Add DrawLine event to TreeView for advanced rendering manipulation

Terminal.Gui/Views/TreeView/Branch.cs

@@ -65,11 +65,10 @@ public virtual void FetchChildren ()
 
 			if (Depth >= tree.MaxDepth) {
 				children = Enumerable.Empty<T> ();
-			}
-			else {
+			} else {
 				children = tree.TreeBuilder.GetChildren (this.Model) ?? Enumerable.Empty<T> ();
 			}
-			
+
 			this.ChildBranches = children.ToDictionary (k => k, val => new Branch<T> (tree, this, val));
 		}
 
@@ -95,6 +94,11 @@ public virtual int GetWidth (ConsoleDriver driver)
 		/// <param name="availableWidth"></param>
 		public virtual void Draw (ConsoleDriver driver, ColorScheme colorScheme, int y, int availableWidth)
 		{
+			var cells = new List<RuneCell> ();
+			int? indexOfExpandCollapseSymbol = null;
+			int indexOfModelText;
+
+
 			// true if the current line of the tree is the selected one and control has focus
 			bool isSelected = tree.IsSelected (Model);
 
@@ -110,15 +114,15 @@ public virtual void Draw (ConsoleDriver driver, ColorScheme colorScheme, int y,
 
 			// if we have scrolled to the right then bits of the prefix will have dispeared off the screen
 			int toSkip = tree.ScrollOffsetHorizontal;
+			var attr = symbolColor;
 
-			driver.SetAttribute (symbolColor);
 			// Draw the line prefix (all parallel lanes or whitespace and an expand/collapse/leaf symbol)
 			foreach (Rune r in prefix) {
 
 				if (toSkip > 0) {
 					toSkip--;
 				} else {
-					driver.AddRune (r);
+					cells.Add (NewRuneCell (attr, r));
 					availableWidth -= r.GetColumns ();
 				}
 			}
@@ -141,23 +145,31 @@ public virtual void Draw (ConsoleDriver driver, ColorScheme colorScheme, int y,
 					color = new Attribute (color.Background, color.Foreground);
 				}
 
-				driver.SetAttribute (color);
+				attr = color;
 			}
 
 			if (toSkip > 0) {
 				toSkip--;
 			} else {
-				driver.AddRune (expansion);
+				indexOfExpandCollapseSymbol = cells.Count;
+				cells.Add (NewRuneCell (attr, expansion));
 				availableWidth -= expansion.GetColumns ();
 			}
 
 			// horizontal scrolling has already skipped the prefix but now must also skip some of the line body
 			if (toSkip > 0) {
+
+				// For the event record a negative location for where model text starts since it
+				// is pushed off to the left because of scrolling
+				indexOfModelText = -toSkip;
+
 				if (toSkip > lineBody.Length) {
 					lineBody = "";
 				} else {
 					lineBody = lineBody.Substring (toSkip);
 				}
+			} else {
+				indexOfModelText = cells.Count;
 			}
 
 			// If body of line is too long
@@ -186,16 +198,47 @@ public virtual void Draw (ConsoleDriver driver, ColorScheme colorScheme, int y,
 				}
 			}
 
-			driver.SetAttribute (modelColor);
-			driver.AddStr (lineBody);
+			attr = modelColor;
+			cells.AddRange (lineBody.Select (r => NewRuneCell (attr, new Rune (r))));
 
 			if (availableWidth > 0) {
-				driver.SetAttribute (symbolColor);
-				driver.AddStr (new string (' ', availableWidth));
+				attr = symbolColor;
+				cells.AddRange (
+					Enumerable.Repeat (
+						NewRuneCell (attr, new Rune (' ')),
+						availableWidth
+						));
 			}
+
+			var e = new DrawTreeViewLineEventArgs<T> {
+				Model = Model,
+				Y = y,
+				RuneCells = cells,
+				Tree = tree,
+				IndexOfExpandCollapseSymbol = indexOfExpandCollapseSymbol,
+				IndexOfModelText = indexOfModelText,
+			};
+			tree.OnDrawLine (e);
+
+			if (!e.Handled) {
+				foreach (var cell in cells) {
+					driver.SetAttribute (cell.ColorScheme.Normal);
+					driver.AddRune (cell.Rune);
+				}
+			}
+
 			driver.SetAttribute (colorScheme.Normal);
 		}
 
+		private static RuneCell NewRuneCell (Attribute attr, Rune r)
+		{
+			return new RuneCell {
+				Rune = r,
+				ColorScheme = new ColorScheme (attr)
+			};
+		}
+
+
 		/// <summary>
 		/// Gets all characters to render prior to the current branches line.  This includes indentation
 		/// whitespace and any tree branches (if enabled).

