rety

“Live” coding without the stress

What is this?

Rety is a library that allows you to record the edits you make on one or more pieces of text (usually code) and replay them later to recreate the same typing flow.

This is particularly useful for orchestrating live demos that run without your presence.

It does not come with any particular UI, the UI is up to you. The UI you see in some of the demos in these docs is not part of Rety.

Here’s an example of using it together with the Inspire.js Live demo plugin to do live demos during a talk:

Background & Motivation

I love live coding as a teaching tool, and over the years it has become part of my trademark speaking style.

When combined with some kind of interactive preview, it allows the speaker to demonstrate not only the final state of a coding snippet, but how you get there, and what the intermediate results are. Live coding is to programming what a blackboard is to math or physics.

However, it does create a unique challenge: My live coded slides don't make sense without me. This may be acceptable for a conference talk, which is usually recorded, but not in other contexts, such as teaching a university course, where all instructors need to be able to teach all lectures, and students need to be able to quickly refer to examples shown.

I didn't want to remove live coding from my slides, as I truly believe it is the perfect implementation of the "show, don’t tell" teaching adage, so I thought instead: what if I could record my live coding, and make it replayable? However, doing so manually seemed like cruel and unusual punishment. And thus, rety was born (pronounced like the "rety" in "retype").

After using it extensively for my course at MIT, I started using it during actual conference talks as well, as it was strictly superior to actual live coding: It offered the same progressive development which is the primary benefit of live coding, but none of the fumbling, delays, or mistakes that often come with it. You can watch the first conference talk I did with it, at CSS Day 2022 here (first live demo at 7:15).

Rety is designed to work well with the code editors of Prism Live and CodeFlask but it should work with any <input>, <textarea> or even compatible custom elements.

Basic Usage

To record edits on a textarea (myTextarea):

import Recorder from "https://rety.verou.me/src/recorder.js";

let recorder = new Recorder(myTextarea);
recorder.start();

recorder.addEventListener("actionschange", evt => {
	// recorder.actions has been updated
	// evt.detail contains the new (or in some cases changed) action
});

To replay an array of actions (actionsArray) on a textarea (editor) with default settings:

import Replayer from "https://rety.verou.me/src/replayer.js";

let replayer = new Replayer(editor);
replayer.runAll(actionsArray);

Instead of importing directly from the CDN, you can also use npm:

npm install rety

API

Rety consists of two classes, residing in correspondingly named modules: Recorder and Replayer.

Recorder class

The Recorder class allows you to record actions on one or more <input>, <textarea>, or any recorder-compatible control.

let recorder = new Recorder(source);
recorder.start();

To record actions from a single editor, source can be a reference to that editor. To record actions from multiple editors, pass in an object literal with identifiers as keys and references to the elements as values.

E.g.

let recorder = new Recorder({
	css: document.querySelector("textarea#css"),
	html: document.querySelector("textarea#html")
});

The identifiers can be anything you want, e.g. to record actions in a multi-file editor environment, the ids could be the filenames.

Call recorder.start() to start recording and recorder.pause() to temporarily pause recording.

You will find any recorded actions in recorder.actions.

recorder is a subclass of EventTarget, meaning it emits events. You can listen to the actionschange event to respond to any change in the recorder.actions array.

Most changes to recorder.actions will be new actions being added at the end of the array. However, there are few cases where instead of adding a new action, the previous action is modified instead. This happens when the caret moves around or the selection is modified without any other action between changes. To preserve all caret changes as separate actions, you can use the preserveCaretChanges option:

let recorder = new Recorder(source, {preserveCaretChanges: true});

You can also provide options when calling start():

recorder.start({preserveCaretChanges: true});

Constructor: new Recorder(editor [, options])

Options:

| Option | Default | Description | |---|---|---| | preserveCaretChanges | false | If true, will not coalesce consecutive caret position changes | | pauseThreshold | 2000 | The delay (in ms) between consecutive actions that will cause a pause action to be inserted. Use 0 or false to disable pause actions entirely. | | pauses | undefined | Set to "ignore" to not record pauses entirely. | pauseCap | undefined | Set to a number of milliseconds to cap pauses to that duration. | keys | undefined | Keystrokes to record, see How do I record custom keystrokes that don’t produce output?

To record custom keystrokes (that don’t result in output), you’d use the keys parameter with strings like "Ctrl + Shift + E". You can specify one or more keystrokes as an array. By default the keyup event is monitored. You can specify a different event by using an object instead of a string, e.g. {key: "Ctrl + Shift + E", event: "keydown"}.

