Innovative Software Technology-Unraveling React Fiber: A Deep Dive into Concurrent Rendering and UI Reconciliation

Unraveling React Fiber: A Deep Dive into Concurrent Rendering and UI Reconciliation

By Yogesh Bamanier

Introduction

React Fiber introduced a revolutionary re-architecture to React’s core algorithm, transforming how the framework handles UI updates, ensuring a smoother and more responsive user experience. For advanced React developers, understanding Fiber is key to truly grasping the framework’s performance capabilities and future directions. This article will demystify React Fiber, exploring concepts like the Work-in-Progress (WIP) tree, the commit phase, incremental and concurrent rendering, and how React manages multiple UI states simultaneously.

The Genesis of Fiber: Current vs. WIP Trees

When a React application initializes using createRoot(...).render(<App />), React establishes a Root Fiber (HostRootFiber). This root serves as the anchor for two primary Fiber trees:

The current Fiber tree: This tree mirrors the UI currently displayed on the screen.
The Work-in-Progress (WIP) Fiber tree: This is a new tree, or a modified copy of the current tree, that React builds in memory as it processes updates. It represents the potential next state of the UI.

Crucially, there’s only one Root Fiber, but it orchestrates the dance between the current and workInProgress trees, allowing React to prepare future UI states without blocking the main thread.

The Lifecycle of the WIP Tree

A common question arises: how does React manage these WIP trees, especially when updates are rapid or work is paused?

Post-Commit Transformation: Once a WIP tree is fully prepared and committed, it replaces the `current` tree, becoming the new active UI.
Dynamic Management: If new updates arrive while a WIP tree is being constructed, React has choices:
- It can resume rendering the existing partial WIP, intelligently integrating the new updates.
- Alternatively, it might discard the half-done WIP and start fresh to prioritize higher-priority updates, ensuring better responsiveness.

It’s important to remember that even if a WIP tree is discarded, no user updates are lost. They remain safely queued within the current Fiber, ready to be processed in a subsequent rendering cycle.

Interruptible Rendering: Pausing and Resuming Work

A cornerstone of React Fiber is its support for interruptible rendering. Unlike previous versions where rendering was a synchronous, all-or-nothing process, Fiber can:

Yield Control: If a rendering task becomes too lengthy, React can pause its work (yield control) to allow the browser to process high-priority tasks like user input or animation frames. This prevents janky UIs.
Intelligent Continuation:
When React regains control, it can either resume the same WIP tree exactly where it left off, or if a more urgent update has arrived, it can discard the old WIP and begin building a new one from the `current` tree. This adaptive behavior ensures immediate feedback, like typing into an input field, feels instantaneous.

Reconciliation: How React Updates the UI

When a component’s state or props change, triggering an update, the reconciliation process doesn’t always restart from the absolute root. Instead, it intelligently begins from the parent of the updated Fiber.

During reconciliation:

The parent’s WIP Fiber is meticulously rebuilt.
React then traverses its children, comparing their current state with the desired next state. It decides whether to reuse an existing Fiber (if props/state are identical) or create a new WIP Fiber (if changes are detected).

This explains why even sibling components, which might not have directly changed, could still be traversed during an update cycle – they are part of the parent’s reconciliation scope.

The Three Phases of the Commit Process

The commit phase is where React’s in-memory WIP tree transitions to become the visible UI. It’s not a single step but a carefully orchestrated sequence of three sub-phases, all executed synchronously and without interruption:

Before Mutation (Snapshot Phase): This phase occurs just before any actual DOM changes. Lifecycle methods like getSnapshotBeforeUpdate are invoked here, allowing React or your components to capture essential layout information (e.g., scroll positions) from the current DOM state.
Mutation Phase: This is where the real DOM manipulation happens. React efficiently creates, updates, or removes DOM nodes to match the new WIP tree. For instance, multiple child DOM nodes are often inserted in a single batched operation for optimal performance.
Layout Phase (Effects): After the DOM is updated, React executes layout effects. This includes running class component lifecycle methods like componentDidMount and componentDidUpdate, as well as scheduling passive effects (e.g., those from `useEffect` without a dependency array).

Only after these three phases are complete does the browser perform its paint, making the new UI visible to the user.

Understanding Multiple WIP Trees

A truly mind-bending aspect of Fiber is its ability to handle multiple versions of the UI concurrently. However, this concurrency exists specifically between commit waves.

At any given moment, your application might have:

The `current` tree: The UI currently rendered on screen.
One or more WIP candidates: Multiple potential future UI states being constructed in memory.

React’s scheduler determines which WIP candidate is the “winner” based on priority and completes it during the commit phase. Other incomplete or lower-priority WIP trees are simply discarded. This mechanism is crucial for high-priority updates to “interrupt” and supersede ongoing lower-priority work, leading to a highly responsive application.

Incremental vs. Concurrent Rendering: A Crucial Distinction

These two terms, often used interchangeably, describe distinct but related capabilities of React Fiber:

Incremental Rendering (Time Slicing)

Concept: Breaks down a single, potentially large rendering task into smaller, manageable “chunks” or “slices.”
Mechanism: Each slice executes for a brief duration (e.g., 5-16ms), after which React can yield control to the browser.
Benefit: Prevents long-running rendering tasks from blocking the main thread, ensuring the UI remains responsive even during heavy computations. If a user interacts with the UI mid-render, React can pause its work to handle the input.

Example: If rendering a <HeavyList /> component takes significant time, React can render a <Sidebar /> first, yield, and then process the <HeavyList /> in chunks. If a user clicks during the HeavyList‘s rendering, React can interrupt, handle the click, and then resume or restart the list rendering.

Concurrent Rendering

Concept: The ability for React to simultaneously prepare and manage multiple WIP trees for the same UI root.
Mechanism: High-priority updates can “interrupt” and take precedence over lower-priority work. React builds a separate WIP tree for the high-priority update while potentially continuing (or discarding) the lower-priority WIP.
Benefit: Ensures that critical user interactions (like typing) receive immediate feedback, even if background tasks (like fetching data for a long list) are still processing. Only the most appropriate and ready WIP tree is eventually committed to the DOM.

Example: When typing into a <SearchBar /> that triggers a heavy <Feed /> update, React can immediately commit the SearchBar‘s update (high-priority) to reflect the keystroke. Concurrently, it starts building a new WIP tree for the Feed based on the new search query, potentially discarding any previous Feed WIP that wasn’t yet committed.

The Key Difference: Incremental rendering is about splitting one render into chunks to keep the UI interactive. Concurrent rendering is about managing multiple possible renders at the same time and deciding which one to commit based on priority.

Key Takeaways

Fiber’s Core Purpose: Enables time-sliced, interruptible rendering for superior UI responsiveness.
WIP Tree Dominance: UI updates are first prepared in an in-memory WIP tree; the DOM is only affected during the commit phase.
Adaptive Rendering: React can intelligently pause, resume, or discard WIP trees based on update priorities.
Ephemeral Concurrency: Multiple WIP trees can coexist briefly between commit cycles, but only one ultimately becomes the `current` tree.
Structured Commit: The commit process is a three-phase synchronous operation (Before Mutation, Mutation, Layout).
Data Integrity: React’s update queues guarantee that no user input is ever lost, even if a WIP tree is abandoned.

Author

👨‍💻 Yogesh Bamanier
🔗 LinkedIn