Add support for if statements.

Sources/Slipstream/Documentation.docc/Views/Fundamentals/Fundamentals.md

@@ -17,3 +17,7 @@ conforms to the ``View`` protocol can act as a view in your website.
 ### Modifying a view
 
 - ``ViewModifier``
+
+### Supporting view types
+
+- ``EmptyView``

Sources/Slipstream/Views/Fundamentals/EmptyView.swift

@@ -0,0 +1,17 @@
+/// A view that doesn't contain any content.
+///
+/// You will rarely, if ever, need to create an `EmptyView` directly. Instead,
+/// `EmptyView` represents the absence of a view.
+///
+/// Slipstream uses `EmptyView` in situations where a Slipstream view type defines one
+/// or more child views with generic parameters, and allows the child views to
+/// be absent. When absent, the child view's type in the generic type parameter
+/// is `EmptyView`.
+@available(iOS 17.0, macOS 14.0, *)
+public struct EmptyView: View {
+  public typealias Content = Never
+
+  /// Creates an empty view.
+  public init() {
+  }
+}

Sources/Slipstream/Views/Fundamentals/View.swift

@@ -115,6 +115,10 @@ extension View where Content == Never {
   public var body: Never {
     fatalError("body should never be executed when a view has declared its Content as Never")
   }
+
+  public func render(_ container: Element, environment: EnvironmentValues) throws {
+    // Do nothing.
+  }
 }
 
 @_documentation(visibility: private)

Sources/Slipstream/Views/Fundamentals/ViewBuilder/ArrayView.swift

@@ -2,6 +2,7 @@ import SwiftSoup
 
 /// A View created from a for loop of View values.
 @_documentation(visibility: private)
+@available(iOS 17.0, macOS 14.0, *)
 public struct ArrayView: View {
   public typealias Content = Never
 

Sources/Slipstream/Views/Fundamentals/ViewBuilder/ConditionalView.swift

@@ -2,6 +2,7 @@ import SwiftSoup
 
 /// A View created from an if/else conditional of View values.
 @_documentation(visibility: private)
+@available(iOS 17.0, macOS 14.0, *)
 public struct ConditionalView<T: View, F: View>: View {
   enum Condition {
     case isTrue(T)

Sources/Slipstream/Views/Fundamentals/ViewBuilder/TupleView.swift

@@ -2,6 +2,7 @@ import SwiftSoup
 
 /// A View created from a swift tuple of View values.
 @_documentation(visibility: private)
+@available(iOS 17.0, macOS 14.0, *)
 public struct TupleView<T>: View {
   public typealias Content = Never
 

Sources/Slipstream/Views/Fundamentals/ViewBuilder/ViewBuilder.swift

@@ -34,6 +34,26 @@ public struct ViewBuilder {
     return TupleView(value: (repeat each content))
   }
 
+  /// An if statement.
+  ///
+  /// An example:
+  ///
+  /// ```swift
+  /// var body: some View {
+  ///   if true {
+  ///     Text("true")
+  ///   }
+  /// }
+  /// ```
+  public static func buildOptional<Content>(_ component: Content?) -> ConditionalView<Content, EmptyView>
+  where Content: View {
+    if let component {
+      return ConditionalView(condition: .isTrue(component))
+    } else {
+      return ConditionalView(condition: .isFalse(EmptyView()))
+    }
+  }
+
   /// An if/else statement, when the condition returns true.
   ///
   /// An example:

Tests/SlipstreamTests/ViewBuilderTests.swift

@@ -15,7 +15,7 @@ private struct TupleBlockView: View {
   }
 }
 
-private struct ConditionalBlockView: View {
+private struct IfElseBlockView: View {
   let bool: Bool
   var body: some View {
     if bool {
@@ -26,6 +26,15 @@ private struct ConditionalBlockView: View {
   }
 }
 
+private struct IfBlockView: View {
+  let bool: Bool
+  var body: some View {
+    if bool {
+      Text("true")
+    }
+  }
+}
+
 private struct ArrayBlockView: View {
   var body: some View {
     for i in 1...3 {
@@ -43,9 +52,14 @@ struct ViewBuilderTests {
     try #expect(renderHTML(TupleBlockView()) == "Hello, world!")
   }
 
-  @Test func conditionalBlock() throws {
-    try #expect(renderHTML(ConditionalBlockView(bool: true)) == "true")
-    try #expect(renderHTML(ConditionalBlockView(bool: false)) == "false")
+  @Test func ifElseBlock() throws {
+    try #expect(renderHTML(IfElseBlockView(bool: true)) == "true")
+    try #expect(renderHTML(IfElseBlockView(bool: false)) == "false")
+  }
+
+  @Test func ifBlock() throws {
+    try #expect(renderHTML(IfBlockView(bool: true)) == "true")
+    try #expect(renderHTML(IfBlockView(bool: false)) == "")
   }
 
   @Test func arrayBlock() throws {