What is React fiber in ReactJS?

React fiber is a reimplementation of React’s core algorithm. The goal of React fiber is to increase its suitability for areas like animation, layout, and gestures. The aim is for React fiber to split rendering work into chunks, pausing work to yield back to the main thread and come back later to continue from where it left off. The key aim here is to avoid blocking visual updates.

Why was React Fiber introduced?

The React team introduced React Fiber to help solve some common problems in UI development with React:

• Animation – Creating smooth animations in React was difficult because React worked synchronously. Fiber allows work to be split into chunks and spread out over multiple frames.
• Layout – React needed better support for detecting layout changes and re-rendering just components affected by layout changes. Fiber adds incremental rendering for better layout handling.
• Gestures – React did not have good support for handling gestures and tracking touch movements. Fiber provides a framework for this.
• The team wanted to improve server-side rendering in React and make it easier to pause, resume, and reuse server-renderer work.
• They wanted to improve initial render performance by splitting work into chunks and parallelizing where possible.

In summary, the React team introduced Fiber to enable features like smooth animations, efficient rendering, server-side rendering, and incremental rendering. The core goal was to make React more flexible and extensible.

How does React Fiber work?

React Fiber works by breaking down rendering work into incremental units of work called “fibers”. These represent a unit of work that can be paused and resumed later if needed.

Some key concepts in React Fiber:

• Fiber – A Fiber is a JavaScript object that contains information about a component, its input and output. Fibers form a tree structure that maps to the component tree.
• Work loop – This is the process that executes the various fibers and performs incremental rendering. It can pause and resume work as needed.
• RequestIdleCallback – This is a browser API that React Fiber uses to pause and resume rendering based on available free time.
• Reconciliation – The algorithm that diffs the old and new versions of the app and determines what changes need to be made to the UI.

At a high level, the Fiber work loop processes units of work incrementally, pausing and resuming as needed. This allows it to spread out rendering over multiple frames for better app responsiveness.

Detailed Fiber rendering process

Here are the key steps in how React Fiber performs incremental rendering:

1. The render phase creates the fiber tree from the component tree.
2. The work loop then processes fibers from the tree, performing incremental work.
3. If higher priority work appears, React Fiber can pause work and come back to it later.
4. React Fiber uses the requestIdleCallback API to figure out chunks of free time to perform work.
5. The work loop continues reconciling and building up the UI incrementally.
6. When updates are processed, the commit phase makes changes to the DOM.
7. Layout effects are run synchronously after the commit phase.
8. UseEffect and other cleanup functions then run asynchronously.

The work loop is the key to understanding Fiber. It can start and stop rendering work to better handle high-priority user interactions.

Virtual DOM differences

React Fiber uses a “split” virtual DOM model compared to React’s previous virtual DOM implementation.

In the old virtual DOM, reconciliation required processing the entire component tree in one single recursive pass. But Fiber processes incremental parts of the virtual DOM in chunks, splitting the work into bite-sized pieces.

This means an update deep inside a component tree doesn’t block other components from rendering. Fiber can pause, work on other components, and come back later to finish reconciliation.

Benefits of React Fiber

Here are some of the key benefits React Fiber provides compared to pre-Fiber React:

• Pausable rendering – React Fiber allows rendering work to be split up and paused/resumed. This prevents a synchronous render from blocking visual updates.
• Incremental rendering – Only small batches of changes are rendered at a time instead of one large update. This lowers the chance of visual stuttering when rendering.
• Better priority handling – High priority user interactions like typing or scrolling can interrupt lower priority rendering. Fiber allows pausing reconciliation to handle high priority renders first.
• Stateful component reordering – Fiber allows stateful components to be split into chunks, avoiding cascading renders when state changes in components deep down the tree.
• Progressive hydration – On the server, Fiber allows pausing and resuming rendering. This enables sending just the HTML for above-the-fold content first.
• Error boundaries – React 16 introduced error boundaries for catching errors. Fiber powers this by splitting error boundaries into a separate fiber for dedicated handling.

In essence, Fiber provides incremental rendering, finer control over scheduling, and better handling for high priority updates. These benefits make React apps more responsive and jank-free.

Use cases for React Fiber

Here are some common use cases where React Fiber is beneficial compared to pre-Fiber React:

Animation

React Fiber enables smooth animations by splitting animation work into small increments. The work loop can pause and resume rendering to avoid blocking visual updates.

