utility: turn observable value into observable map with one K,V pair

src/observable/map/ObservableValueMap.ts

@@ -0,0 +1,53 @@
+/*
+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 {BaseObservableMap} from "./BaseObservableMap";
+import {BaseObservableValue} from "../value/BaseObservableValue";
+import {SubscriptionHandle} from "../BaseObservable";
+
+export class ObservableValueMap<K, V> extends BaseObservableMap<K, V> {
+    private subscription?: SubscriptionHandle;
+
+    constructor(private readonly key: K, private readonly observableValue: BaseObservableValue<V>) {
+        super();
+    }
+
+    onSubscribeFirst() {
+        this.subscription = this.observableValue.subscribe(value => {
+            this.emitUpdate(this.key, value, undefined);
+        });
+        super.onSubscribeFirst();
+    }
+
+    onUnsubscribeLast() {
+        this.subscription!();
+        super.onUnsubscribeLast();
+    }
+
+    *[Symbol.iterator](): Iterator<[K, V]> {
+        yield [this.key, this.observableValue.get()];
+    }
+    
+    get size(): number {
+        return 1;
+    }
+
+    get(key: K): V | undefined {
+        if (key == this.key) {
+            return this.observableValue.get();
+        }
+    }
+}