Immer-based Optimistic UI

Purpose

This subtopic addresses the challenge of implementing optimistic UI updates in React applications while maintaining immutable state management. It leverages Immer, a library that enables writing immutable update logic in a mutable style, to simplify handling complex state changes during optimistic updates. Unlike basic optimistic UI patterns, which may rely on manual immutable updates or direct state mutation, this approach ensures immutability transparently and safely, reducing boilerplate and potential bugs.

Functionality

The core workflow involves an optimistic update to the cached data immediately upon user interaction (e.g., submitting a new item), before the server confirms the change. Immer’s produce function is used to create a new immutable state by applying mutable operations on a draft copy of the cached data. This draft mutation is written naturally, enhancing developer ergonomics while preserving immutability guarantees.

Key steps in the workflow:

1. Optimistic Cache Update:
   When the user submits new data, the cache for the relevant key (/api/data) is updated optimistically using mutate. The update applies Immer’s produce to immutably add the new item to the cached list synchronously, reflecting the update instantly in the UI without waiting for the server response.

2. Server Mutation Request:
   A POST request is sent to the API endpoint with the new data. This request represents the authoritative mutation on the backend.

3. Cache Revalidation:
   After the server responds, mutate is called again with the fresh server data, updating the cache to reflect the confirmed state. This ensures eventual consistency between client and server, correcting any optimistic assumptions if the server response differs.

4. Error Handling Considerations:
   If the server request fails or returns unexpected data, the UI state may temporarily be inconsistent until the next successful fetch. This highlights the trade-off in optimistic UI approaches for responsiveness versus guaranteed consistency.

Code Illustration

mutate("/api/data", produce(draftData => {
  draftData.push(text)
}), false)

This snippet performs an immediate optimistic update by pushing the new item into a draft copy of the cached data using Immer’s produce. The third argument false prevents revalidation at this step.

Following this:

mutate('/api/data', await fetch('/api/data', {
  method: 'POST',
  body: JSON.stringify({ text })
}))

The cache is updated again with the result from the server, ensuring data correctness.

Integration

This subtopic extends the parent topic’s optimistic UI examples by incorporating Immer to manage immutable state updates conveniently. It complements the basic optimistic UI approach by addressing the complexity and verbosity often associated with immutable update patterns.

Within the broader SWR ecosystem:

• It uses the core mutate function from SWR to manipulate the data cache directly.

• It fits naturally with SWR’s revalidation and caching mechanisms, leveraging mutate’s ability to accept both synchronous updates (for optimistic UI) and asynchronous data fetching.

• It can be combined with middleware that handles error rollbacks or retry logic, further enhancing robustness.

• This approach also aligns well with React’s state immutability principles, facilitating easier integration with other state management tools or patterns.

Diagram

flowchart TD
    U[User submits new item] --> O[Optimistic cache update using Immer produce]
    O --> R[Render UI with optimistic data]
    U --> S[Send POST request to server]
    S --> SR[Receive server response]
    SR --> C[Cache revalidation with server data]
    C --> R

This flowchart visualizes the optimistic update cycle: user action triggers an immediate cache update with Immer, UI renders the optimistic state, server request proceeds asynchronously, and upon response, the cache is updated again with the authoritative data.