Use `subscribe` to listen to `beforeAnnotationCreated` events

This removes the last remaining usage of the `events` property of
`Delegator`, enabling that code to be removed.

src/annotator/plugin/document.js

@@ -69,15 +69,26 @@ export default class DocumentMeta extends Plugin {
   constructor(element, options) {
     super(element, options);
 
-    this.events = {
-      beforeAnnotationCreated: 'beforeAnnotationCreated',
-    };
-
     this.metadata = createMetadata();
 
     this.baseURI = options.baseURI || baseURI;
     this.document = options.document || document;
     this.normalizeURI = options.normalizeURI || normalizeURI;
+
+    /**
+     * Event handler that adds document metadata to new annotations.
+     */
+    this.beforeAnnotationCreated = annotation => {
+      annotation.document = this.metadata;
+    };
+
+    // @ts-expect-error - Method comes from CoffeeScript base class.
+    this.subscribe('beforeAnnotationCreated', this.beforeAnnotationCreated);
+  }
+
+  destroy() {
+    // @ts-expect-error - Method comes from CoffeeScript base class.
+    this.unsubscribe('beforeAnnotationCreated', this.beforeAnnotationCreated);
   }
 
   pluginInit() {
@@ -114,14 +125,6 @@ export default class DocumentMeta extends Plugin {
     return Object.keys(uniqueUrls);
   }
 
-  /**
-   * Hook that augments new annotations with metadata about the document they
-   * came from.
-   */
-  beforeAnnotationCreated(annotation) {
-    annotation.document = this.metadata;
-  }
-
   /**
    * Return metadata for the current page.
    */

src/annotator/plugin/test/document-test.js

@@ -31,14 +31,27 @@ describe('DocumentMeta', function () {
       return normalizeURI(url, base);
     });
 
-    testDocument = new DocumentMeta(tempDocument, {
+    // Root element to use for the `Delegator` event bus. This can be different
+    // than the document from which metadata is gathered.
+    const rootElement = document.body;
+
+    testDocument = new DocumentMeta(rootElement, {
       document: tempDocument,
       normalizeURI: fakeNormalizeURI,
     });
     testDocument.pluginInit();
   });
 
-  afterEach(() => $(document).unbind());
+  afterEach(() => {
+    testDocument.destroy();
+    $(document).unbind();
+  });
+
+  it('attaches document metadata to new annotations', () => {
+    const annotation = {};
+    testDocument.publish('beforeAnnotationCreated', [annotation]);
+    assert.equal(annotation.document, testDocument.metadata);
+  });
 
   describe('annotation should have some metadata', function () {
     let metadata = null;