The React Reconciliation Process is the mechanism React uses to efficiently update the real DOM when the application state or props change. React uses a virtual DOM to perform this update in a way that minimizes performance overhead by calculating the differences (or “diffing”) between the old and new versions of the virtual DOM and applying those changes in the most efficient way possible.
In React 16, the introduction of React Fiber completely re-architected the reconciliation algorithm, offering significant performance improvements and more flexibility for developers. The introduction of Fiber allows React to handle more complex updates asynchronously, without blocking the UI thread.
1. What is React Reconciliation?
Reconciliation refers to the process React follows to determine how to update the UI (real DOM) when the state or props of a component change. The primary goal of reconciliation is to efficiently calculate the minimal number of changes required to keep the UI in sync with the application state.
React achieves this by comparing the previous virtual DOM (snapshot) to the new one and applying only the necessary changes to the real DOM. This process is also called diffing.
Key Concepts:
- Virtual DOM: A lightweight representation of the actual DOM that React uses for efficient updates.
- Diffing Algorithm: A mechanism for comparing the previous and current versions of the virtual DOM to determine the minimum number of updates required.
2. Challenges with the Old Reconciliation Algorithm
Before the introduction of Fiber, React used a simple, synchronous reconciliation algorithm that had a few significant drawbacks:
- Blocking UI Updates: The process of updating the virtual DOM and performing the diffing operation was synchronous, which meant that long or complex updates could block the main thread, making the app feel sluggish or unresponsive.
- Lack of Prioritization: React didn’t have a built-in way to prioritize important updates (like user interactions or animations) over less urgent ones (like background data fetching).
- No Ability to Interrupt Work: Once React started reconciling the virtual DOM, it would finish the entire process before rendering any updates to the real DOM. This caused performance issues, especially with large updates.
3. What is React Fiber?
React Fiber is a complete rewrite of the React reconciliation engine, introduced in React 16. It aims to solve the performance issues mentioned above and gives React more control over how and when updates are processed. The key feature of React Fiber is that it introduces an asynchronous rendering mechanism that allows React to break the work into smaller chunks, prioritize them, and spread them out over time.
Fiber uses a fiber node structure to manage the reconciliation process, where each component in the React tree is represented as a “fiber.” These fiber nodes allow React to break up updates into smaller pieces that can be scheduled and executed independently.
4. How Does React Fiber Work?
The Fiber reconciliation algorithm works by breaking the rendering work into units of work, where each unit can be processed independently and asynchronously. This allows React to optimize the rendering process and improve performance by prioritizing important tasks.
Fiber Node Structure
Each component in the virtual DOM is represented as a Fiber Node. A Fiber Node contains:
- Type: The type of the component (class, function, etc.).
- Props: The component’s props.
- State: The internal state of the component.
- Effect List: The list of effects (like DOM updates) to be applied after the reconciliation.
- Parent and Child Pointers: These form a linked list, creating a tree-like structure of components.
Reconciliation Phases in Fiber
Fiber introduces two main phases in the reconciliation process:
- Render Phase:
- This is the phase where React builds the new fiber tree. React determines what changes need to be made to the UI but doesn’t apply those changes yet. This phase can be interrupted and resumed later, allowing React to prioritize other tasks (e.g., user input or animations).
- The render phase can be thought of as the “planning” phase where React computes which components need to be updated.
- Commit Phase:
- After the render phase, React enters the commit phase, where it applies the changes to the DOM. This is when the actual updates happen — the real DOM is mutated based on the changes detected during the render phase.
- This phase is synchronous because it directly interacts with the DOM.
Fiber’s Scheduling and Prioritization
One of the most powerful features of Fiber is scheduling. React can assign different priority levels to updates, allowing more important updates to be processed first, such as user interactions or animations. Lower-priority tasks, like background data fetching, can be deferred.
- Immediate Priority: Critical updates like user inputs or animations.
- Normal Priority: Updates that aren’t time-sensitive but still need to be processed soon.
- Low Priority: Background tasks that can be deferred or run in idle time.
This scheduling mechanism helps ensure the UI remains responsive, especially when dealing with large updates or complex components.
5. Key Benefits of React Fiber
React Fiber offers several improvements over the old reconciliation algorithm:
a) Asynchronous Rendering
Fiber introduces asynchronous rendering, allowing React to pause and resume rendering work, making the UI more responsive. For example, React can perform expensive updates incrementally and avoid blocking the main thread.
b) Incremental Work
With Fiber, React can break the work of rendering into smaller chunks, allowing each chunk to be executed independently. This avoids blocking the main thread for extended periods of time.
c) Prioritization of Updates
Fiber allows React to prioritize different types of updates based on their importance. For instance, it can prioritize user interactions (clicks, typing) over less critical updates like background data fetching.
d) Error Boundaries
React Fiber makes it easier to catch errors in components by introducing error boundaries. These components can catch errors in their subtree and prevent the entire application from crashing.
e) Time Slicing
Fiber can slice the work and process the most critical updates first. If the browser is under heavy load, React can pause rendering and continue when the main thread is idle, preventing UI freezes.
6. Example of Fiber in Action
Consider an example where a user interacts with a button in the UI. Without Fiber, React would update the virtual DOM, perform the diffing, and apply all updates synchronously, potentially blocking the UI if the updates are large.
With Fiber, React can:
- Prioritize the user interaction and immediately update the UI in response to the button click.
- Defer less critical tasks, such as background data fetching, until the UI is idle.
- Break up rendering work into smaller chunks and continue processing them as needed without blocking the UI thread.
This makes the user experience smoother and more responsive, even during large or complex updates.
7. React Fiber and Concurrent Mode
Fiber also lays the foundation for Concurrent Mode, an experimental feature in React that allows for even more sophisticated scheduling and prioritization. In Concurrent Mode, React can work on multiple tasks simultaneously and interrupt the rendering process if necessary.
With Concurrent Mode:
- React can work on multiple updates in parallel, allowing for more granular control over the rendering process.
- React can pause and resume rendering based on the priority of the update, ensuring that critical updates happen immediately.
Though Concurrent Mode is still experimental and not yet widely adopted, it demonstrates the power of React Fiber in handling complex, high-priority updates efficiently.