Inside Fiber: An Overview of React's Reconciliation Algorithm

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With React 16.0, Facebook has released an update to the React core reconciliation algorithm which was named ""Fiber"". Fiber allows React to break the limits of the call stack and pause/start rendering work at will.


In this talk, we will explore why Fiber was necessary, cover some of the internal implementation details of Fiber, and see Fiber in action with React's experimental Concurrent Mode.

Elad Tzemach
Elad Tzemach
20 min
14 May, 2021

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Video Summary and Transcription

React Fiber is a reconciliation algorithm introduced in React 16 to address laggy input fields and heavy rendering. The old stack reconciler caused a laggy experience by re-rendering the entire subtree immediately. React Fiber solves this by breaking work into incremental units and assigning priorities. It introduces concurrent mode to make apps responsive and adaptable. The useDeferredValue hook is commonly used to keep the interface responsive by rendering components immediately and others at a later time.

1. Introduction to Fiber and Demo

Short description:

Hi, I'm Elad Zamek, a software developer at Viva Systems. Today, we'll talk about Fiber, React's newest reconciliation algorithm introduced in React 16. We'll cover what React had before Fiber, why Fiber was necessary, and what we can achieve with it. Let's start with a quick demo. I have a simple app with an input field and a list. Whatever I type is rendered in the list. The list component waits for three milliseconds to simulate a heavy render.

Hi everyone, my name is Elad Zamek and I'm a software developer at Viva Systems in Toronto, Canada. Viva provides cloud-based products for the global life sciences industry and I currently work there as the lead UI developer for the Safety AI product.

Today, we'll be talking about Fiber, React's newest reconciliation algorithm that was introduced in React 16. We'll talk about what React had before Fiber, why Fiber was necessary, and some of the things we can achieve with Fiber. We have a lot to cover so let's get started.

I'll start with a quick demo here and as you can see, I have a pretty simple app that has an input field. Whatever I type into the input field is then being rendered. We have 80 elements in a list that's being rendered with the value that we typed into the field. If I type in a lot, we're going to get 80 elements with a text a lot. My slow list is a component that's being rendered. If I go into it, we can see it returns us an array of 80 elements of the list item component which is a component that waits for three milliseconds and then returns an LI element with the children prompt which is essentially the text that we passed. The reason that I'm waiting here for three milliseconds is just to simulate a component that's heavy that takes a long time to render because it's very complex.

2. Laggy Input Fields and Heavy Rendering

Short description:

When typing into the input field, there is a noticeable lag before the value is rendered in the list. This is due to a complex heavy component, MySOLIST, that renders as the typing occurs. Many applications experience this issue with laggy input fields.

Now, if I type something into the input field, so let's say I'm going to type React conference, typing and stop typing now and then I can see the value in the input field and in the list. So I'll do that again, React conference, stop typing now, and then I see it. So I hope that you noticed that lag that I had. It was like about a second or two, between the moment I finished typing React conference into the input field, and the moment I saw the value being populated in the input field itself and being rendered on the list. And that's because when I typed in the value, I had another component, the MySOLIST component that rendered as I was typing it. And I'm sure that like most of you at least, I've seen this type of issue happening in your applications where you have like a laggy input field because there is another complex heavy component that needs to be rendered based on the value that you have in the input field or something similar to that.

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