How do I make a WPF TextBlock show my text on multiple lines?

i have a wpf window where i have a stackpanel with two viewports in it - each Viewport with a textblock in it.

<Grid>
    <StackPanel VerticalAlignment="Center" Orientation="Vertical" >
        <Viewbox Margin="100,0,100,0">
            <TextBlock x:Name="headerText" Text="Lorem ipsum dolor" Foreground="Black"/>
        </Viewbox>
        <Viewbox Margin="150,0,150,0">
            <TextBlock x:Name="subHeaderText" Text="Lorem ipsum dolor, Lorem ipsum dolor, lorem isum dolor, Lorem ipsum dolor, Lorem ipsum dolor, lorem isum dolor, " TextWrapping="Wrap" Foreground="Gray" />
        </Viewbox>
    </StackPanel>
</Grid>

What i would like to achieve is that the top textblock is the heading with a bigger text. The second textblock is the sub heading with a smaller text. No matter how much text there is for either the heading or the subheading the font should dynamic become smaller/bigger. My problem is that i would like the subheading to be fixed width. This means that, the font should be a percentage (70%) of the heading and wrap to multiple lines, depending on how much text i have. I enclosed the code I have thus far... im missing something with that subheading, cant figure out what. Cheers

Edit Basically what i want achieve is that the sub header wraps the text so it can expand it downwards with the font being a 70% of the heading - no matter how big that font is.

Nesting a stackpanel will cause the textbox to wrap properly:

<Viewbox Margin="120,0,120,0">
    <StackPanel Orientation="Vertical" Width="400">
        <TextBlock x:Name="subHeaderText" 
                   FontSize="20" 
                   TextWrapping="Wrap" 
                   Foreground="Black"
                   Text="Lorem ipsum dolor, lorem isum dolor,Lorem ipsum dolor sit amet, lorem ipsum dolor sit amet " />
    </StackPanel>
</Viewbox>

Use the property TextWrapping of the TextBlock element:

<TextBlock Text="StackOverflow Forum"
           Width="100"
           TextWrapping="WrapWithOverflow"/>

Use Linebreak:

<TextBlock>
        <Run Text="Line1"/>
        <LineBreak/>
        <Run Text="Line2" FontStyle="Italic" FontSize="9"/>
        <LineBreak/>
        <Run Text="Line3"/>
    </TextBlock>

Refer this: https://social.msdn.microsoft.com/Forums/vstudio/en-US/51a3ffe4-ec82-404a-9a99-6672f2a6842b/how-to-give-multiline-in-textblock?forum=wpf

Thanks,

RDV

This gets part way there. There is no ActualFontSize property but there is an ActualHeight and that would relate to the FontSize. Right now this only sizes for the original render. I could not figure out how to register the Converter as resize event. Actually maybe need to register the FontSize as a resize event. Please don't mark me down for an incomplete answer. I could not put code sample in a comment.

    <Window.Resources>
        <local:WidthConverter x:Key="widthConverter"/>
    </Window.Resources>
    <Grid>
        <Grid>
            <StackPanel VerticalAlignment="Center" Orientation="Vertical" >
                <Viewbox Margin="100,0,100,0">
                    <TextBlock x:Name="headerText" Text="Lorem ipsum dolor" Foreground="Black"/>
                </Viewbox>
                <TextBlock Margin="150,0,150,0" FontSize="{Binding ElementName=headerText, Path=ActualHeight, Converter={StaticResource widthConverter}}" x:Name="subHeaderText" Text="Lorem ipsum dolor, Lorem ipsum dolor, lorem isum dolor, Lorem ipsum dolor, Lorem ipsum dolor, lorem isum dolor, " TextWrapping="Wrap" Foreground="Gray" />
            </StackPanel>
        </Grid>
    </Grid>        

Converter

    [ValueConversion(typeof(double), typeof(double))]
    public class WidthConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            double width = (double)value*.7;
            return width; // columnsCount;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    } 

If you just want to have your header font a little bit bigger then the rest, you can use ScaleTransform. so you do not depend on the real fontsize.

 <TextBlock x:Name="headerText" Text="Lorem ipsum dolor">
                <TextBlock.LayoutTransform>
                    <ScaleTransform ScaleX="1.1" ScaleY="1.1" />
                </TextBlock.LayoutTransform>
  </TextBlock>