Set maximum scale of ViewBox - wpf

Question

Is it somehow possible to set a maximum scale of the content in a ViewBox? If I have this code for instance, I want to ensure that the text is not scaled more than 200%
<Grid>
<Viewbox>
<TextBox Text="Hello world" />
</Viewbox>
</Grid>

Answer 1

For future readers, I've written an even more elegant approach based on user2250152's answer that uses an attached property:
public static class ViewboxExtensions
{
public static readonly DependencyProperty MaxZoomFactorProperty =
DependencyProperty.Register("MaxZoomFactor", typeof(double), typeof(ViewboxExtensions), new PropertyMetadata(1.0, OnMaxZoomFactorChanged));
private static void OnMaxZoomFactorChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
var viewbox = d as Viewbox;
if (viewbox == null)
return;
viewbox.Loaded += OnLoaded;
}
private static void OnLoaded(object sender, RoutedEventArgs e)
{
var viewbox = sender as Viewbox;
var child = viewbox?.Child as FrameworkElement;
if (child == null)
return;
child.SizeChanged += (o, args) => CalculateMaxSize(viewbox);
CalculateMaxSize(viewbox);
}
private static void CalculateMaxSize(Viewbox viewbox)
{
var child = viewbox.Child as FrameworkElement;
if (child == null)
return;
viewbox.MaxWidth = child.ActualWidth * GetMaxZoomFactor(viewbox);
viewbox.MaxHeight = child.ActualHeight * GetMaxZoomFactor(viewbox);
}
public static void SetMaxZoomFactor(DependencyObject d, double value)
{
d.SetValue(MaxZoomFactorProperty, value);
}
public static double GetMaxZoomFactor(DependencyObject d)
{
return (double)d.GetValue(MaxZoomFactorProperty);
}
}
The attached property can be added to a viewbox like this:
<Viewbox extensions:ViewboxExtensions.MaxZoomFactor="2.0">...</Viewbox>

Answer 2

You need to set MaxWidth and MaxHeight on ViewBox:
<Grid>
<Viewbox x:Name="MyViewBox">
<TextBox x:Name="MyTextBox" Text="Hello world" />
</Viewbox>
</Grid>
and in code behind:
MyViewBox.MaxWidth = MyTextBox.ActualWidth * 2d;
MyViewBox.MaxHeight = MyTextBox.ActualHeight * 2d;
Or maybe better solution with converter:
public class ViewBoxMaxWidthOrHeightConverter : IValueConverter
{
public object Convert(object value, System.Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
double textBoxWidthOrHeight = (double) value;
return textBoxWidthOrHeight*2d;
}
public object ConvertBack(object value, System.Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
throw new System.NotImplementedException();
}
}
and XAML:
<local:ViewBoxMaxWidthOrHeightConverter x:Key="ViewBoxMaxWidthOrHeightConverter"/>
<Grid>
<Viewbox x:Name="MyViewBox"
MaxWidth="{Binding ElementName=MyTextBox, Path=ActualWidth, Converter={StaticResource ViewBoxMaxWidthOrHeightConverter}, Mode=OneWay}"
MaxHeight="{Binding ElementName=MyTextBox, Path=ActualHeight, Converter={StaticResource ViewBoxMaxWidthOrHeightConverter}, Mode=OneWay}">
<TextBox x:Name="MyTextBox" Text="Hello world" />
</Viewbox>
</Grid>