fix: Dispatch all emsg boxes, even if they are ID3

lib/media/streaming_engine.js

@@ -1982,7 +1982,6 @@ shaka.media.StreamingEngine = class {
     const messageData = box.reader.readBytes(
         box.reader.getLength() - box.reader.getPosition());
 
-
     // See DASH sec. 5.10.3.3.1
     // If a DASH client detects an event message box with a scheme that is not
     // defined in MPD, the client is expected to ignore it.
@@ -1992,23 +1991,8 @@ shaka.media.StreamingEngine = class {
       // A special scheme in DASH used to signal manifest updates.
       if (schemeId == 'urn:mpeg:dash:event:2012') {
         this.playerInterface_.onManifestUpdate();
-      } else if (schemeId == 'https://aomedia.org/emsg/ID3' ||
-        schemeId == 'https://developer.apple.com/streaming/emsg-id3') {
-        // See https://aomediacodec.github.io/id3-emsg/
-        const frames = shaka.util.Id3Utils.getID3Frames(messageData);
-        if (frames.length && reference) {
-          /** @private {shaka.extern.ID3Metadata} */
-          const metadata = {
-            cueTime: reference.startTime,
-            data: messageData,
-            frames: frames,
-            dts: reference.startTime,
-            pts: reference.startTime,
-          };
-          this.playerInterface_.onMetadata(
-              [metadata], /* offset= */ 0, reference.endTime);
-        }
       } else {
+        // All other schemes are dispatched as a general 'emsg' event.
         /** @type {shaka.extern.EmsgInfo} */
         const emsg = {
           startTime: startTime,
@@ -2026,7 +2010,38 @@ shaka.media.StreamingEngine = class {
         const eventName = shaka.util.FakeEvent.EventName.Emsg;
         const data = (new Map()).set('detail', emsg);
         const event = new shaka.util.FakeEvent(eventName, data);
+        // A user can call preventDefault() on a cancelable event.
+        event.cancelable = true;
+
         this.playerInterface_.onEvent(event);
+
+        if (event.defaultPrevented) {
+          // If the caller uses preventDefault() on the 'emsg' event, don't
+          // process any further, and don't generate an ID3 'metadata' event
+          // for the same data.
+          return;
+        }
+
+        // Additionally, ID3 events generate a 'metadata' event.  This is a
+        // pre-parsed version of the metadata blob already dispatched in the
+        // 'emsg' event.
+        if (schemeId == 'https://aomedia.org/emsg/ID3' ||
+            schemeId == 'https://developer.apple.com/streaming/emsg-id3') {
+          // See https://aomediacodec.github.io/id3-emsg/
+          const frames = shaka.util.Id3Utils.getID3Frames(messageData);
+          if (frames.length && reference) {
+            /** @private {shaka.extern.ID3Metadata} */
+            const metadata = {
+              cueTime: reference.startTime,
+              data: messageData,
+              frames: frames,
+              dts: reference.startTime,
+              pts: reference.startTime,
+            };
+            this.playerInterface_.onMetadata(
+                [metadata], /* offset= */ 0, reference.endTime);
+          }
+        }
       }
     }
   }

lib/player.js

@@ -99,7 +99,9 @@ goog.requireType('shaka.routing.Payload');
 
 /**
  * @event shaka.Player.EmsgEvent
- * @description Fired when a non-typical emsg is found in a segment.
+ * @description Fired when an emsg box is found in a segment.
+ *   If the application calls preventDefault() on this event, further parsing
+ *   will not happen, and no 'metadata' event will be raised for ID3 payloads.
  * @property {string} type
  *   'emsg'
  * @property {shaka.extern.EmsgInfo} detail

test/media/streaming_engine_unit.js

@@ -3130,21 +3130,45 @@ describe('StreamingEngine', () => {
       expect(onManifestUpdate).toHaveBeenCalled();
     });
 
-    it('triggers metadata event', async () => {
+    it('triggers both emsg event and metadata event for ID3', async () => {
       setSegment0(emsgSegmentV0ID3);
       videoStream.emsgSchemeIdUris = [id3SchemeUri];
 
+      onEvent.and.callFake((emsgEvent) => {
+        expect(emsgEvent.type).toBe('emsg');
+      });
+
       // Here we go!
       streamingEngine.switchVariant(variant);
       streamingEngine.switchTextStream(textStream);
       await streamingEngine.start();
       playing = true;
       await runTest();
 
-      expect(onEvent).not.toHaveBeenCalled();
+      expect(onEvent).toHaveBeenCalled();
       expect(onMetadata).toHaveBeenCalled();
     });
 
+    it('only triggers emsg event for ID3 if event canceled', async () => {
+      setSegment0(emsgSegmentV0ID3);
+      videoStream.emsgSchemeIdUris = [id3SchemeUri];
+
+      onEvent.and.callFake((emsgEvent) => {
+        expect(emsgEvent.type).toBe('emsg');
+        emsgEvent.preventDefault();
+      });
+
+      // Here we go!
+      streamingEngine.switchVariant(variant);
+      streamingEngine.switchTextStream(textStream);
+      await streamingEngine.start();
+      playing = true;
+      await runTest();
+
+      expect(onEvent).toHaveBeenCalled();
+      expect(onMetadata).not.toHaveBeenCalled();
+    });
+
     it('event start matches presentation time', async () => {
       // This box has a non-zero event time, which doesn't matter.
       setSegment0(emsgSegmentV1NonZeroStart);