Flows of energy

by Marijn Haverbeke (license)

ProseMirror

Tuesday, July 7, 2015 javascript prosemirror

Sometimes I lie awake at night, feverishly searching for new ways to load myself down with more poorly-paying responsibilities. And then it comes to me: I should start another open-source project!

Well, that's not really what happens. But the effect is the same. I keep building complex, demanding pieces of code and then giving them away. The actual mechanism is usually that I think of some technical concept, find out that it hasn't been done yet, and through some mix of curiosity and ego, just have to see if I can do it.

Here's the newest damage: ProseMirror, a browser-based rich text editor. Though I'm not giving it away per se, but running a crowd-funder to open-source it, and have thought a bit about how to make after-release maintenance sustainable.

An Editor?

Didn't I just talk about implementing things that have “not been done yet”? And aren't there at least a hundred browser-based rich text editors out there?

Yes, and yes. But none of the existing projects take the approach that I think would be ideal. Many of them are firmly rooted in the old paradigm of relying on contentEditable elements and then trying to sort of clean up the resulting mess. This gives us very little control over what the user and the browser are doing to our document.

What do we need control for? For one thing, it makes it much easier to keep the document in a sane state. If the document is only modified by your code, you can define these modifications so that they preserve the invariants you want to preserve, and you can ensure that the same thing happens on different browsers.

But more importantly, it allows you to represent these modification in a more abstract way than “something changed around here, and this is the new document state”. And that is extremely helpful when implementing collaborative editing—to effectively merge conflicting changes from multiple users, it helps to have an accurate representation of the intent of the changes.

General Approach

ProseMirror does create a contentEditable element that it displays its document in. This gives us all the logic related to focus and cursor motion for free, and makes it much, much easier to support screen readers and bidirectional text.

Any actual modifications made to the document are captured by handling the appropriate browser events, and converted to our own representation of these modifications. Assuming relatively modern browsers, this is easy for most types of changes. We can handle key events to capture typed text and things like backspace and enter. We can handle clipboard events to make copy, cut, and paste work. Drag and drop are also exposed through events. Even IME input fires relatively usable composition events.

Unfortunately, there are a few cases where browsers don't fire events that describe user intent, and all you get is an after-the-fact input event. This happens when picking a correction from the spell check menu, for example, and also when using key combinations to type special characters (for example “Multi-e =” to type “€” on Linux). Fortunately, all such cases that I have run across so far involve simple, character-level input. We can inspect the DOM, compare it to our representation of the document, and derive the intended modification from that.

When a modification is made the editor's representation of the document is changed, and then the display (the DOM element on the screen) is updated to reflect the new document. By using a persistent data structure for the document—making a change creates a new document object, without mutating the old one—we can use a very fast document-diffing algorithm to make only the DOM updates that are actually necessary. This is somewhat similar to what React and its various spin-offs do, except that ProseMirror works with its own representation of the document, not with a general-purpose DOM-like data structure.

The Document

This document representation is explicitly not HTML. It is also a “semantic” representation of the document, and a tree-shaped data structure that describes the structure of the text in terms of paragraphs, headings, lists, emphasis, links, and so on. It can be rendered as a DOM tree, but also as Markdown text, and any number of other formats that happen to be able to express the concepts it encodes.

The outer part of this representation, the part that deals with paragraphs, headings, lists, and so on, is especially DOM-like in its structure—it consists of nodes with child nodes. The content of paragraphs nodes (and other block elements such as headings) is represented as a flat sequence of inline elements, each of which has a set of styles associated with it. This works better than using a tree structure all the way, as the DOM does. It makes it easier to enforce invariants like not allowing text to be emphasized twice, and allows us to represent positions in paragraphs as simple character offsets, which are easier to reason about than positions in a tree.

Outside of paragraphs, we are forced to work with a tree. So a position in the document is represented by a path, which is a sequence of integers denoting the child indices at each level of the tree, and an offset into the node at the end of this path. This is how the cursor position is represented, and how the positions at which modifications should happen are stored.

ProseMirror's current document model mirrors that of Markdown, supporting precisely the things that can be expressed in that format. In the future, you will be able to extend and customize the document model you want to use in a given editor instance.

Interface

The editor currently comes with a two styles of user interface. One is the classical toolbar at the top. The other shows tooltips above your selection to do inline-level styling, and a menu button to the right of the currently selected paragraph for block-level actions. I rather like the latter, since it gets out of your way when you are not using it, but I expect many people will prefer the familiar toolbar.

But these interfaces are implemented as modules outside of the editor core, and other interface styles can be implemented on top of the same API.

Key bindings are also configurable, closely following CodeMirror's model. The functionality that keys are bound to is available as named commands, and can also be run from scripts using the execCommand method.

Finally, there is a module called inputrules that can be used to specify that something should happen when text matching a given pattern is typed. It can be used for things like “smart quotes”, but also to make a list appear if you type “1.” and press space.

Collaboration

I mentioned collaboration before. A significant amount of the work that went into this project went into making it support collaborative real-time editing. I wrote another blog post about the technical details, but the concepts is roughly this:

When a modification is applied to the document, it creates a new document as well as a position map, which maps positions in the old document to positions in the new document. This is needed to, for example, move the cursor in response to modifications.

Being able to map positions makes it possible to “rebase” modifications on top of other modifications by mapping the position(s) at which they should be applied. There's more to this, and I rewrote this aspect of the system several times before getting it right, but I am pretty confident I ended up with something that works.

In a collaborative scenario, when a client makes modifications, they are buffered locally and sent to the server. If another client submitted its modifications before ours arrive, the server responds with “nope, apply these modifications first”, and the client takes the modifications, rebases its own ones on top of them, and tries again. When they go through, they are broadcast to all other clients, making sure everybody stays synchronized.

Target Audience

Who do I expect to use this?

Sounds interesting? See how the crowd funding campaign to open-source this is doing.