utility: observable value that emits when event is fired

src/observable/value/EventObservableValue.ts

@@ -0,0 +1,45 @@
+/*
+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 {BaseObservableValue} from "./BaseObservableValue";
+import {EventEmitter} from "../../utils/EventEmitter";
+
+export class EventObservableValue<T, V extends EventEmitter<T>> extends BaseObservableValue<V> {
+    private eventSubscription: () => void;
+
+    constructor(
+        private readonly value: V,
+        private readonly eventName: keyof T
+    ) {
+        super();
+    }
+
+    onSubscribeFirst(): void {
+        this.eventSubscription = this.value.disposableOn(this.eventName, () => {
+            this.emit(this.value);
+        });
+        super.onSubscribeFirst();
+    }
+
+    onUnsubscribeLast(): void {
+        this.eventSubscription!();
+        super.onUnsubscribeLast();
+    }
+
+    get(): V {
+        return this.value;
+    }
+}