Honor the GFM delimiter row's column alignment in tables

Sources/MarkdownText/Block/Table+.swift

@@ -38,6 +38,15 @@ extension Markdown.Table: BlockConvertible {
     let headerColumnCount = self.head.childCount
     let isWellFormed = self.body.children.allSatisfy { $0.childCount == headerColumnCount }
     let rawMarkdown = isWellFormed ? self.format() : ""
-    return .table(id: self.id, headers: headerCells, rows: rows, rawMarkdown: rawMarkdown)
+    let alignments = (0..<headerCells.count).map { index -> MarkdownColumnAlignment in
+      guard index < self.columnAlignments.count else { return .leading }
+      switch self.columnAlignments[index] {
+      case .left: return .leading
+      case .center: return .center
+      case .right: return .trailing
+      case .none: return .leading
+      }
+    }
+    return .table(id: self.id, headers: headerCells, rows: rows, alignments: alignments, rawMarkdown: rawMarkdown)
   }
 }

Sources/MarkdownText/Models/MarkdownRenderable.swift

@@ -13,6 +13,15 @@ import AppKit
 /// Markdown element representation that is ready to be rendered by a SwiftUI View
 /// The representation already have all the parsing and processing completed to minimize rendering overhead on UI thread.
 /// This data structure is not thread safe due to the usage of `NSMutableAttributedString`, this needs to be addressed as a future improvement
+/// Per-column text alignment declared by a GFM table's delimiter row
+/// (`:---`, `:---:`, `---:`). `.leading` is the GFM default for a column that
+/// declares nothing.
+enum MarkdownColumnAlignment: Equatable, Sendable {
+  case leading
+  case center
+  case trailing
+}
+
 indirect enum MarkdownRenderable: Identifiable, Equatable, @unchecked Sendable {
 
   /// To be rendered as a paragraph
@@ -34,7 +43,7 @@ indirect enum MarkdownRenderable: Identifiable, Equatable, @unchecked Sendable {
   case codeBlock(id: String, language: String?, code: String)
 
   /// To be rendered as a table
-  case table(id: String, headers: [NSMutableAttributedString], rows: [[NSMutableAttributedString]], rawMarkdown: String)
+  case table(id: String, headers: [NSMutableAttributedString], rows: [[NSMutableAttributedString]], alignments: [MarkdownColumnAlignment], rawMarkdown: String)
 
   /// To be rendered as thematic break
   case thematicBreak(id: String)
@@ -54,7 +63,7 @@ indirect enum MarkdownRenderable: Identifiable, Equatable, @unchecked Sendable {
     case .orderedList(let id, _): return id
     case .unorderedList(let id, _, _): return id
     case .codeBlock(let id, _, _): return id
-    case .table(let id, _, _, _): return id
+    case .table(let id, _, _, _, _): return id
     case .thematicBreak(let id): return id
     case .blockQuote(let id, _): return id
     case .image(let id, _): return id

Sources/MarkdownText/Models/RenderableDocument.swift

@@ -92,7 +92,7 @@ extension MarkdownRenderable {
       return items.plainText(separator: "\n")
     case .codeBlock(_, _, let code):
       return code
-    case .table(_, let headers, let rows, _):
+    case .table(_, let headers, let rows, _, _):
       let headerLine = headers.map { $0.string }.joined(separator: "\t")
       let rowLines = rows.map { row in row.map { $0.string }.joined(separator: "\t") }
       return ([headerLine] + rowLines).joined(separator: "\n")
@@ -135,7 +135,7 @@ extension MarkdownRenderable {
       return items.flatMap { $0.attributedStrings() }
     case .unorderedList(_, let items, _):
       return items.flatMap { $0.attributedStrings() }
-    case .table(_, let headers, let rows, _):
+    case .table(_, let headers, let rows, _, _):
       return headers + rows.flatMap { $0 }
     default:
       return []

