Create a UIStore to track important data

This helper should hold data related to the UI and access save in a smart to avoid performance pitfalls in other parts of the application

src/stores/UIStore.ts

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+/*
+Copyright 2020 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 EventEmitter from "events";
+
+export enum UI_EVENTS {
+    Resize = "resize"
+}
+
+export type ResizeObserverCallbackFunction = (entries: ResizeObserverEntry[]) => void;
+
+
+export default class UIStore extends EventEmitter {
+    private static _instance: UIStore = null;
+
+    private resizeObserver: ResizeObserver;
+
+    public windowWith: number;
+    public windowHeight: number;
+
+    constructor() {
+        super();
+
+        this.windowWith = window.innerWidth;
+        this.windowHeight = window.innerHeight;
+
+        this.resizeObserver = new ResizeObserver(this.resizeObserverCallback);
+        this.resizeObserver.observe(document.body);
+    }
+
+    public static get instance(): UIStore {
+        if (!UIStore._instance) {
+            UIStore._instance = new UIStore();
+        }
+        return UIStore._instance;
+    }
+
+    public static destroy(): void {
+        if (UIStore._instance) {
+            UIStore._instance.resizeObserver.disconnect();
+            UIStore._instance.removeAllListeners();
+            UIStore._instance = null;
+        }
+    }
+
+    private resizeObserverCallback = (entries: ResizeObserverEntry[]) => {
+        const { width, height } = entries
+            .find(entry => entry.target === document.body)
+            .contentRect;
+
+        this.windowWith = width;
+        this.windowHeight = height;
+
+        this.emit(UI_EVENTS.Resize, entries);
+    }
+}