With FormattedText class, the formatted text can be first created and its size evaluated, so you know the space it takes in a first step,
If it's too long, it's up to you to split in separate lines.
Then in a second step, it could be drawn.
Everything could happen on the DrawingContext object in the following method :
protected override void OnRender(System.Windows.Media.DrawingContext dc)
Here is the CustomControl solution :
[ContentProperty("Text")]
public class TextBlockLineSplitter : FrameworkElement
{
public FontWeight FontWeight
{
get { return (FontWeight)GetValue(FontWeightProperty); }
set { SetValue(FontWeightProperty, value); }
}
public static readonly DependencyProperty FontWeightProperty =
DependencyProperty.Register("FontWeight", typeof(FontWeight), typeof(TextBlockLineSplitter), new PropertyMetadata(FontWeight.FromOpenTypeWeight(400)));
public double FontSize
{
get { return (double)GetValue(FontSizeProperty); }
set { SetValue(FontSizeProperty, value); }
}
public static readonly DependencyProperty FontSizeProperty =
DependencyProperty.Register("FontSize", typeof(double), typeof(TextBlockLineSplitter), new PropertyMetadata(10.0));
public String FontFamily
{
get { return (String)GetValue(FontFamilyProperty); }
set { SetValue(FontFamilyProperty, value); }
}
public static readonly DependencyProperty FontFamilyProperty =
DependencyProperty.Register("FontFamily", typeof(String), typeof(TextBlockLineSplitter), new PropertyMetadata("Arial"));
public String Text
{
get { return (String)GetValue(TextProperty); }
set { SetValue(TextProperty, value); }
}
public static readonly DependencyProperty TextProperty =
DependencyProperty.Register("Text", typeof(String), typeof(TextBlockLineSplitter), new PropertyMetadata(null));
public double Interline
{
get { return (double)GetValue(InterlineProperty); }
set { SetValue(InterlineProperty, value); }
}
public static readonly DependencyProperty InterlineProperty =
DependencyProperty.Register("Interline", typeof(double), typeof(TextBlockLineSplitter), new PropertyMetadata(3.0));
public List<String> Lines
{
get { return (List<String>)GetValue(LinesProperty); }
set { SetValue(LinesProperty, value); }
}
public static readonly DependencyProperty LinesProperty =
DependencyProperty.Register("Lines", typeof(List<String>), typeof(TextBlockLineSplitter), new PropertyMetadata(new List<String>()));
protected override void OnRender(DrawingContext drawingContext)
{
base.OnRender(drawingContext);
Lines.Clear();
if (!String.IsNullOrWhiteSpace(Text))
{
string remainingText = Text;
string textToDisplay = Text;
double availableWidth = ActualWidth;
Point drawingPoint = new Point();
// put clip for preventing writing out the textblock
drawingContext.PushClip(new RectangleGeometry(new Rect(new Point(0, 0), new Point(ActualWidth, ActualHeight))));
FormattedText formattedText = null;
// have an initial guess :
formattedText = new FormattedText(textToDisplay,
Thread.CurrentThread.CurrentUICulture,
FlowDirection.LeftToRight,
new Typeface(FontFamily),
FontSize,
Brushes.Black);
double estimatedNumberOfCharInLines = textToDisplay.Length * availableWidth / formattedText.Width;
while (!String.IsNullOrEmpty(remainingText))
{
// Add 15%
double currentEstimatedNumberOfCharInLines = Math.Min(remainingText.Length, estimatedNumberOfCharInLines * 1.15);
do
{
textToDisplay = remainingText.Substring(0, (int)(currentEstimatedNumberOfCharInLines));
formattedText = new FormattedText(textToDisplay,
Thread.CurrentThread.CurrentUICulture,
FlowDirection.LeftToRight,
new Typeface(FontFamily),
FontSize,
Brushes.Black);
currentEstimatedNumberOfCharInLines -= 1;
} while (formattedText.Width > availableWidth);
Lines.Add(textToDisplay);
System.Diagnostics.Debug.WriteLine(textToDisplay);
System.Diagnostics.Debug.WriteLine(remainingText.Length);
drawingContext.DrawText(formattedText, drawingPoint);
if (remainingText.Length > textToDisplay.Length)
remainingText = remainingText.Substring(textToDisplay.Length);
else
remainingText = String.Empty;
drawingPoint.Y += formattedText.Height + Interline;
}
foreach (var line in Lines)
{
System.Diagnostics.Debug.WriteLine(line);
}
}
}
}
Usage of that control (border is here to show effective clipping) :
<Border BorderThickness="1" BorderBrush="Red" Height="200" VerticalAlignment="Top">
<local:TextBlockLineSplitter>Alice was beginning to get very tired of sitting by her sister on the bank, and of having nothing to do. Once or twice she had peeped into the book her sister was reading, but it had no pictures or conversations in it, "and what is the use of a book," thought Alice, ...</local:TextBlockLineSplitter>
</Border>