Terminal.Gui/Views/TreeView/DrawTreeViewLineEventArgs.cs

@@ -0,0 +1,66 @@
+// This code is based on http://objectlistview.sourceforge.net (GPLv3 tree/list controls 
+// by [email protected]). Phillip has explicitly granted permission for his design
+// and code to be used in this library under the MIT license.
+
+using System.Collections.Generic;
+
+namespace Terminal.Gui {
+	/// <summary>
+	/// Event args for the <see cref="TreeView{T}.DrawLine"/> event
+	/// </summary>
+	/// <typeparam name="T"></typeparam>
+	public class DrawTreeViewLineEventArgs<T> where T : class {
+
+		/// <summary>
+		/// The object at this line in the tree
+		/// </summary>
+		public T Model { get; init; }
+
+		/// <summary>
+		/// The <see cref="TreeView{T}"/> that is performing the
+		/// rendering.
+		/// </summary>
+		public TreeView<T> Tree { get; init; }
+
+		/// <summary>
+		/// The line within tree view bounds that is being rendered
+		/// </summary>
+		public int Y { get; init; }
+
+		/// <summary>
+		/// Set to true to cancel drawing (e.g. if you have already manually
+		/// drawn content).
+		/// </summary>
+		public bool Handled { get; set; }
+
+		/// <summary>
+		/// The rune and color of each symbol that will be rendered.  Note
+		/// that only <see cref="ColorScheme.Normal"/> is respected.  You
+		/// can modify these to change what is rendered.
+		/// </summary>
+		/// <remarks>
+		/// Changing the length of this collection may result in corrupt rendering
+		/// </remarks>
+		public List<RuneCell> RuneCells { get; init; }
+
+		/// <summary>
+		/// The notional index in <see cref="RuneCells"/> which contains the first
+		/// character of the <see cref="TreeView{T}.AspectGetter"/> text (i.e.
+		/// after all branch lines and expansion/collapse sybmols).
+		/// </summary>
+		/// <remarks>
+		/// May be negative or outside of bounds of <see cref="RuneCells"/> if the view
+		/// has been scrolled horizontally.
+		/// </remarks>
+
+		public int IndexOfModelText { get; init; }
+
+		/// <summary>
+		/// If line contains a branch that can be expanded/collapsed then this is
+		/// the index in <see cref="RuneCells"/> at which the symbol is (or null for
+		/// leaf elements).
+		/// </summary>
+		public int? IndexOfExpandCollapseSymbol { get; init; }
+
+	}
+}

Terminal.Gui/Views/TreeView/TreeView.cs

@@ -169,6 +169,12 @@ public Key ObjectActivationKey {
 		/// </summary>
 		public event EventHandler<SelectionChangedEventArgs<T>> SelectionChanged;
 
+		/// <summary>
+		/// Called once for each visible row during rendering.  Can be used
+		/// to make last minute changes to color or text rendered
+		/// </summary>
+		public event EventHandler<DrawTreeViewLineEventArgs<T>> DrawLine;
+
 		/// <summary>
 		/// The root objects in the tree, note that this collection is of root objects only.
 		/// </summary>
@@ -557,10 +563,9 @@ internal IReadOnlyCollection<Branch<T>> BuildLineMap ()
 			List<Branch<T>> toReturn = new List<Branch<T>> ();
 
 			foreach (var root in roots.Values) {
-				
+
 				var toAdd = AddToLineMap (root, false, out var isMatch);
-				if(isMatch)
-				{
+				if (isMatch) {
 					toReturn.AddRange (toAdd);
 				}
 			}
@@ -574,41 +579,38 @@ internal IReadOnlyCollection<Branch<T>> BuildLineMap ()
 
 		private bool IsFilterMatch (Branch<T> branch)
 		{
-			return Filter?.IsMatch(branch.Model) ?? true;
+			return Filter?.IsMatch (branch.Model) ?? true;
 		}
 
-		private IEnumerable<Branch<T>> AddToLineMap (Branch<T> currentBranch,bool parentMatches, out bool match)
+		private IEnumerable<Branch<T>> AddToLineMap (Branch<T> currentBranch, bool parentMatches, out bool match)
 		{
-			bool weMatch = IsFilterMatch(currentBranch);
+			bool weMatch = IsFilterMatch (currentBranch);
 			bool anyChildMatches = false;
-			
-			var toReturn = new List<Branch<T>>();
-			var children = new List<Branch<T>>();
+
+			var toReturn = new List<Branch<T>> ();
+			var children = new List<Branch<T>> ();
 
 			if (currentBranch.IsExpanded) {
 				foreach (var subBranch in currentBranch.ChildBranches.Values) {
 
 					foreach (var sub in AddToLineMap (subBranch, weMatch, out var childMatch)) {
-
-						if(childMatch)
-						{
-							children.Add(sub);
+
+						if (childMatch) {
+							children.Add (sub);
 							anyChildMatches = true;
 						}
 					}
 				}
 			}
 
-			if(parentMatches || weMatch || anyChildMatches)
-			{
+			if (parentMatches || weMatch || anyChildMatches) {
 				match = true;
-				toReturn.Add(currentBranch);
-			}
-			else{
+				toReturn.Add (currentBranch);
+			} else {
 				match = false;
 			}
-			
-			toReturn.AddRange(children);
+
+			toReturn.AddRange (children);
 			return toReturn;
 		}
 
@@ -1421,8 +1423,17 @@ protected virtual void OnSelectionChanged (SelectionChangedEventArgs<T> e)
 		{
 			SelectionChanged?.Invoke (this, e);
 		}
-	}
 
+		/// <summary>
+		/// Raises the DrawLine event
+		/// </summary>
+		/// <param name="e"></param>
+		internal void OnDrawLine (DrawTreeViewLineEventArgs<T> e)
+		{
+			DrawLine?.Invoke (this, e);
+		}
+
+	}
 	class TreeSelection<T> where T : class {
 
 		public Branch<T> Origin { get; }