Understanding React’s Fiber Architecture

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We've heard a lot about React's Fiber Architecture, but it feels like few of us understand it in depth (or have the time to). In this talk, Tejas will go over his best attempt at understanding Fiber (reviewed by other experts), and present it in an 'explain-like-I'm-five years old' way.

Tejas Kumar
Tejas Kumar
29 min
21 Oct, 2022

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Video Summary and Transcription

This Talk explores React's internal jargon, specifically fiber, which is an internal unit of work for rendering and committing. Fibers facilitate efficient updates to elements and play a crucial role in the reconciliation process. The work loop, complete work, and commit phase are essential steps in the rendering process. Understanding React's internals can help with optimizing code and pull request reviews. React 18 introduces the work loop sync and async functions for concurrent features and prioritization. Fiber brings benefits like async rendering and the ability to discard work-in-progress trees, improving user experience.

1. Understanding React's Internal Jargon

Short description:

Welcome back after lunch. This talk is about React's internal jargon, specifically fiber. Fiber is an internal thing that React uses as a single unit of work for rendering and committing. It's not exposed to users, unlike React elements. Elements describe desired UI state, while fibers are internal and ephemeral. They work together to build great apps.

Welcome back after lunch. I hope you're ready for some deepness in this very advanced conference. Let me just start by saying this is a talk that nobody needs to hear. Like, for real, like nobody needs to hear this. I hope some of you want to hear this. Do you want to? Okay. But nobody, because here's the thing, like this is about React's internal jargon. It's stuff that you may be interested in, like me, if you're curious. You do not need to know this stuff at all to build great apps with React.

React is the best way to build user interfaces on the Web. And you don't need to know the internals for that. Who wants to dive into the internals today just for fun? Yes, that's right. Let's get this. Let's get this show on the road. We're going to be talking about something called fiber. Fiber is something that I don't have a lot of on my... But what is it in the React context? Succinctly put, fiber... And by the way, we're going to be diving into the fiber architecture in 20 minutes. Imagine that! Fiber in React is a single unit of work. There's a single unit of work that React does. And you might be thinking, OK, what is the work? The work is essentially rendering your stuff and putting it on the screen. Render and commit. Fiber is an internal thing. Again, so you don't need to know about it at all to build great React apps. And as such it's not exposed to the users. So maybe an easier way of reasoning about it is to think of something that is exposed and go from there. OK? What is exposed in React? The humble element. We all write React elements. We write an angular bracket with a name and that calls react.create element. And we have elements. Elements are what we work with daily. And I'm not going to dive deep on elements because I did that once. That was a relatively popular talk about React elements and how they exist and how they are created and all this. Feel free to watch that in your free time. But what I want to do is compare and contrast elements and fibers so we get, you know, some sense of how they work together. Essentially what I want to summarize that talk in saying for this talk is a React element describes desired UI state. A React element allows you to describe, like, declaratively, here's my tree of React elements. This is my app. And then React does the work of making that the truth in a host environment. So in a browser, in a mobile device with React Native, in a command line interface with React, so your elements describe what you want. React makes that happen. Is that clear? Awesome. So how do fibers come into this picture? For starters, elements are public, meaning they're external. We write them. But fibers are internal. We don't talk about them because we don't need to. There's a whole team that works with these daily for our sake, on our behalf, so we can build great apps. Elements are ephemeral, so they don't live long. You can throw away elements. You can create new ones.

2. Understanding Elements and Fibers

Short description:

Elements are ephemeral, while fibers are long-lived, stateful data structures used by React to track app behavior and appearance. Fibers facilitate efficient updates to elements, preventing unnecessary re-rendering. Reconciliation is the process of aligning the UI description with the host environment, and fibers play a crucial role in this.

They're just a representation. If you speak DevOps, they're like a Kubernetes manifest. You just describe the state you want, and then a system brings it to fruition. Or like a Terraform file. Any DevOps people in the house? Oh, cool. Hey. One person.

Elements are ephemeral. Fibers are long-lived, stateful data structures that React uses to keep track of what your app is supposed to do. What you want it to do, and what you want it to look like. And that is React's value proposition at all.

So the way to build user interfaces that makes efficient and proper updates to your elements such that it doesn't throw one out and give you a new one every time. Can you imagine losing your input focus state on keystroke? Nasty. So fibers facilitate that. And third, elements are immutable. Meaning, you don't change them. You throw them away. You create new ones. But fibers are long-lived, mutable, stateful data structures used in the process of reconciliation. What is reconciliation? Reconciliation is when you take a tree of elements, a description, a road map of what you want your UI to look like and how you want it to respond to events and what effects you want it. You take this list of elements and reconcile them to a host environment. The browser or a mobile device or CLI. That is reconciliation. It's essentially taking your description and making it the truth. Fibers are instrumental in this.

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