Question

Time for my first question 🙂

I have the following:

public class BuilderViewModel : INotifyPropertyChanged
{
    #region Implementation of INotifyPropertyChanged

    public event PropertyChangedEventHandler PropertyChanged;

    #endregion

    private double _contentScale = 1.0;

    public double ContentScale
    {
        get { return _contentScale; }
        set
        {
            _contentScale = value;
            NotifyPropertyChanged("ContentScale");
        }
    }

    public void NotifyPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    } 

    #region Commands

    bool CanZoomIn() { return true; }
    void ZoomInExecute()
    {
        ContentScale += 1.0;
    }

    public ICommand ZoomIn { get { return new RelayCommand(ZoomInExecute, CanZoomIn); } }

    #endregion
}

And the corresponding view:

<UserControl x:Class="PS_IDE.FormBuilder.View.Builder"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:local="clr-namespace:PS_IDE.FormBuilder.ViewModel">
    <UserControl.DataContext>
        <local:BuilderViewModel />
    </UserControl.DataContext>

    <TextBox Text="{Binding ContentScale}" Width="100" />

</UserControl>

I’m trying to have the ZoomIn command in BuilderViewModel update the text box value in it’s view. The command is being fired from another user control, UIBuilder, which includes Builder. If I debug and fire the command from UIBuilder, I can see it updating ContentScale properly.

However, my text box value does not get updated (it only says “1”, which is the initial value of ContentScale).

I know I’m missing something and hope someone can point me in the right direction.

EDIT: Added the control that is firing the command

<UserControl x:Class="PS_IDE.FormBuilder.UIBuilder"
         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:local="clr-namespace:PS_IDE.FormBuilder"
         xmlns:ViewModel="clr-namespace:PS_IDE.FormBuilder.ViewModel"
         xmlns:View="clr-namespace:PS_IDE.FormBuilder.View" mc:Ignorable="d">  
    <UserControl.DataContext>
        <ViewModel:BuilderViewModel />
    </UserControl.DataContext>
    <DockPanel LastChildFill="True">

        ....

        <ToolBarTray DockPanel.Dock="Bottom" HorizontalAlignment="Right">
            <ToolBar>
                <Button Height="24" Width="24" ToolTip="Zoom In" Command="{Binding ZoomIn}">
                    <Image Source="Images/ZoomIn.png" Height="16"/>
                </Button>

                ....

            </ToolBar>
       </ToolBarTray>
       <View:Builder x:Name="builder" />
    </DockPanel>
</UserControl>

Answer 1

With the setting in both view:

<UserControl.DataContext>
    <local:BuilderViewModel />
</UserControl.DataContext>

you are basically creating two viewmodels, one for each view. So when your Command updates the property it does it on one of the viewmodel but your textbox is bound to a different viewmodel.

To resolve it remove the DataContext setting from the Builder.xaml

Additionally you need to pass your DataContext to your Builder control (with this both view will share the same viewmodel).

So modify your UIBuilder.xaml:

<View:Builder x:Name="builder" DataContext="{Binding}" />