For example, when changing a progress bar width from 0% to 100%, Fiber allows doing it over multiple frames instead of one big update. This prevents stuttering and blocking.

Scrolling

For content like chat conversations or timelines, Fiber allows small incremental renders so scrolling performance is smooth. Expensive reconciliations can be paused to keep scroll handling responsive.

Typing

For text inputs or contenteditable elements, Fiber allows pausing reconciliation work to prioritize handling text input. This keeps the UI speedy and responsive as the user types.

Layout changes

React Fiber can incrementally handle layout changes in small chunks instead of one big render. For example, when window size changes only the affected components re-render.

Data fetching

Fiber enables flexibility in data fetching – it allows starting render with initial data, and incrementally updating as async data arrives from the server.

Server-side rendering

Fiber enables pausing and resuming renderer work on the server. This allows sending HTML progressively as it is created on the server-side.

How to use React Fiber

React Fiber is utilized automatically starting in React 16. The key is understanding how to best leverage the asynchronous rendering capabilities that Fiber enables.

Here are some tips for using React Fiber effectively in your apps:

• Use the useTransition hook for transitions and animations. This allows marking them as lower priority work that can yield to high priority interactions.
• Load data asynchronously where possible, and use incremental patterns like useState and useReducer to update state as data streams in.
• Use windowing techniques like virtualization for long lists. This allows rendering only small chunks at a time.
• Profile and measure render times to find optimizations needed. Chrome DevTools profiler is excellent for this.
• Consider time slicing – splitting work into chunks using setTimeout or requestIdleCallback to spread out expensive computations.
• Use React.memo and techniques like memoization to avoid unnecessary re-renders.

The key mindset is to focus on incremental rendering patterns instead of synchronous big updates. The React DevTools profiler is invaluable for detecting when more optimizations are needed.

Examples of React Fiber usage

Here are some examples of React components that leverage Fiber’s incremental rendering capabilities:

Animated progress bar

“`jsx
function ProgressBar({ percentage }) {
const [prog, setProg] = useState(0);

useEffect(() => {
let curr = 0;
const interval = setInterval(() => {
curr += 1;
setProg(curr);
if (curr >= percentage) clearInterval(interval);
}, 16);
}, [percentage]);

return (

);
}
“`

This animates progress by incrementally increasing it across frames. Fiber allows pausing between increments to keep UI responsive.

Virtualized list

“`jsx
function VirtualList({ list }) {
const rowHeight = 50;
const [start, setStart] = useState(0);

let visibleRows = list.slice(start, start + 20);

return (

);
}
“`

This implements incremental rendering by only showing small chunks of rows. Fiber allows pausing reconciliation to handle scrolling and row rendering.

Async data fetch

“`jsx
function DataComponent() {
const [data, setData] = useState(null);

useEffect(() => {
fetchData().then(setData);
}, []);

return (

);
}
“`

Here data is fetched async, and the component incrementally renders more as data arrives. Fiber allows reconciling in multiple passes.

Limitations of React Fiber

While Fiber improves rendering performance, it also comes with some limitations:

• More conceptual complexity – Fiber makes React’s reconciler algorithm more complex to understand.
• Increased memory usage – Fiber requires more memory to store additional objects like fibers.
• Differences across environments – Browsers handle requestIdleCallback differently so performance varies.
• Non-blocking semantics – Since work can be paused/resumed, component state can be inconsistent during renders.
• Debugging challenges – Asynchronous actions make debugging more difficult compared to synchronous rendering.

Additional debugging tools and DevTools support help mitigate some of these challenges. But in general Fiber adds complexity that can make profiling and debugging React apps more difficult.

Conclusion

React Fiber is a reimplementation of React’s core rendering algorithm aimed at improving animation, layout, and responsiveness. It works by splitting rendering into incremental chunks of work that can be paused and resumed.

Fiber helps enable smooth UI experiences by prioritizing work, allowing pausing for high priority interactions, and incremental updating. Useful for animations, scrolling, data fetching, and more.

While more complex, the async capabilities of Fiber provide a leap forward in React’s capabilities. Care is needed to leverage Fiber effectively, but the results are often worth the tradeoff.