terminal: add audio/visual feedback when no user program

Characters typed into the terminal are only sent to the hub when a user
program is running. This provides audio and visual feedback to indicate
to the user that keystrokes have no effect when the user program is not
running.

CHANGELOG.md

@@ -4,6 +4,9 @@
 
 ## [Unreleased]
 
+### Added
+- Added audio and visual feedback when typing into terminal while user program is not running.
+
 ## [2.0.1] - 2022-12-21
 
 ### Fixed

src/terminal/Terminal.tsx

@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: MIT
 // Copyright (c) 2020-2022 The Pybricks Authors
 
+import 'xterm/css/xterm.css';
+import './terminal.scss';
 import { Menu, MenuDivider, MenuItem, ResizeSensor } from '@blueprintjs/core';
 import { ContextMenu2, ContextMenu2ContentProps } from '@blueprintjs/popover2';
 import React, { useContext, useEffect, useMemo, useRef } from 'react';
@@ -13,7 +15,12 @@ import { TerminalContext } from './TerminalContext';
 import { receiveData } from './actions';
 import { useI18n } from './i18n';
 
-import 'xterm/css/xterm.css';
+// Source: https://freesound.org/people/altemark/sounds/45759/
+// This sound is released under the Creative Commons Attribution 3.0 Unported
+// (CC BY 3.0) license. It was created by 'altemark'. No modifications have been
+// made, apart from the conversion to base64.
+const BELL_SOUND =
+    'data:audio/mp3;base64,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';
 
 function handleKeyEvent(event: KeyboardEvent): boolean {
     if (
@@ -186,12 +193,46 @@ const Terminal: React.FC = (_props) => {
         return () => removeEventListener('pb-terminal-focus', listener);
     }, [xterm]);
 
+    // audio and visual notification of bell
+
+    const bellRef = useRef<HTMLAudioElement>(null);
+    const bellOverlayRef = useRef<HTMLDivElement>(null);
+    const bellTimeoutRef = useRef<ReturnType<typeof setTimeout>>();
+
+    useEffect(() => {
+        const audioElement = bellRef.current;
+        const overlayElement = bellOverlayRef.current;
+
+        if (process.env.NODE_ENV === 'test' || !audioElement || !overlayElement) {
+            return;
+        }
+
+        const subscription = xterm.onBell(() => {
+            if (bellTimeoutRef.current) {
+                clearTimeout(bellTimeoutRef.current);
+            }
+
+            audioElement.play();
+            overlayElement.classList.add('pb-bell');
+
+            bellTimeoutRef.current = setTimeout(() => {
+                overlayElement.classList.remove('pb-bell');
+            }, 150);
+        });
+
+        return () => subscription.dispose();
+    }, [xterm, bellRef, bellOverlayRef, bellTimeoutRef]);
+
     return (
         <ContextMenu2
             className="h-100"
             content={contextMenu}
             popoverProps={{ onClosed: () => xterm.focus() }}
         >
+            <audio hidden preload="auto" ref={bellRef}>
+                <source src={BELL_SOUND} />
+            </audio>
+            <div className="pb-terminal-bell-overlay" ref={bellOverlayRef} />
             <ResizeSensor onResize={(): void => fitAddon.fit()}>
                 <div ref={terminalRef} className="h-100" />
             </ResizeSensor>

src/terminal/sagas.ts

@@ -79,6 +79,8 @@ function* receiveTerminalData(): Generator {
         // it should be fine as long a the logic for the state doesn't change.
         if (!isUserProgramRunning) {
             // if no user program is running, input goes to /dev/null
+            // print the BEL character (^G) to notify the user that the input was ignored
+            yield* put(sendData('\x07'));
             continue;
         }
 

src/terminal/terminal.scss

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2022 The Pybricks Authors
+
+@use '@blueprintjs/core/lib/scss/variables' as bp;
+
+.pb-terminal-bell-overlay {
+    position: absolute;
+    top: 0;
+    left: 0;
+    width: 100%;
+    height: 100%;
+    z-index: bp.$pt-z-index-content - 1;
+    pointer-events: none;
+
+    &.pb-bell {
+        background-color: adjust-color(bp.$pt-intent-danger, $alpha: -0.5);
+        transition: linear 100ms;
+    }
+}