Rewrite inject_dartpad and expose some functionality in library

pkgs/inject_dartpad/example/inject_dartpad_example.dart

@@ -0,0 +1,22 @@
+import 'package:inject_dartpad/inject_dartpad.dart';
+import 'package:web/web.dart' as web;
+
+void main() async {
+  final dartPad = EmbeddedDartPad.create(
+    iframeId: 'my-dartpad',
+    theme: DartPadTheme.light,
+  );
+
+  await dartPad.initialize(
+    addToDocument: (iframe) {
+      iframe.style.height = '560';
+
+      web.document.body!.append(iframe);
+    },
+  );
+
+  dartPad.updateCode('''
+void main() {
+  print("Hello, I'm Dash!");
+}''');
+}

pkgs/inject_dartpad/lib/inject_dartpad.dart

@@ -0,0 +1,243 @@
+// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:async';
+import 'dart:js_interop';
+
+import 'package:web/web.dart' as web;
+
+/// An iframe-embedded DartPad that can be injected into a web page,
+/// then have its source code updated.
+///
+/// Example usage:
+///
+/// ```dart
+/// import 'package:inject_dartpad/inject_dartpad.dart';
+/// import 'package:web/web.dart' as web;
+///
+/// void main() async {
+///   final dartPad = EmbeddedDartPad.create(
+///     iframeId: 'my-dartpad',
+///     theme: DartPadTheme.light,
+///   );
+///
+///   await dartPad.initialize(
+///     addToDocument: (iframe) {
+///       iframe.style.height = '560';
+///
+///       web.document.body!.append(iframe);
+///     },
+///   );
+///
+///   dartPad.updateCode('''
+/// void main() {
+///   print("Hello, I'm Dash!");
+/// }''');
+/// }
+/// ```
+final class EmbeddedDartPad {
+  /// The unique identifier that's used to identify the created DartPad iframe.
+  ///
+  /// This ID is used both as the HTML element `id` and
+  /// as the iframe's `name` attribute for message targeting.
+  final String iframeId;
+
+  /// The full URL of the DartPad iframe including
+  /// all path segments and query parameters.
+  final String _iframeUrl;
+
+  /// Tracks the initialization state of the embedded DartPad.
+  ///
+  /// Completes when the DartPad iframe has loaded and
+  /// sent a 'ready' message indicating it can receive code updates.
+  final Completer<void> _initializedCompleter = Completer();
+
+  /// Creates an embedded DartPad instance with
+  /// the specified [iframeId] and [iframeUrl].
+  EmbeddedDartPad._({required this.iframeId, required String iframeUrl})
+    : _iframeUrl = iframeUrl;
+
+  /// Creates a new embedded DartPad element with the specified configuration.
+  ///
+  /// Once created, the DartPad must be initialized by
+  /// calling and awaiting [initialize].
+  ///
+  /// The [iframeId] is used to identify the created DartPad iframe.
+  /// It must be unique within the document and a valid HTML element ID.
+  ///
+  /// The [scheme] and [host] are used to construct the DartPad iframe URL.
+  /// [scheme] defaults to 'https' and [host] defaults to 'dartpad.dev'.
+  ///
+  /// To control the appearance of the embedded DartPad,
+  /// you can switch to the [embedLayout] and choose a specific [theme].
+  factory EmbeddedDartPad.create({
+    required String iframeId,
+    String? scheme,
+    String? host,
+    bool? embedLayout,
+    DartPadTheme? theme = DartPadTheme.auto,
+  }) {
+    final dartPadUrl = Uri(
+      scheme: scheme ?? 'https',
+      host: host ?? 'dartpad.dev',
+      queryParameters: <String, String>{
+        if (embedLayout ?? true) 'embed': '$embedLayout',
+        if (theme != DartPadTheme.auto) 'theme': '$theme',
+      },
+    ).toString();
+
+    return EmbeddedDartPad._(iframeId: iframeId, iframeUrl: dartPadUrl);
+  }
+
+  /// Creates and initializes the embedded DartPad iframe.
+  ///
+  /// Must be called and awaited before interacting with this instance,
+  /// such as updating the DartPad editor's current source code.
+  ///
+  /// The created iframe is passed to the [addToDocument] callback,
+  /// which should be used to add the iframe to the document and
+  /// further configure its attributes, such as classes and size.
+  ///
+  /// For example, if you want to embed the DartPad in
+  /// a container with an ID of 'dartpad-container':
+  ///
+  /// ```dart
+  /// await dartPad.initialize(
+  ///   addToDocument: (iframe) {
+  ///     document.getElementById('dartpad-container')!.append(iframe);
+  ///   },
+  /// );
+  /// ```
+  Future<void> initialize({
