Voice Broadcast live state / extract RelationsHelper (#9432)

* Extract RelationsHelper

* Make RelationsHelper.relations optional

src/events/RelationsHelper.ts

@@ -0,0 +1,98 @@
+/*
+Copyright 2022 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { MatrixClient, MatrixEvent, MatrixEventEvent, RelationType } from "matrix-js-sdk/src/matrix";
+import { Relations, RelationsEvent } from "matrix-js-sdk/src/models/relations";
+import { TypedEventEmitter } from "matrix-js-sdk/src/models/typed-event-emitter";
+
+import { IDestroyable } from "../utils/IDestroyable";
+
+export enum RelationsHelperEvent {
+    Add = "add",
+}
+
+interface EventMap {
+    [RelationsHelperEvent.Add]: (event: MatrixEvent) => void;
+}
+
+/**
+ * Helper class that manages a specific event type relation for an event.
+ * Just create an instance and listen for new events for that relation.
+ * Optionally receive the current events by calling emitCurrent().
+ * Clean up everything by calling destroy().
+ */
+export class RelationsHelper
+    extends TypedEventEmitter<RelationsHelperEvent, EventMap>
+    implements IDestroyable {
+    private relations?: Relations;
+
+    public constructor(
+        private event: MatrixEvent,
+        private relationType: RelationType,
+        private relationEventType: string,
+        private client: MatrixClient,
+    ) {
+        super();
+        this.setUpRelations();
+    }
+
+    private setUpRelations = (): void => {
+        this.setRelations();
+
+        if (this.relations) {
+            this.relations.on(RelationsEvent.Add, this.onRelationsAdd);
+        } else {
+            this.event.once(MatrixEventEvent.RelationsCreated, this.onRelationsCreated);
+        }
+    };
+
+    private onRelationsCreated = (): void => {
+        this.setRelations();
+
+        if (this.relations) {
+            this.relations.on(RelationsEvent.Add, this.onRelationsAdd);
+            this.emitCurrent();
+        } else {
+            this.event.once(MatrixEventEvent.RelationsCreated, this.onRelationsCreated);
+        }
+    };
+
+    private setRelations(): void {
+        const room = this.client.getRoom(this.event.getRoomId());
+        this.relations = room?.getUnfilteredTimelineSet()?.relations?.getChildEventsForEvent(
+            this.event.getId(),
+            this.relationType,
+            this.relationEventType,
+        );
+    }
+
+    private onRelationsAdd = (event: MatrixEvent): void => {
+        this.emit(RelationsHelperEvent.Add, event);
+    };
+
+    public emitCurrent(): void {
+        this.relations?.getRelations()?.forEach(e => this.emit(RelationsHelperEvent.Add, e));
+    }
+
+    public destroy(): void {
+        this.removeAllListeners();
+        this.event.off(MatrixEventEvent.RelationsCreated, this.onRelationsCreated);
+
+        if (this.relations) {
+            this.relations.off(RelationsEvent.Add, this.onRelationsAdd);
+        }
+    }
+}