How to Update Fiber Architecture in React

React Fiber is the core reconciliation engine responsible for managing updates, scheduling rendering, and driving the user interface in modern React applications. While developers do not directly rewrite React’s internal Fiber source code, updating and interacting with the Fiber architecture involves triggering state changes, managing update priorities, and leveraging concurrent rendering features. This article explains how the Fiber architecture processes updates, how developers can control this behavior using modern React APIs, and the underlying mechanisms that execute these updates.

Understanding the Fiber Reconciliation Process

The Fiber architecture works by representing the component tree as a linked list of “Fiber” nodes. Each node contains information about a component’s state, props, and DOM relationships. When an update occurs, React performs reconciliation using a two-phase process:

  1. The Render Phase: React builds a “work-in-progress” Fiber tree. This phase is asynchronous and interruptible. React can pause, resume, or discard work on this tree based on browser idle time and update priorities.
  2. The Commit Phase: React applies the calculated changes to the actual DOM. This phase is synchronous and uninterruptible to prevent a partially updated, inconsistent UI from being displayed to the user.

React uses a technique called double-buffering. It maintains a “current” tree (what is currently visible on the screen) and a “work-in-progress” tree. Once the work-in-progress tree is fully computed during the render phase, React swaps the pointers in the commit phase, instantly updating the UI.

How State Updates Trigger Fiber Changes

Every time you call a state updater function (such as useState or dispatch from useReducer), React schedules a new “update” object on the corresponding Fiber node.

The update process follows these steps: * Creation of an Update Object: React captures the new state or action and attaches it to an update queue on the Fiber node. * Lane Assignment: React assigns a “lane” to the update. Lanes represent the priority of the update (e.g., discrete events like clicks have higher priority than background data syncing). * Scheduling: React schedules a task with the Scheduler package to process the Fiber tree, starting from the root down to the affected node.

Controlling Fiber Priorities with Concurrent Features

In older versions of React, all updates were synchronous and blocking. With the release of modern concurrent features, developers can now directly influence how the Fiber architecture schedules and interrupts updates.

1. Transition APIs (useTransition)

By wrapping a state update in startTransition, you mark it as a non-urgent transition. This tells the Fiber engine that it can pause rendering the transition if a high-priority update (like user input) occurs.

import { useState, useTransition } from 'react';

function SearchComponent() {
  const [query, setQuery] = useState('');
  const [results, setResults] = useState([]);
  const [isPending, startTransition] = useTransition();

  function handleChange(e) {
    // Urgent update: Show the typed character immediately
    setQuery(e.target.value);

    // Non-urgent update: Allow Fiber to pause this render if the user types again
    startTransition(async () => {
      const data = await fetchResults(e.target.value);
      setResults(data);
    });
  }
}

2. Deferred Values (useDeferredValue)

The useDeferredValue hook allows you to defer updating a specific part of the UI. React will attempt to render the deferred value immediately, but if the render takes too long, the Fiber scheduler will deprioritize it to keep the main thread responsive.

import { useDeferredValue } from 'react';

function List({ input }) {
  const deferredInput = useDeferredValue(input);
  // React will defer rendering this list if 'input' changes rapidly
  return <ExpensiveList query={deferredInput} />;
}

The Fiber Work Loop

At the heart of Fiber’s ability to update is the cooperative work loop. Instead of rendering the entire application recursively in a single block of execution, React uses a loop that checks how much time is left in the current frame.

Using the Scheduler package, React executes a function called workLoopConcurrent. This loop processes one Fiber node at a time and checks whether it needs to yield control back to the browser:

function workLoopConcurrent() {
  // Perform work on a single Fiber node until there is no work left,
  // or until the host environment (browser) needs to handle an event.
  while (workInProgress !== null && !shouldYield()) {
    performUnitOfWork(workInProgress);
  }
}

If shouldYield() returns true, React pauses the reconciliation of the Fiber tree, allows the browser to paint or handle user inputs, and then resumes the update precisely where it left off.