AvalonEdit: Add TextDocument constructor that takes ITextSource.

This allows creating a document from a RopeTextDocument or from another TextDocument while sharing unchanged parts of the document text.

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Document/TextDocument.cs

@@ -105,6 +105,31 @@ namespace ICSharpCode.AvalonEdit.Document
 			undoStack.AttachToDocument(this);
 			FireChangeEvents();
 		}
+		
+		/// <summary>
+		/// Create a new text document with the specified initial text.
+		/// </summary>
+		public TextDocument(ITextSource initialText)
+			: this(GetTextFromTextSource(initialText))
+		{
+		}
+		
+		// gets the text from a text source, directly retrieving the underlying rope where possible
+		static IEnumerable<char> GetTextFromTextSource(ITextSource textSource)
+		{
+			if (textSource == null)
+				throw new ArgumentNullException("textSource");
+			
+			RopeTextSource rts = textSource as RopeTextSource;
+			if (rts != null)
+				return rts.GetRope();
+			
+			TextDocument doc = textSource as TextDocument;
+			if (doc != null)
+				return doc.rope;
+			
+			return textSource.Text;
+		}
 		#endregion
 		
 		#region Text

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/TextEditor.cs

@@ -58,18 +58,16 @@ namespace ICSharpCode.AvalonEdit
 			
 			textArea.TextView.Services.AddService(typeof(TextEditor), this);
 			
-			SetCurrentPropertyValue(OptionsProperty, textArea.Options);
-			SetCurrentPropertyValue(DocumentProperty, new TextDocument());
+			SetCurrentValue(OptionsProperty, textArea.Options);
+			SetCurrentValue(DocumentProperty, new TextDocument());
 		}
 		
-		void SetCurrentPropertyValue(DependencyProperty property, object value)
+		#if !DOTNET4
+		void SetCurrentValue(DependencyProperty property, object value)
 		{
-			#if DOTNET4
-			SetCurrentValue(property, value);
-			#else
 			SetValue(property, value);
-			#endif
 		}
+		#endif
 		#endregion
 		
 		/// <inheritdoc/>
@@ -440,7 +438,7 @@ namespace ICSharpCode.AvalonEdit
 			if (e.PropertyName == "IsOriginalFile") {
 				TextDocument document = this.Document;
 				if (document != null) {
-					SetCurrentPropertyValue(IsModifiedProperty, Boxes.Box(!document.UndoStack.IsOriginalFile));
+					SetCurrentValue(IsModifiedProperty, Boxes.Box(!document.UndoStack.IsOriginalFile));
 				}
 				return true;
 			} else {