+    required void Function(web.HTMLIFrameElement iframe) addToDocument,
+  }) async {
+    if (_initialized) return;
+
+    late final JSExportedDartFunction readyHandler;
+    readyHandler = (web.MessageEvent event) {
+      if (event.data case _EmbedReadyMessage(type: 'ready', :final sender?)) {
+        if (sender != iframeId) {
+          return;
+        }
+
+        web.window.removeEventListener('message', readyHandler);
+        if (!_initialized) {
+          _initializedCompleter.complete();
+        }
+      }
+    }.toJS;
+
+    web.window.addEventListener('message', readyHandler);
+
+    final iframe = web.HTMLIFrameElement()
+      ..src = _iframeUrl
+      ..id = iframeId
+      ..name = iframeId
+      ..loading = 'lazy'
+      ..allow = 'clipboard-write';
+    addToDocument(iframe);
+
+    await _initializedCompleter.future;
+  }
+
+  /// Updates the source code displayed in the embedded DartPad's editor
+  /// with the specified Dart [code].
+  ///
+  /// The [code] should generally be valid Dart code for
+  /// the latest stable versions of Dart and Flutter.
+  ///
+  /// Should only be called after [initialize] has completed,
+  /// otherwise throws.
+  void updateCode(String code) {
+    if (!_initialized) {
+      throw StateError(
+        'EmbeddedDartPad.initialize must be called and awaited '
+        'before updating the embedded source code.',
+      );
+    }
+
+    _underlyingIframe.contentWindowCrossOrigin?.postMessage(
+      _MessageToDartPad.updateSource(code),
+      _anyTargetOrigin,
+    );
+  }
+
+  /// Whether the DartPad instance has been successfully initialized.
+  ///
+  /// Returns `true` if [initialize] has been called and awaited,
+  /// and the embedded DartPad has signaled that it's ready to receive messages.
+  bool get _initialized => _initializedCompleter.isCompleted;
+
+  /// Retrieves the iframe element from the current page by
+  /// searching with its ID of [iframeId].
+  ///
+  /// If the iframe can't be found, the method throws.
+  /// The often means it wasn't added to the DOM or was removed.
+  web.HTMLIFrameElement get _underlyingIframe {
+    final frame =
+        web.document.getElementById(iframeId) as web.HTMLIFrameElement?;
+    if (frame == null) {
+      throw StateError(
+        'Failed to find iframe with an '
+        'id of $iframeId in the document. '
+        'Have you added the iframe to the document?',
+      );
+    }
+    return frame;
+  }
+}
+
+/// The themes available for an embedded DartPad instance.
+enum DartPadTheme {
+  /// Light theme with a bright background.
+  light,
+
+  /// Dark theme with a dark background.
+  dark,
+
+  /// Theme that relies on DartPad's built-in theme handling.
+  auto,
+}
+
+/// The target origin to be used for cross-frame messages sent to
+/// the DartPad iframe's content window.
+///
+/// Uses '*' to enable communication with DartPad instances
+/// regardless of their actual origin.
+final JSString _anyTargetOrigin = '*'.toJS;
+
+/// Represents a ready message received from the DartPad iframe.
+///
+/// Sent by DartPad when it has finished loading and is ready to
+/// receive code updates by sending it a cross-frame message.
+extension type _EmbedReadyMessage._(JSObject _) {
+  /// The message type, which should be 'ready' for initialization messages.
+  external String? get type;
+
+  /// The sender ID to identify which DartPad instance sent the message.
+  external String? get sender;
+}
+
+/// Represents DartPad's expected format for receiving cross-frame messages
+/// from its parent window, usually the [EmbeddedDartPad] host.
+@anonymous
+extension type _MessageToDartPad._(JSObject _) implements JSObject {
+  /// Creates a JavaScript object with the expected structure for
+  /// updating the source code in an embedded DartPad's editor.
+  external factory _MessageToDartPad._updateSource({
+    required String sourceCode,
+    String type,
+  });
+
+  /// Creates a message to update that can be sent to
+  /// update the source code in an embedded DartPad instance.
+  ///
+  /// The [sourceCode] should generally be valid Dart code for
+  /// the latest stable versions of Dart and Flutter.
+  factory _MessageToDartPad.updateSource(String sourceCode) =>
+      _MessageToDartPad._updateSource(
+        sourceCode: sourceCode,
+        type: 'sourceCode',
+      );
+}