improve user experience around mic/camera permission

- ask for permission when trying to use a feature, not on startup
- if permission is denied, explain the consequence and provide a hint
  for fixing it

src/main/index.js

@@ -18,22 +18,48 @@ const isDevelopment = process.env.NODE_ENV !== 'production';
 // global window references prevent them from being garbage-collected
 const _windows = {};
 
-const createWindow = ({search = null, url = 'index.html', ...browserWindowOptions}) => {
-    const window = new BrowserWindow({
-        useContentSize: true,
-        show: false,
-        webPreferences: {
-            nodeIntegration: true
-        },
-        ...browserWindowOptions
-    });
-    const webContents = window.webContents;
-
-    if (isDevelopment) {
-        webContents.openDevTools({mode: 'detach', activate: true});
+const displayPermissionDeniedWarning = (browserWindow, permissionType) => {
+    let title;
+    let message;
+    switch (permissionType) {
+    case 'camera':
+        title = 'Camera Permission Denied';
+        message = 'Permission to use the camera has been denied. ' +
+            'Scratch will not be able to take a photo or use video sensing blocks.';
+        break;
+    case 'microphone':
+        title = 'Microphone Permission Denied';
+        message = 'Permission to use the microphone has been denied. ' +
+            'Scratch will not be able to record sounds or detect loudness.';
+        break;
+    default: // shouldn't ever happen...
+        title = 'Permission Denied';
+        message = 'A permission has been denied.';
     }
 
-    const fullUrl = formatUrl(isDevelopment ?
+    let instructions;
+    switch (process.platform) {
+    case 'darwin':
+        instructions = 'To change Scratch permissions, please check "Security & Privacy" in System Preferences.';
+        break;
+    default:
+        instructions = 'To change Scratch permissions, please check your system settings and restart Scratch.';
+        break;
+    }
+    message = `${message}\n\n${instructions}`;
+
+    dialog.showMessageBox(browserWindow, {type: 'warning', title, message});
+};
+
+/**
+ * Build an absolute URL from a relative one, optionally adding search query parameters.
+ * The base of the URL will depend on whether or not the application is running in development mode.
+ * @param {string} url - the relative URL, like 'index.html'
+ * @param {*} search - the optional "search" parameters (the part of the URL after '?'), like "route=about"
+ * @returns {string} - an absolute URL as a string
+ */
+const makeFullUrl = (url, search = null) =>
+    encodeURI(formatUrl(isDevelopment ?
         { // Webpack Dev Server
             hostname: 'localhost',
             pathname: url,
@@ -47,7 +73,77 @@ const createWindow = ({search = null, url = 'index.html', ...browserWindowOption
             search,
             slashes: true
         }
-    );
+    ));
+
+const handlePermissionRequest = async (webContents, permission, callback, details) => {
+    if (webContents !== _windows.main.webContents) {
+        // deny: request came from somewhere other than the main window's web contents
+        return callback(false);
+    }
+    if (!details.isMainFrame) {
+        // deny: request came from a subframe of the main window, not the main frame
+        return callback(false);
+    }
+    if (permission !== 'media') {
+        // deny: request is for some other kind of access like notifications or pointerLock
+        return callback(false);
+    }
+    const requiredBase = makeFullUrl('/');
+    if (details.requestingUrl.indexOf(requiredBase) !== 0) {
+        // deny: request came from a URL outside of our "sandbox"
+        return callback(false);
+    }
+    let askForMicrophone = false;
+    let askForCamera = false;
+    for (const mediaType of details.mediaTypes) {
+        switch (mediaType) {
+        case 'audio':
+            askForMicrophone = true;
+            break;
+        case 'video':
+            askForCamera = true;
+            break;
+        default:
+            // deny: unhandled media type
+            return callback(false);
+        }
+    }
+    const parentWindow = _windows.main; // if we ever allow media in non-main windows we'll also need to change this
+    if (askForMicrophone) {
+        const microphoneResult = await systemPreferences.askForMediaAccess('microphone');
+        if (!microphoneResult) {
+            displayPermissionDeniedWarning(parentWindow, 'microphone');
+            return callback(false);
+        }
+    }
+    if (askForCamera) {
+        const cameraResult = await systemPreferences.askForMediaAccess('camera');
+        if (!cameraResult) {
+            displayPermissionDeniedWarning(parentWindow, 'camera');
+            return callback(false);
+        }
+    }
+    return callback(true);
+};
+
+const createWindow = ({search = null, url = 'index.html', ...browserWindowOptions}) => {
+    const window = new BrowserWindow({
+        useContentSize: true,
+        show: false,
+        webPreferences: {
+            nodeIntegration: true
+        },
+        ...browserWindowOptions
+    });
+    const webContents = window.webContents;
+
+    webContents.session.setPermissionRequestHandler(handlePermissionRequest);
+
+    if (isDevelopment) {
+        webContents.openDevTools({mode: 'detach', activate: true});
+    }
+
+    const fullUrl = makeFullUrl(url, search);
     window.loadURL(fullUrl);
 
     return window;
@@ -140,10 +236,6 @@ const createMainWindow = () => {
 if (process.platform === 'darwin') {
     const osxMenu = Menu.buildFromTemplate(MacOSMenu(app));
     Menu.setApplicationMenu(osxMenu);
-    (async () => {
-        await systemPreferences.askForMediaAccess('microphone');
-        await systemPreferences.askForMediaAccess('camera');
-    })();
 } else {
     // disable menu for other platforms
     Menu.setApplicationMenu(null);