AlignmentID

Sources/TerminalUI/Modifiers/Layout/FixedFrame.swift

@@ -3,9 +3,14 @@ extension View {
 
   public func frame(
     width: Horizontal? = nil,
-    height: Vertical? = nil
+    height: Vertical? = nil,
+    alignment: Alignment = .center
   ) -> some View {
-    FixedFrame(content: self, width: width, height: height)
+    FixedFrame(
+      content: self,
+      width: width,
+      height: height,
+      alignment: alignment)
   }
 }
 
@@ -14,6 +19,7 @@ private struct FixedFrame<Content: View>: Builtin, View {
   let content: Content
   let width: Horizontal?
   let height: Vertical?
+  let alignment: Alignment
 
   func size(
     for proposal: ProposedSize,

Sources/TerminalUI/Primitives/Alignment.swift

@@ -25,18 +25,83 @@ extension Alignment {
   public static var bottomTrailing: Self { Self(horizontal: .trailing, vertical: .bottom) }
 }
 
+// MARK: - AlignmentID
+
+public protocol AlignmentID: Equatable {
+  static func defaultValue(in size: Size) -> Int
+}
+
+// MARK: - AlignmentKey
+
+public struct AlignmentKey: Equatable, Hashable, Sendable {
+
+  fileprivate let id: any AlignmentID.Type
+
+  public static func == (lhs: AlignmentKey, rhs: AlignmentKey) -> Bool {
+    String(describing: lhs.id) == String(describing: rhs.id)
+  }
+
+  public func hash(into hasher: inout Hasher) {
+    hasher.combine(String(describing: id))
+  }
+}
+
 // MARK: - Horizontal Alignment
 
-public enum HorizontalAlignment: Equatable, Hashable, Sendable {
-  case leading
-  case center
-  case trailing
+public struct HorizontalAlignment: Equatable, Hashable, Sendable {
+  let id: AlignmentKey
+}
+
+extension HorizontalAlignment {
+  public init(_ id: any AlignmentID.Type) {
+    self.id = AlignmentKey(id: id)
+  }
+}
+
+extension HorizontalAlignment {
+  public static var leading: Self { Self(HorizontalLeading.self) }
+  public static var center: Self { Self(HorizontalCenter.self) }
+  public static var trailing: Self { Self(HorizontalTrailing.self) }
+}
+
+private enum HorizontalLeading: AlignmentID {
+  static func defaultValue(in size: Size) -> Int { 1 }
+}
+
+private enum HorizontalCenter: AlignmentID {
+  static func defaultValue(in size: Size) -> Int { Int(size.width) / 2 }
+}
+
+private enum HorizontalTrailing: AlignmentID {
+  static func defaultValue(in size: Size) -> Int { Int(size.width) }
 }
 
 // MARK: - Vertical Alignment
 
-public enum VerticalAlignment: Equatable, Hashable, Sendable {
-  case top
-  case center
-  case bottom
+public struct VerticalAlignment: Equatable, Hashable, Sendable {
+  let id: AlignmentKey
+}
+
+extension VerticalAlignment {
+  public init(_ id: any AlignmentID.Type) {
+    self.id = AlignmentKey(id: id)
+  }
+}
+
+extension VerticalAlignment {
+  public static var top: Self { Self(VerticalTop.self) }
+  public static var center: Self { Self(VerticalCenter.self) }
+  public static var bottom: Self { Self(VerticalBottom.self) }
+}
+
+private enum VerticalTop: AlignmentID {
+  static func defaultValue(in size: Size) -> Int { 1 }
+}
+
+private enum VerticalCenter: AlignmentID {
+  static func defaultValue(in size: Size) -> Int { Int(size.height) / 2 }
+}
+
+private enum VerticalBottom: AlignmentID {
+  static func defaultValue(in size: Size) -> Int { Int(size.height) }
 }

Sources/TerminalUI/Primitives/Dimensions.swift

@@ -118,3 +118,9 @@ extension Vertical {
     Vertical(-vertical.value)
   }
 }
+
+extension Int {
+  init(_ vertical: Vertical) {
+    self = vertical.value
+  }
+}

Sources/TerminalUI/Primitives/Size.swift

@@ -1,10 +1,10 @@
 
-package struct Size {
+public struct Size {
 
-  package let width: Horizontal
-  package let height: Vertical
+  public let width: Horizontal
+  public let height: Vertical
 
-  package init(width: Horizontal, height: Vertical) {
+  public init(width: Horizontal, height: Vertical) {
     self.width = width
     self.height = height
   }

Tests/TerminalUITests/Modifiers/Layout/FixedFrameTests.swift

@@ -131,4 +131,28 @@ struct FixedFrameTests {
       Position(x: 2, y: 2): pixel,
     ])
   }
+
+  @Test("alignment", arguments: Array<(
+    Alignment,        Horizontal, Vertical
+  )>([
+    (.topLeading,     1,          1),
+    (.top,            2,          1),
+    (.topTrailing,    3,          1),
+    (.leading,        1,          2),
+    (.center,         2,          2),
+    (.trailing,       3,          2),
+    (.bottomLeading,  1,          3),
+    (.bottom,         2,          3),
+    (.bottomTrailing, 3,          3),
+  ]))
+  func alignment(alignment: Alignment, x: Horizontal, y: Vertical) {
+
+    canvas.render {
+      view
+        .frame(width: 1, height: 1)
+        .frame(width: 3, height: 3, alignment: alignment)
+    }
+
+    #expect(canvas.pixels == [Position(x: x, y: y): pixel])
+  }
 }