fix(linenumbers): Adjust Gutter Background, Add Trailing Padding (#272)

- Updates the gutter to consider an inset for where to draw it's
background.
- Adds some padding to the trailing edge when line wrap is disabled.
This is not an opened issue, but while I was adjusting padding might as
well.

### Related Issues

* closes #271

.swiftlint.yml

@@ -5,6 +5,7 @@ disabled_rules:
   - todo
   - trailing_comma
   - nesting
+  - optional_data_string_conversion
 
 type_name:
   excluded:

Example/CodeEditSourceEditorExample/CodeEditSourceEditorExample/Documents/CodeEditSourceEditorExampleDocument.swift

@@ -29,7 +29,7 @@ struct CodeEditSourceEditorExampleDocument: FileDocument {
     }
 
     func fileWrapper(configuration: WriteConfiguration) throws -> FileWrapper {
-        let data = text.data(using: .utf8)!
+        let data = Data(text.utf8)
         return .init(regularFileWithContents: data)
     }
 }

Sources/CodeEditSourceEditor/Controller/TextViewController.swift

@@ -93,6 +93,10 @@ public class TextViewController: NSViewController {
         didSet {
             textView.layoutManager.wrapLines = wrapLines
             scrollView.hasHorizontalScroller = !wrapLines
+            textView.edgeInsets = HorizontalEdgeInsets(
+                left: textView.edgeInsets.left,
+                right: textViewTrailingInset // Refresh this value, see docs
+            )
         }
     }
 
@@ -194,6 +198,11 @@ public class TextViewController: NSViewController {
         return max(inset, .zero)
     }
 
+    /// The trailing inset for the editor. Grows when line wrapping is disabled.
+    package var textViewTrailingInset: CGFloat {
+        wrapLines ? 1 : 48
+    }
+
     // MARK: Init
 
     init(
@@ -315,6 +324,6 @@ public class TextViewController: NSViewController {
 extension TextViewController: GutterViewDelegate {
     public func gutterViewWidthDidUpdate(newWidth: CGFloat) {
         gutterView?.frame.size.width = newWidth
-        textView?.edgeInsets = HorizontalEdgeInsets(left: newWidth, right: 0)
+        textView?.edgeInsets = HorizontalEdgeInsets(left: newWidth, right: textViewTrailingInset)
     }
 }

Sources/CodeEditSourceEditor/Gutter/GutterView.swift

@@ -13,6 +13,24 @@ public protocol GutterViewDelegate: AnyObject {
     func gutterViewWidthDidUpdate(newWidth: CGFloat)
 }
 
+/// The gutter view displays line numbers that match the text view's line indexes.
+/// This view is used as a scroll view's ruler view. It sits on top of the text view so text scrolls underneath the
+/// gutter if line wrapping is disabled.
+///
+/// If the gutter needs more space (when the number of digits in the numbers increases eg. adding a line after line 99),
+/// it will notify it's delegate via the ``GutterViewDelegate/gutterViewWidthDidUpdate(newWidth:)`` method. In
+/// `CodeEditSourceEditor`, this notifies the ``TextViewController``, which in turn updates the textview's edge insets
+/// to adjust for the new leading inset.
+///
+/// This view also listens for selection updates, and draws a selected background on selected lines to keep the illusion
+/// that the gutter's line numbers are inline with the line itself.
+///
+/// The gutter view has insets of it's own that are relative to the widest line index. By default, these insets are 20px
+/// leading, and 12px trailing. However, this view also has a ``GutterView/backgroundEdgeInsets`` property, that pads
+/// the rect that has a background drawn. This allows the text to be scrolled under the gutter view for 8px before being
+/// overlapped by the gutter. It should help the textview keep the cursor visible if the user types while the cursor is
+/// off the leading edge of the editor.
+///
 public class GutterView: NSView {
     struct EdgeInsets: Equatable, Hashable {
         let leading: CGFloat
@@ -32,6 +50,9 @@ public class GutterView: NSView {
     @Invalidating(.display)
     var edgeInsets: EdgeInsets = EdgeInsets(leading: 20, trailing: 12)
 
+    @Invalidating(.display)
+    var backgroundEdgeInsets: EdgeInsets = EdgeInsets(leading: 0, trailing: 8)
+
     @Invalidating(.display)
     var backgroundColor: NSColor? = NSColor.controlBackgroundColor
 
@@ -44,6 +65,7 @@ public class GutterView: NSView {
     @Invalidating(.display)
     var selectedLineColor: NSColor = NSColor.selectedTextBackgroundColor.withSystemEffect(.disabled)
 
+    /// The required width of the entire gutter, including padding.
     private(set) public var gutterWidth: CGFloat = 0
 
     private weak var textView: TextView?
@@ -118,6 +140,17 @@ public class GutterView: NSView {
         }
     }
 
+    private func drawBackground(_ context: CGContext) {
+        guard let backgroundColor else { return }
+        let xPos = backgroundEdgeInsets.leading
+        let width = gutterWidth - backgroundEdgeInsets.trailing
+
+        context.saveGState()
+        context.setFillColor(backgroundColor.cgColor)
+        context.fill(CGRect(x: xPos, y: 0, width: width, height: frame.height))
+        context.restoreGState()
+    }
+
     private func drawSelectedLines(_ context: CGContext) {
         guard let textView = textView,
               let selectionManager = textView.selectionManager,
@@ -126,10 +159,14 @@ public class GutterView: NSView {
             return
         }
         context.saveGState()
+
         var highlightedLines: Set<UUID> = []
         context.setFillColor(selectedLineColor.cgColor)
-        for selection in selectionManager.textSelections
-        where selection.range.isEmpty {
+
+        let xPos = backgroundEdgeInsets.leading
+        let width = gutterWidth - backgroundEdgeInsets.trailing
+
+        for selection in selectionManager.textSelections where selection.range.isEmpty {
             guard let line = textView.layoutManager.textLineForOffset(selection.range.location),
                   visibleRange.intersection(line.range) != nil || selection.range.location == textView.length,
                   !highlightedLines.contains(line.data.id) else {
@@ -138,13 +175,14 @@ public class GutterView: NSView {
             highlightedLines.insert(line.data.id)
             context.fill(
                 CGRect(
-                    x: 0.0,
+                    x: xPos,
                     y: line.yPos,
-                    width: maxWidth + edgeInsets.horizontal,
+                    width: width,
                     height: line.height
                 )
             )
         }
+
         context.restoreGState()
     }
 
@@ -204,8 +242,8 @@ public class GutterView: NSView {
         CATransaction.begin()
         superview?.clipsToBounds = false
         superview?.layer?.masksToBounds = false
-        layer?.backgroundColor = backgroundColor?.cgColor
         updateWidthIfNeeded()
+        drawBackground(context)
         drawSelectedLines(context)
         drawLineNumbers(context)
         CATransaction.commit()