Reactive MVC and the Virtual DOM | Hacker News Reader

Reactive MVC and the Virtual DOM

futurice.com

113 pointsjessaustin22 comments

> The gist is to frequently re-render a complete and lightweight representation of the DOM, then apply a difference filter to detect the minimum changes that need to be made to the DOM. A similar technique has existed in game development long before React: re-render the game screen in every game loop, but only update the minimum portion of the screen which changed compared to the previously rendered screen.

Practically every desktop UI works that way (Windows API, X Windows, Java UI libs). It's funny to see how web programming in 2015 slowly reinvents the wheels known since 1970s.

The browser that is rendering the DOM also, ultimately, works that way, with various portions of the browser screen redrawing while others remain static.

So it's kinda funny that the modern state of web dev is adding another layer of that same approach on top of layers of that approach.

Yes, yes! I also think about this.

React's virtual DOM (and other virtual DOM implementations) aren't just "adding another layer of that same approach," because the browser's DOM doesn't give you any way of diffing against a previous DOM state then performing the minimal operations needed to realize the new state.

The point is that the browser internally maintains an analogue of the virtual DOM in React, and compares the diffs so only some parts of the screen must be updated.

The missing part in the DOM is:

  freeze()

  doLotsOfUpdates()

  thaw()

So one would have to reinvent this. Kind of reminds of propeties in some languages, runtimes and their UI's:

  someForm.setWidth(500); // And it's updated right away on the screen, instead of being deferred, or put in some freeze()/thaw() mode. (disableUpdates() / enabledUpdates()).

Is there any ongoing effort to add 'freeze' and 'thaw' to the standard? Or any browsers that have added them?

Isn't requestAnimationFrame basically that?

Our scripts are executed in a freeze mode.

When we modify DOM it becomes "dirty", and after our script finishes, reflow/re-render happens. But also, if our script queries DOM properties, like elem.clientHeight, DOM may need to perform the calculations in order to return us correct values, reflecting all the recent property changes. For example, if we access element.clientHeight browser may need to calculate re-flow (I recently encountered very slow clientHeight calculation by FireFox).

So, if we only modify DOM properties, all the re-rendering is deferred till the end of script.

Google about "DOM reflow"

To be fair, the DOM is that abstraction layer already. It just performs horribly.

I've been wondering about additional benefits to abstracting away the DOM, about how practical it would be to then have UIs constructed in different environments that runs off the same base logic.

I suppose that's the value proposition of React Native; write your javascript logic once and just rewrite the render function for Android, iOS, web, any any future target that implements the API.

The value proposition is declarative UI definition, vdom is how that proposition performs acceptably (or more than acceptably as it allows further optimisation due to pervasive use of pure functions) when layered on an imperative underlying structure.

Yeah I don't specifically mean the virtual DOM, I'm thinking more about the fact that your interactivity isn't coupled to the actual markup. If you avoid browser specific functionality, it would be super easy to port to another platform.

I don't think it performs poorly, it's simply missing functionality to stop update right away, but rather sit there wait for a bunch of commands to come in, and then update.

It's possibly missing this "mode" because of other factors (what if you never put it back in "updates enabled" mode?)

> Flux attempts to be reactive by making Stores listen to Dispatcher events, and Controller-Views listen to Store events. However, the centralized Dispatcher is imperatively controlled by Actions, rather than taking the responsibility to observe Actions.

This can definitely be disorienting since by default you use imperative APIs to create the actions, and then switch to working with events and subscribers from the dispatcher up until the re-render. I've not had too much difficulty with this, at least after some initial confusion.

> Flux dictates that inter-model dependency should live in the Dispatcher, but in MVI, those dependencies are defined inside each Model.

Maybe this should be edited. Flux stores are the analog of models, and each store can define which other stores have to finish updating before it responds to a given action (in that big case statement where it registers with the dispatcher).

The arguments for RxJS, testable view functions, and avoiding internal component state are important. The dispatcher being a singleton doesn't seem to affect much in practice, since that code is never really touched. Looks like performance could be better with virtual-dom but I'd be interested to see the different optimization strategies for each architecture and how they affect performance. Some of my own notes about optimizing React/Flux are here if you're curious: http://guscost.com/2015/05/27/react-js-and-flux-ideas-for-pr...

Also I think that reusable components is a really nice feature and I would encourage you to continue working on including this functionality. All in all a very nice piece, thanks for writing it up.

I like the idea of moving that arrow the way he does.

Everything else was just less convincing. The real power of React is composable components that are easy to think about. I don't think the author really "got" this; if he was building "complex state machines ... and [mixing] multiple concerns in one component", he was just doing it wrong.

I also don't love the idea of stores depending on each other. In my experience, it leads to hard-to-maintain tangled interdependencies, especially on the frontend. I actually tried to use Bacon.js as a Flux replacement because of the reactive philosophy (this article was a source of inspiration), and while it was nice, the purported benefits of Rx fell flat and store interdepencies remained a problem.

Now, I use NuclearJS [0] to avoid store interdependency altogether. It's actually what I was trying to build with Reactive programming, but done better.

[0] https://github.com/optimizely/nuclear-js

Completely agree. Looking at his eventual realization of these ideas, Cycle.js, it's just a complete mess in terms of declarative, easy to understand components. The beauty of React, beyond its simple lifecycle API and virtual DOM, is that you can glance at a component and very quickly reason what it does and how it works. I can't say the same for anything that came out of this article.

There's a tendency in JavaScript development especially to overthink, over-engineer, and over-abstract. We're still in the middle of the woods with JavaScript application development. I hope we get out soon!

Interestingly, the sliders in the MVI example seem to react way more slowly...

Same for me.

Good write-up and similar to some of my findings. In fact I like the event emitting / messaging pattern so much I wrote a little library called msngr.js (https://github.com/KrisSiegel/msngr.js) to do the same type of thing.

I'm not entirely convinced of the separate "renderer" or really the act of updating a virtual dom but I do like the model-view-intent. What I've done is something pretty similar though typically I combine the model and the view in all cases but the main application itself. Basically the breakdown is like this:

Application controller - initializes application components / authorization / anything necessary for startup

Web components - Either through webcomponents.org polyfills or polymer these are the individual components that make up the user interface of the web application. Each web component is entirely responsible for its interaction with the user. It subscribes to messages when it should take in updates and it emits the data the user inputs.

Application libraries - Handles the other end of messaging; has zero interaction with anything within the DOM. Subscribes to data that gets emitted from the web components and emits data that the web components need to display to the user.

I try to keep things as basic as possible. I'm still experimenting and working on an example web application using msngr.js that I can actually release as open source but I am a fan of this type of pattern. The best part of this is I can do full unit testing on the application libraries with zero DOM and the web component side of things can be fully tested with mocked results.

Looks to be the same idea as Cycle.

https://github.com/staltz/cycle

He wrote Cycle

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