Methods

| Member | Description | |---|---| | recorder.start() | Start listening to edits | | recorder.pause() | Temporarily stop listening to edits |

Properties and accessors

| Member | Description | |---|---| | recorder.actions | The array of actions recorded so far.

Replayer class

The Replayer class allows you to run a single action or a sequence of actions on an <input>, <textarea>, or any replayer-compatible control.

Constructor: new Replayer(dest [, options])

dest is the same type as the first argument to the Recorder constructor. To replay actions on a single editor element, dest would be a reference to that element. To replay actions that span multiple editors, dest would be an object literal that maps ids to editor elements.

Options:

| Option | Default | Description | |---|---|---| | delay | 140 | Delay between consecutive actions when runAll() is used | | pauses | "delay" | What to do with pause actions? "delay" will just pause by that amount of time, "pause" will pause playback, "ignore" will discard them. You can also provide a function that decides which of these keywords to return based on the action specifics | | animated_selection | true | Should selections be animated or happen at once? |

Methods

| Member | Description | |---|---| | async replayer.runAll(actions) | Run a sequence of actions. Returns a promise that resolves when all actions have ran or the replayer has been paused. If another sequence of actions is currently being played, it will stop it first, then replace the rest of its queue. | async replayer.queueAll(actions) | Just like runAll() but instead of replacing the queue, it will add the actions to the existing queue. | async replayer.next() | Runs the next action in the queue | async replayer.run([action]) | Run a single action (except pauses, since this is pretty low-level and does not handle timing). | replayer.pause() | Finish the action currently executing (if any), then pause. | async replayer.resume() | Resumes playing the current queue.

Properties and Accessors

| Member | Description | |---|---| | recorder.queue | Contains the actions that have been queued up for playing, but have not been played yet. Can also be set, and the array it is set to will be (shallowly) cloned. | | recorder.paused | true if the Replayer is paused or stopped, false if playing, undefined if the Replayer has not yet been used in any way. | | recorder.played | Array with actions that have already been played from the current queue. These actions have been removed from the queue. |

FAQ

How do I record a demo from an arbitrary page, e.g. a live coded slide?

Drag this bookmarklet to your bookmarks toolbar: ⏺ Rety. Then, when you’re ready to record, press it. It will insert a button at the top right corner that you press to stop recording. When you stop recording, it will log the actions it recorded in the console, for your copying convenience. It will log them in two ways: both as an object, as well as a minified JSON serialization. Unfortunately, it does not yet allow customizing recording options.

What is the browser support?

Generally: all modern browsers. No IE11 or pre-Chromium Edge. More details:

• Recorder makes heavy use of evt.inputType so it supports browsers that support that
• Replayer makes heavy use of document.execCommand(), which limits Firefox support to Firefox 89+.
• Both are written with well-supported modern ES features, such as private members. Since these generally have better support than evt.inputType, I did not bother transpiling.

What about unit tests?

I’m still trying to decide what's the best testing framework for mocking interactions with a textarea. If you have suggestions, please weigh in!

Where is the minified version?

This is currently a tiny codebase, so minifying is more trouble than it’s worth. No, it makes zero difference if you save one KB. If in the future the code grows enough that minifying adds value, there will be a minified version.

If you really can’t live with a non-minified asset, you can always use the generated version by jsdelivr.

I want to record actions from a custom element, not a built-in <input> or <textarea>

Recorder will work fine with any control that meets the following requirements:

• Implements selectionStart and selectionEnd properties
• Implements a value property
• Emits input events with suitable inputType properties
• Emits select events when the selection changes

I want to run actions on a custom element, not a built-in <input> or <textarea>

Replayer will work fine with any control that meets the following requirements:

• Implements writable selectionStart and selectionEnd properties
• Works well with document.execCommand() (the actions used are insertText, delete, forwardDelete, undo, redo)

How do I record custom keystrokes that don’t produce output?

When constructing Recorder objects, you can pass in a keys parameter, which is an array of custom keystrokes to record. These keystrokes can be specified as strings, like Ctrl + Enter or objects (like {key: "Ctrl + Enter", event: "keydown"}), if you also want to specify an event (other than keyup which is the default).

Recorder will then record key actions that match this keystroke. Replayer does not need to be taught about custom keystrokes, it replicates any key action it finds.