[video_player_web] Listen to loadedmetadata event from video element. (flutter#5289)

This PR configures the underlying HTMLVideoElement used by the plugin so it triggers a `VideoEventType.initialized` both for `canplay` (current behavior) and `loadedmetadata` events (fixes iOS 17).

It also sets `src` as the last configuration value when creating the VideoElement, which is common practice when listening to events from HTMLElements, like `img` or even `iframe`s.

### Issues

* Fixes: **P1** flutter#137023

### Tests

* Added a small integration test to ensure `loadedmetadata` triggers the expected VideoPlayer event.
* Deployed changes to: https://dit-videoplayer-tests.web.app for manual verification on an actual iOS device.

Author: ditman

packages/video_player/video_player_web/CHANGELOG.md

@@ -1,3 +1,9 @@
+## 2.1.2
+
+* Listens to `loadedmetadata` as an event that marks that initialization is
+  complete. (Fixes playback in Safari iOS 17).
+* Sets the `src` of the underlying video element after every other attribute.
+
 ## 2.1.1
 
 * Ensures that the `autoplay` attribute of the underlying video element is set

packages/video_player/video_player_web/example/integration_test/video_player_test.dart

@@ -197,6 +197,23 @@ void main() {
         expect(events[0].eventType, VideoEventType.initialized);
       });
 
+      // Issue: https://github.com/flutter/flutter/issues/137023
+      testWidgets('loadedmetadata dispatches initialized',
+          (WidgetTester tester) async {
+        video.dispatchEvent(html.Event('loadedmetadata'));
+        video.dispatchEvent(html.Event('loadedmetadata'));
+
+        final Future<List<VideoEvent>> stream = timedStream
+            .where((VideoEvent event) =>
+                event.eventType == VideoEventType.initialized)
+            .toList();
+
+        final List<VideoEvent> events = await stream;
+
+        expect(events, hasLength(1));
+        expect(events[0].eventType, VideoEventType.initialized);
+      });
+
       // Issue: https://github.com/flutter/flutter/issues/105649
       testWidgets('supports `Infinity` duration', (WidgetTester _) async {
         setInfinityDuration(video);

packages/video_player/video_player_web/lib/src/video_player.dart

@@ -58,7 +58,17 @@ class VideoPlayer {
   /// This method sets the required DOM attributes so videos can [play] programmatically,
   /// and attaches listeners to the internal events from the [html.VideoElement]
   /// to react to them / expose them through the [VideoPlayer.events] stream.
-  void initialize() {
+  ///
+  /// The [src] parameter is the URL of the video. It is passed in from the plugin
+  /// `create` method so it can be set in the VideoElement *last*. This way, all
+  /// the event listeners needed to integrate the videoElement with the plugin
+  /// are attached before any events start firing (events start to fire when the
+  /// `src` attribute is set).
+  ///
+  /// The `src` parameter is nullable for testing purposes.
+  void initialize({
+    String? src,
+  }) {
     _videoElement
       ..autoplay = false
       ..controls = false;
@@ -68,14 +78,11 @@ class VideoPlayer {
     // This property is not exposed through dart:html so we use the
     // HTML Boolean attribute form (when present with any value => true)
     // See: https://developer.mozilla.org/en-US/docs/Glossary/Boolean/HTML
-    _videoElement.setAttribute('playsinline', 'true');
+    _videoElement.setAttribute('playsinline', true);
 
-    _videoElement.onCanPlay.listen((dynamic _) {
-      if (!_isInitialized) {
-        _isInitialized = true;
-        _sendInitialized();
-      }
-    });
+    _videoElement.onCanPlay.listen(_onVideoElementInitialization);
+    // Needed for Safari iOS 17, which may not send `canplay`.
+    _videoElement.onLoadedMetadata.listen(_onVideoElementInitialization);
 
     _videoElement.onCanPlayThrough.listen((dynamic _) {
       setBuffering(false);
@@ -122,6 +129,12 @@ class VideoPlayer {
       setBuffering(false);
       _eventController.add(VideoEvent(eventType: VideoEventType.completed));
     });
+
+    // The `src` of the _videoElement is the last property that is set, so all
+    // the listeners for the events that the plugin cares about are attached.
+    if (src != null) {
+      _videoElement.src = src;
+    }
   }
 
   /// Attempts to play the video.
@@ -252,6 +265,20 @@ class VideoPlayer {
     _videoElement.load();
   }
 
+  // Handler to mark (and broadcast) when this player [_isInitialized].
+  //
+  // (Used as a JS event handler for "canplay" and "loadedmetadata")
+  //
+  // This function can be called multiple times by different JS Events, but it'll
+  // only broadcast an "initialized" event the first time it's called, and ignore
+  // the rest of the calls.
+  void _onVideoElementInitialization(Object? _) {
+    if (!_isInitialized) {
+      _isInitialized = true;
+      _sendInitialized();
+    }
+  }
+
   // Sends an [VideoEventType.initialized] [VideoEvent] with info about the wrapped video.
   void _sendInitialized() {
     final Duration? duration =

packages/video_player/video_player_web/lib/video_player_web.dart

@@ -75,7 +75,6 @@ class VideoPlayerPlugin extends VideoPlayerPlatform {
 
     final VideoElement videoElement = VideoElement()
       ..id = 'videoElement-$textureId'
-      ..src = uri
       ..style.border = 'none'
       ..style.height = '100%'
       ..style.width = '100%';
@@ -85,7 +84,9 @@ class VideoPlayerPlugin extends VideoPlayerPlatform {
         'videoPlayer-$textureId', (int viewId) => videoElement);
 
     final VideoPlayer player = VideoPlayer(videoElement: videoElement)
-      ..initialize();
+      ..initialize(
+        src: uri,
+      );
 
     _videoPlayers[textureId] = player;
 

packages/video_player/video_player_web/pubspec.yaml

@@ -2,7 +2,7 @@ name: video_player_web
 description: Web platform implementation of video_player.
 repository: https://github.com/flutter/packages/tree/main/packages/video_player/video_player_web
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+video_player%22
-version: 2.1.1
+version: 2.1.2
 
 environment:
   sdk: ">=3.1.0 <4.0.0"