extract RetainedValue from MemberList

src/matrix/room/members/MemberList.js

@@ -15,15 +15,15 @@ limitations under the License.
 */
 
 import {ObservableMap} from "../../../observable/map/ObservableMap.js";
+import {RetainedValue} from "../../../utils/RetainedValue.js";
 
-export class MemberList {
+export class MemberList extends RetainedValue {
     constructor({members, closeCallback}) {
+        super(closeCallback);
         this._members = new ObservableMap();
         for (const member of members) {
             this._members.add(member.userId, member);
         }
-        this._closeCallback = closeCallback;
-        this._retentionCount = 1;
     }
 
     afterSync(memberChanges) {
@@ -35,15 +35,4 @@ export class MemberList {
     get members() {
         return this._members;
     }
-
-    retain() {
-        this._retentionCount += 1;
-    }
-
-    release() {
-        this._retentionCount -= 1;
-        if (this._retentionCount === 0) {
-            this._closeCallback();
-        }
-    }
 }

src/utils/RetainedValue.js

@@ -0,0 +1,33 @@
+/*
+Copyright 2021 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.
+*/
+
+export class RetainedValue {
+    constructor(freeCallback) {
+        this._freeCallback = freeCallback;
+        this._retentionCount = 1;
+    }
+
+    retain() {
+        this._retentionCount += 1;
+    }
+
+    release() {
+        this._retentionCount -= 1;
+        if (this._retentionCount === 0) {
+            this._freeCallback();
+        }
+    }
+}