How to bind a form in a grid that is bound to a tree which is bound to a different collection in wpf

Question

Ok so this is kinda hard to explain. I have a view which has a tree and has a form. The tree is bound to 2 different observable collections.

<TreeView FontFamily="Calibri" FontSize="16" Name="tree" Margin="3" Grid.Row="1" Grid.Column="0"  Height="950" VerticalAlignment="Top" Grid.ColumnSpan="2"

              ItemsSource="{Binding Path=Org, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" SelectedItemChanged="tree_SelectedItemChanged"  >             

    <TreeView.ItemTemplate>
            <HierarchicalDataTemplate ItemsSource="{Binding Path=Org_Sredstva}">
                <TextBlock Text="{Binding Path=Organizacije_Naziv}"/>
                <HierarchicalDataTemplate.ItemTemplate>
                    <DataTemplate>
                        <TextBlock Text="{Binding Path=Ossr_Naziv}"/>
                    </DataTemplate>
                </HierarchicalDataTemplate.ItemTemplate>
            </HierarchicalDataTemplate>
        </TreeView.ItemTemplate>
    </TreeView>

Now then next to the tree there is a form. And the form is supposed to be bound to a third observable collection that changes depending on selecteditem of tree.

<Grid Grid.Row="2" Grid.Column="2" Margin="5" DataContext="{Binding ElementName=tree, Path=SelectedItem}">

As you can see the binding is to the tree which is bound to a different observable collection.

 <extToolkit:WatermarkTextBox x:Name="RadnoVrijeme" Grid.Row="1"
                     Grid.Column="1" 
                     Grid.ColumnSpan="2"
                     HorizontalAlignment="Left"
                     VerticalAlignment="Bottom"
                       Width="100"
                     Height="25"
                     Padding="3" 
                     Margin="3"
                     AcceptsReturn="True" 
                     Validation.ErrorTemplate="{StaticResource ValidationTemplate}"
                     Validation.Error="Validation_Error">
                    <extToolkit:WatermarkTextBox.Watermark>
                        <StackPanel Orientation="Horizontal">
                            <TextBox Text="Radno Vrijeme" Margin="4,2,2,0" FontWeight="Regular" Foreground="Silver" BorderThickness="0"/>
                        </StackPanel>
                    </extToolkit:WatermarkTextBox.Watermark>
                    <extToolkit:WatermarkTextBox.Text>
                        <Binding Path="Opr_RadnoVrijeme" UpdateSourceTrigger="PropertyChanged" NotifyOnValidationError="True">
                            <Binding.ValidationRules>
                                <validation:InputLenghtRule MinLength="10" MaxLength="250" ValidatesOnTargetUpdated="True"/>
                            </Binding.ValidationRules>
                        </Binding>
                    </extToolkit:WatermarkTextBox.Text>
                </extToolkit:WatermarkTextBox>

This is a textbox and it is bound to a property on the third collection but it doesn't work obviously because that collection is never set as itemssource anywhere.

The workaround I found is to in codebehind set every textbox individually so

this.View.RadnoVrijeme.Text = opr.Opr_RadnoVrijeme;

Where the left side of the equation is the text property of the textbox and the right side is the collection.property. This works but I don't like it and was hoping there was a way to actually bind this?

Answer 1

You should definetely have a look at MVVM. It utilize modular design, where your View is consisting of multiple Views controlled by ViewModels. Then it's easy to solve any kind of dependency (relation? selection?) problem.

In your case you are trying to bind to SelectedItem directly, while in MVVM you will handle selection and then you have possibility to rise notification of related property change, which will cause update of corresponding View element automatically.

Here is a generic example:

ItemVM _selectedItem;
public ItemVM SelectedItem
{
    get { return _selectedItem; }
    set
    {
        _selectedItem = value;
        OnPropertyChanged();
        // trigger update in the view
        OnPropertyChanged(nameof(SomeRelatedProperty));
    }
}

// bind TextBlock.Text to that property
public string SomeRelatedProperty
{
    get
    {
        // some logic based on selected item
        if(SelectedItem.SomeProperty == SomeValue)
            return "a";
        ...
        return "b";
    }
}

The cooler thing if SelectedItem already provides value as one of property (because it's a ViewModel), then assuming all ViewModels implement INotifyPropertyChanged you can simply bind to that property directly and whenever selected item is changed the view will be updated to display that property too:

<TextBlock Text="{Binding SelectedItem.SomeProperty} ... / >

---

As for error you are getting:

System.Windows.Data Error: 40 : BindingExpression path error: 'Opr_RadnoVrijeme' property not found on 'object' ''Organizacije' (HashCode=23451234)'. BindingExpression:Path=Opr_RadnoVrijeme; DataItem='Organizacije' (HashCode=23451234); target element is 'WatermarkTextBox' (Name='RadnoVrijeme'); target property is 'Text' (type 'String')

Try binding (pretty unsure, you didn't provide ViewModels to figure that out):

<WatermarkTextBox Text="{Binding SelectedItem.Opr_RadnoVrijeme, RelativeSource={RelativeSource FindAncestor, AncestorType=TreeView}}" ... />