Why ActualWidth is always 0 after DrawingContext drew?

Recently I have been trying to rewrite TextBlock for letterspacing. Now I almost achieved this by the DrawingContext.

Here is my code:

  public class STextBlock : Control
{
public string Text
{
get { return (string)GetValue(TextProperty); }
set { SetValue(TextProperty, value); }
}
// Using a DependencyProperty as the backing store for Text.  This enables animation, styling, binding, etc...
public static readonly DependencyProperty TextProperty =
DependencyProperty.Register("Text", typeof(string), typeof(STextBlock), new FrameworkPropertyMetadata("", FrameworkPropertyMetadataOptions.AffectsRender));
public double LetterSpacing
{
get { return (double)GetValue(LetterSpacingProperty); }
set { SetValue(LetterSpacingProperty, value); }
}
// Using a DependencyProperty as the backing store for LetterSpacing.  This enables animation, styling, binding, etc...
public static readonly DependencyProperty LetterSpacingProperty =
DependencyProperty.Register("LetterSpacing", typeof(double), typeof(STextBlock), new FrameworkPropertyMetadata(0.0, FrameworkPropertyMetadataOptions.AffectsRender));
public FontStyle FontStyle
{
get { return (FontStyle)GetValue(FontStyleProperty); }
set { SetValue(FontStyleProperty, value); }
}
// Using a DependencyProperty as the backing store for FontStyle.  This enables animation, styling, binding, etc...
public static readonly DependencyProperty FontStyleProperty =
DependencyProperty.Register("FontStyle", typeof(FontStyle), typeof(STextBlock), new FrameworkPropertyMetadata(FontStyles.Normal, FrameworkPropertyMetadataOptions.AffectsRender));
public Brush Foreground
{
get { return (Brush)GetValue(ForegroundProperty); }
set { SetValue(ForegroundProperty, value); }
}
// Using a DependencyProperty as the backing store for Foreground.  This enables animation, styling, binding, etc...
public static readonly DependencyProperty ForegroundProperty =
DependencyProperty.Register("Foreground", typeof(Brush), typeof(STextBlock), new FrameworkPropertyMetadata(new SolidColorBrush(Colors.Black), FrameworkPropertyMetadataOptions.AffectsRender));
public FontStretch FontStretch
{
get { return (FontStretch)GetValue(FontStretchProperty); }
set { SetValue(FontStretchProperty, value); }
}
// Using a DependencyProperty as the backing store for FontStretch.  This enables animation, styling, binding, etc...
public static readonly DependencyProperty FontStretchProperty =
DependencyProperty.Register("FontStretch", typeof(FontStretch), typeof(STextBlock), new FrameworkPropertyMetadata(FontStretches.Normal, FrameworkPropertyMetadataOptions.AffectsRender));
public FontFamily FontFamily
{
get { return (FontFamily)GetValue(FontFamilyProperty); }
set { SetValue(FontFamilyProperty, value); }
}
// Using a DependencyProperty as the backing store for FontFamily.  This enables animation, styling, binding, etc...
public static readonly DependencyProperty FontFamilyProperty =
DependencyProperty.Register("FontFamily", typeof(FontFamily), typeof(STextBlock), new FrameworkPropertyMetadata(new FontFamily(), FrameworkPropertyMetadataOptions.AffectsRender));
public double FontSize
{
get { return (double)GetValue(FontSizeProperty); }
set { SetValue(FontSizeProperty, value); }
}
// Using a DependencyProperty as the backing store for FontSize.  This enables animation, styling, binding, etc...
public static readonly DependencyProperty FontSizeProperty =
DependencyProperty.Register("FontSize", typeof(double), typeof(STextBlock), new FrameworkPropertyMetadata(12.0, FrameworkPropertyMetadataOptions.AffectsRender));
public FontWeight FontWeight
{
get { return (FontWeight)GetValue(FontWeightProperty); }
set { SetValue(FontWeightProperty, value); }
}
// Using a DependencyProperty as the backing store for FontWeight.  This enables animation, styling, binding, etc...
public static readonly DependencyProperty FontWeightProperty =
DependencyProperty.Register("FontWeight", typeof(FontWeight), typeof(STextBlock), new FrameworkPropertyMetadata(FontWeights.Normal, FrameworkPropertyMetadataOptions.AffectsRender));
protected override void OnRender(DrawingContext drawingContext)
{
base.OnRender(drawingContext);
List<FormattedTextToDrawModel> FormattedTextToDrawList = new List<FormattedTextToDrawModel>();
double PositionX = Padding.Left;
double PositionY = Padding.Top;            
if (!string.IsNullOrEmpty(Text))
{                            
foreach (var i in Text)
{
var FT = new FormattedText(i.ToString(), CultureInfo.CurrentUICulture, FlowDirection.LeftToRight, new Typeface(FontFamily, FontStyle, FontWeight, FontStretch), FontSize, Foreground);
FormattedTextToDrawList.Add(new FormattedTextToDrawModel() { FormattedText = FT, DrawPoint = new Point(PositionX, PositionY) });
PositionX += FT.Width + LetterSpacing;
if (PositionX > ActualSize.Width)
{
ActualSize.Width = PositionX;
}
if (PositionY + FT.Height > ActualSize.Height)
{
ActualSize.Height = PositionY + FT.Height;
}
}
}
ActualSize.Width += Padding.Right;
ActualSize.Height += Padding.Top;
drawingContext.DrawRectangle(Background, new Pen(), new Rect(ActualSize));
foreach (var i in FormattedTextToDrawList)
{
drawingContext.DrawText(i.FormattedText, i.DrawPoint);
}
}
public struct FormattedTextToDrawModel
{
public FormattedText FormattedText { get;init;}
public Point DrawPoint { get; init; }
}
Size ActualSize = new Size(0, 0);             
}

I test the code above like this:

<Border VerticalAlignment="Center" HorizontalAlignment="Center">
<Test:STextBlock Margin="10" x:Name="TB" Text="123123" Padding="20" LetterSpacing="4" FontFamily="Comic Sans MS" FontSize="20" Background="Red"></Seal:STextBlock>
</Border>

Now I have a problem. After the DrawingContext drew the background and text. The ActualWidth and ActualHeight not only of the STextBlock self but also of its parent is still being 0. What's wrong with it?

Is there any way I can set the ActualWidth and ActualHeight by myself?

Actual width and height are calculated internally as part of the measure arrange layout pass.

Each control being laid out is asked for it's desired size.

How much space would it like?

That's then compared to what it can have from parent containers etc and it may not get what it desires.

As an example.

A textblock which cannot have the width it would like will have it's text truncated or it will overflow onto the next line.

FrameworkElement.ActualWidth is read only

Note only {get;}

https://learn.microsoft.com/en-us/dotnet/api/system.windows.frameworkelement.actualwidth?view=windowsdesktop-7.0

You don't set that directly yourself.

You should instead override measure. I think you've sort of done those calculations.

protected override Size MeasureOverride(Size constraint)
{
Size desiredSize = new Size();
// Calculate how big your text wants to be
return desiredSize;
}