag="{Binding}"
DisplayMemberPath="Key" SelectionChanged="AddTermsItem_SelectionChanged"
Background="#FFD1D1D1" SelectedItem="{Binding SelectedItem, Mode=TwoWay}" />
<TextBox x:Name="TermsInputTxt1" Text="{Binding FilterText, Mode=TwoWay}"
BorderBrush="#FF727272" Background="#FFD1D1D1" HorizontalAlignment="Left"
Width="200" FontSize="16" VerticalAlignment="Center"
Visibility="{Binding IsTxtForm, Converter={StaticResource BooleanToVisibilityConverter}}"
Height="45"/>
<TextBox x:Name="TermsInputTxt2" Text="{Binding FilterText, Mode=TwoWay}"
BorderBrush="#FF727272" Background="#FFD1D1D1" HorizontalAlignment="Left"
Width="200" FontSize="16" VerticalAlignment="Center"
Visibility="{Binding IsTxtForm, Converter={StaticResource BooleanToVisibilityConverter}}"
Height="45"/>
<Button x:Name="TrashBtn" HorizontalAlignment="Left" VerticalAlignment="Stretch"
BorderBrush="#FF575757" Foreground="#FF494949" BorderThickness="2" Tapped="TrashBtn_Tapped"
Style="{StaticResource DiscardAppBarButtonStyle}" Height="45" Width="55"
RenderTransformOrigin="0.5,0.5">
<Button.RenderTransform>
<CompositeTransform ScaleX="-1" />
</Button.RenderTransform>
</Button>
</StackPanel>
It's have a combobox and two TextBox. At default, the combobox don's show anything, just a blank item, I want when user type some text to the TermsInputTxt1 or TermsInputTxt2, the combobox will select the first item.
I try to bind the selectedindex of combobox with the lenght of those two textbox with a converter but look like XAML doesn't support multi binding like that.
Any suggest? Thank you!

Answer 1

So, this is a bit of a difficult situation. You have a couple of ways of going about it:
Bind the TermsInputTxt1 and TermsInputTxt2 to the ViewModel, with the Setter on the ViewModel controlling an ObservableCollection, adding and removing items as they get set in the property.
Create a Model for each entry with a Value field (string) and bind the Terms... to accessors in the ViewModel. Then, the ComboBox items will automatically change.
I suggest doing the first one. You'll have to have a SelectedItem/SelectedIndex property in the ViewModel which the ComboBox will bind to. You may also want to change the method by which the binding on the TextBoxs are updated, either so that they are updated immediately (lots of processing) or after it loses focus (less processing, but won't update the UI immediately).