Added custom inline nodes support

Package.swift

@@ -1,12 +1,12 @@
-// swift-tools-version:5.6
+// swift-tools-version:5.7
 
 import PackageDescription
 
 let package = Package(
   name: "swift-markdown-ui",
   platforms: [
     .macOS(.v12),
-    .iOS(.v15),
+    .iOS(.v16),
     .tvOS(.v15),
     .macCatalyst(.v15),
     .watchOS(.v8),
@@ -15,19 +15,19 @@ let package = Package(
     .library(
       name: "MarkdownUI",
       targets: ["MarkdownUI"]
-    )
+    ),
   ],
   dependencies: [
     .package(url: "https://github.com/gonzalezreal/NetworkImage", from: "6.0.0"),
     .package(url: "https://github.com/pointfreeco/swift-snapshot-testing", from: "1.10.0"),
-    .package(url: "https://github.com/swiftlang/swift-cmark", from: "0.4.0"),
+    .package(id: "spm-external.cmark-gfm", exact: "0.5.0+3ccff77b2dc5b96b77db3da0d68d28068593fa53"),
   ],
   targets: [
     .target(
       name: "MarkdownUI",
       dependencies: [
-        .product(name: "cmark-gfm", package: "swift-cmark"),
-        .product(name: "cmark-gfm-extensions", package: "swift-cmark"),
+        .product(name: "cmark-gfm", package: "spm-external.cmark-gfm"),
+        .product(name: "cmark-gfm-extensions", package: "spm-external.cmark-gfm"),
         .product(name: "NetworkImage", package: "NetworkImage"),
       ]
     ),

Sources/MarkdownUI/DSL/Blocks/MarkdownContent.swift

@@ -59,7 +59,7 @@ public protocol MarkdownContentProtocol {
 ///   }
 /// }
 /// ```
-public struct MarkdownContent: Equatable, MarkdownContentProtocol {
+public struct MarkdownContent: Hashable, MarkdownContentProtocol, Sendable {
   /// Returns a Markdown content value with the sum of the contents of all the container blocks
   /// present in this content.
   ///
@@ -87,8 +87,8 @@ public struct MarkdownContent: Equatable, MarkdownContentProtocol {
 
   /// Creates a Markdown content value from a Markdown-formatted string.
   /// - Parameter markdown: A Markdown-formatted string.
-  public init(_ markdown: String) {
-    self.init(blocks: .init(markdown: markdown))
+  public init(_ markdown: String, extensions: [CmarkExtension] = []) {
+    self.init(blocks: .init(markdown: markdown, extensions: extensions))
   }
 
   /// Creates a Markdown content value composed of any number of blocks.

Sources/MarkdownUI/Parser/CmarkExtension.swift

@@ -0,0 +1,26 @@
+/// Describes extension to parse custom inline nodes in cmark.
+public struct CmarkExtension: Sendable {
+  /// Extension name to pass to cmark
+  public var name: String
+
+  /// Registers extension in cmark
+  public var register: @Sendable () -> Void
+
+  /// Name to identify cmark node
+  public var nodeName: String
+
+  /// This is pointer to `cmark_node`
+  public var makeNode: @Sendable (UnsafeMutableRawPointer) -> CustomInline?
+
+  public init(
+    name: String,
+    register: @escaping @Sendable () -> Void,
+    nodeName: String,
+    makeNode: @escaping @Sendable (UnsafeMutableRawPointer) -> CustomInline?,
+  ) {
+    self.name = name
+    self.register = register
+    self.nodeName = nodeName
+    self.makeNode = makeNode
+  }
+}

Sources/MarkdownUI/Parser/CustomInline.swift

@@ -0,0 +1,34 @@
+import SwiftUI
+
+/// Describes custom inline node.
+/// As this is inline node, it is type erased to SwiftUI's `Text`
+/// to properly render inside surrounding text.
+public struct CustomInline: Hashable {
+  /// Must uniquely identify renderers provided.
+  /// Failure to do so will result in undefined behavior.
+  public var id: String
+
+  /// Sync render must be as lightweight as possible.
+  public var renderSync: @Sendable () -> Text
+
+  /// Optional async renderer for heavy operations like image render/load.
+  public var renderAsync: (@Sendable () async -> Text)?
+
+  public init(
+    id: String,
+    renderSync: @escaping @Sendable () -> Text,
+    renderAsync: (@Sendable () async -> Text)? = nil,
+  ) {
+    self.id = id
+    self.renderSync = renderSync
+    self.renderAsync = renderAsync
+  }
+
+  public static func == (lhs: CustomInline, rhs: CustomInline) -> Bool {
+    lhs.id == rhs.id
+  }
+
+  public func hash(into hasher: inout Hasher) {
+    hasher.combine(self.id)
+  }
+}

Sources/MarkdownUI/Parser/InlineNode.swift

@@ -11,6 +11,7 @@ enum InlineNode: Hashable, Sendable {
   case strikethrough(children: [InlineNode])
   case link(destination: String, children: [InlineNode])
   case image(source: String, children: [InlineNode])
+  case custom(CustomInline)
 }
 
 extension InlineNode {