Sources/MarkdownText/UI/BlockView.swift

@@ -62,9 +62,10 @@ struct SingleBlockView: View {
                       code: code)
       case .thematicBreak:
         ThematicBreakView()
-      case .table(_, let headers, let rows, let rawMarkdown):
+      case .table(_, let headers, let rows, let alignments, let rawMarkdown):
         TableView(headings: headers,
                   rows: rows,
+                  alignments: alignments,
                   rawMarkdown: rawMarkdown)
       case .blockQuote(_, let item):
         BlockQuoteView(item: item)

Sources/MarkdownText/UI/TableView.swift

@@ -22,6 +22,7 @@ struct TableView: View {
 
   let headings: [AttributedString]
   let rows: [[RowContent]]
+  let alignments: [MarkdownColumnAlignment]
   let columnMaxWidths: [Int: CGFloat]
 
   private let defaultMaxColumnWidth: CGFloat = 200
@@ -32,7 +33,8 @@ struct TableView: View {
 
   private let rawMarkdown: String
 
-  init(headings: [NSMutableAttributedString], rows: [[NSMutableAttributedString]], columnMaxWidths: [Int: CGFloat] = [:], rawMarkdown: String = "") {
+  init(headings: [NSMutableAttributedString], rows: [[NSMutableAttributedString]], alignments: [MarkdownColumnAlignment] = [], columnMaxWidths: [Int: CGFloat] = [:], rawMarkdown: String = "") {
+    self.alignments = alignments
     self.headings = headings.map { AttributedString($0) }
     self.rows = rows.map { row in
       row.map { content in
@@ -52,13 +54,36 @@ struct TableView: View {
     return rows.count
   }
 
+  /// The GFM delimiter row's alignment for a column, defaulting to `.leading`
+  /// for a table that declares none.
+  private func alignment(forColumn index: Int) -> MarkdownColumnAlignment {
+    guard index < alignments.count else { return .leading }
+    return alignments[index]
+  }
+
+  private func textAlignment(forColumn index: Int) -> TextAlignment {
+    switch alignment(forColumn: index) {
+    case .leading: return .leading
+    case .center: return .center
+    case .trailing: return .trailing
+    }
+  }
+
+  private func frameAlignment(forColumn index: Int) -> Alignment {
+    switch alignment(forColumn: index) {
+    case .leading: return .topLeading
+    case .center: return .top
+    case .trailing: return .topTrailing
+    }
+  }
+
   private func headerView(colIdx: Int) -> some View {
     HStack(spacing: 0) {
       Text(headings[colIdx])
         .foregroundStyle(config.tableStyle.headerTextColor)
         .lineLimit(nil)
-        .multilineTextAlignment(.leading)
-        .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .leading)
+        .multilineTextAlignment(textAlignment(forColumn: colIdx))
+        .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: frameAlignment(forColumn: colIdx))
         .if(config.shouldAnimateText) { view in
           view.fadeInTextTransition(attributedString: headings[colIdx])
         }
@@ -106,7 +131,7 @@ struct TableView: View {
     case .containsAttachment(let nsAttributedString):
       HStack(spacing: 0) {
         ParagraphView(contents: applyTypographyThemingAndGetContent(nsAttributedString))
-          .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .topLeading)
+          .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: frameAlignment(forColumn: colIdx))
           .accessibilityValue(String.itemPositionInTable(rowIndex: rowIdx + 2, totalRow: numOfRows + 1, columnIndex: colIdx + 1, totalColumn: headings.count))
         Spacer()
       }
@@ -119,8 +144,8 @@ struct TableView: View {
         Text(attributedString)
           .foregroundStyle(config.tableStyle.regularTextColor)
           .lineLimit(nil)
-          .multilineTextAlignment(.leading)
-          .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .topLeading)
+          .multilineTextAlignment(textAlignment(forColumn: colIdx))
+          .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: frameAlignment(forColumn: colIdx))
           .if(config.shouldAnimateText) { view in
             view.fadeInTextTransition(attributedString: attributedString)
           }