Convert preload.js to Typescript so that it gets copied to lib where we expect it

src/preload.ts

@@ -0,0 +1,71 @@
+/*
+Copyright 2018, 2019, 2021 New Vector Ltd
+
+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 { ipcRenderer, desktopCapturer, contextBridge, IpcRendererEvent, SourcesOptions } from 'electron';
+
+// Expose only expected IPC wrapper APIs to the renderer process to avoid
+// handing out generalised messaging access.
+
+const CHANNELS = [
+    "app_onAction",
+    "before-quit",
+    "check_updates",
+    "install_update",
+    "ipcCall",
+    "ipcReply",
+    "loudNotification",
+    "preferences",
+    "seshat",
+    "seshatReply",
+    "setBadgeCount",
+    "update-downloaded",
+    "userDownloadCompleted",
+    "userDownloadOpen",
+];
+
+contextBridge.exposeInMainWorld(
+    "electron",
+    {
+        on(channel: string, listener: (event: IpcRendererEvent, ...args: any[]) => void) {
+            if (!CHANNELS.includes(channel)) {
+                console.error(`Unknown IPC channel ${channel} ignored`);
+                return;
+            }
+            ipcRenderer.on(channel, listener);
+        },
+        send(channel: string, ...args: any[]) {
+            if (!CHANNELS.includes(channel)) {
+                console.error(`Unknown IPC channel ${channel} ignored`);
+                return;
+            }
+            ipcRenderer.send(channel, ...args);
+        },
+        async getDesktopCapturerSources(options: SourcesOptions) {
+            const sources = await desktopCapturer.getSources(options);
+            const desktopCapturerSources = [];
+
+            for (const source of sources) {
+                desktopCapturerSources.push({
+                    id: source.id,
+                    name: source.name,
+                    thumbnailURL: source.thumbnail.toDataURL(),
+                });
+            }
+
+            return desktopCapturerSources;
+        },
+    },
+);