added ForEach element (#26)

* added ForEach

* use ForEach in example

Examples/HummingbirdDemo/Sources/App/Elementary+Hummingbird.swift

@@ -1,6 +1,12 @@
 import Elementary
 import Hummingbird
 
+// ----------------------
+// NOTICE
+// For integrating with Hummingbird, please use https://github.com/hummingbird-community/hummingbird-elementary
+// You will not need the code in this file.
+// ----------------------
+
 extension MainLayout: ResponseGenerator {}
 
 struct HTMLResponseBodyWriter: HTMLStreamWriter {

Examples/HummingbirdDemo/Sources/App/Pages.swift

@@ -58,7 +58,7 @@ struct GreetingPage: HTML {
             }
         } else {
             ul {
-                for i in 0 ..< greetingCount {
+                ForEach(0 ..< greetingCount) { i in
                     li {
                         "Hello there, \(name)!"
                         // demo of conditional styling (should be done with CSS in real life)

Sources/Elementary/Core/ForEach.swift

@@ -0,0 +1,48 @@
+/// An element that lazily renders HTML for each element in a sequence.
+///
+/// Using `ForEach` instead of `for ... in` is potentially more efficient when rendering a large number of elements,
+/// as the result does not need to be collected into an array before rendering.
+///
+/// ```swift
+/// ForEach(1 ... 100) { index in
+///    li { "Item \(index)" }
+/// }
+/// ```
+public struct ForEach<Data, Content>: HTML
+    where Data: Sequence, Content: HTML
+{
+    var sequence: Data
+    // TODO: Swift 6 - @Sendable is not ideal here, but currently the response generators for hummingbird/vapor require sendable HTML types
+    // also, currently there is no good way to conditionally apply Sendable conformance based on closure type
+    var contentBuilder: @Sendable (Data.Element) -> Content
+
+    /// Creates a new `ForEach` element with the given sequence and content builder closure.
+    ///
+    /// - Parameters:
+    ///  - sequence: A sequence of data to render.
+    ///  - contentBuilder: A closure that builds the HTML content for each element in the sequence.
+    public init(_ sequence: Data, @HTMLBuilder content contentBuilder: @escaping @Sendable (Data.Element) -> Content) {
+        self.sequence = sequence
+        self.contentBuilder = contentBuilder
+    }
+
+    @_spi(Rendering)
+    public static func _render<Renderer: _HTMLRendering>(_ html: consuming Self, into renderer: inout Renderer, with context: consuming _RenderingContext) {
+        context.assertNoAttributes(self)
+
+        for element in html.sequence {
+            Content._render(html.contentBuilder(element), into: &renderer, with: copy context)
+        }
+    }
+
+    @_spi(Rendering)
+    public static func _render<Renderer: _AsyncHTMLRendering>(_ html: consuming Self, into renderer: inout Renderer, with context: consuming _RenderingContext) async throws {
+        context.assertNoAttributes(self)
+
+        for element in html.sequence {
+            try await Content._render(html.contentBuilder(element), into: &renderer, with: copy context)
+        }
+    }
+}
+
+extension ForEach: Sendable where Data: Sendable, Content: Sendable {}

Tests/ElementaryTests/CompositionRenderingTest.swift

@@ -15,6 +15,24 @@ final class CompositionRenderingTests: XCTestCase {
             #"<ul><li id="1">one</li><li class="selected" id="2">two</li></ul>"#
         )
     }
+
+    func testRendersForEachWithRange() async throws {
+        try await HTMLAssertEqual(
+            ForEach(1 ... 3) { index in
+                li { "Item \(index)" }
+            },
+            #"<li>Item 1</li><li>Item 2</li><li>Item 3</li>"#
+        )
+    }
+
+    func testRendersForEachWithLazyMap() async throws {
+        try await HTMLAssertEqual(
+            ForEach([1, 2, 3].lazy.map { $0 * 10 }) { index in
+                li { "Item \(index)" }
+            },
+            #"<li>Item 10</li><li>Item 20</li><li>Item 30</li>"#
+        )
+    }
 }
 
 struct MyPage: HTMLDocument {