การปรับปรุงข้อมูลเชิงคาดการณ์

Imagine clicking 'Like' on a post. You expect it to show 'Liked' instantly, right? That's the magic of Optimistic Updates!

Optimistic updates make your app feel incredibly fast and responsive. They improve the user experience by giving immediate feedback.

An optimistic update means updating the user interface (UI) before the server has confirmed the change. You 'optimistically' assume the server request will succeed.

• Instant UI update: The user sees their action immediately reflected.
• Background server call: The actual data change is sent to the backend.
• Rollback on error: If the server fails, the UI reverts to its previous state.

tRPC, combined with a client-side caching library like React Query (or TanStack Query), provides powerful tools for managing server state and implementing optimistic updates.

React Query's useMutation hook is central to this. It allows you to define callbacks for when a mutation starts, succeeds, or fails, enabling precise control over the UI.

When you trigger a mutation, React Query calls the onMutate function before sending the request to the server. This is where you perform the optimistic UI update.

Inside onMutate, you typically:

• Cancel any ongoing fetches for the data you're about to change.
• Snapshot the current data so you can roll back if needed.
• Update the cache with the new, optimistic data.

Let's look at a simplified example for 'liking' a post. We'll use queryClient.setQueryData to update the cache optimistically.

If the server mutation fails, you need to revert the UI to its previous state. This is handled in the onError callback.

Here, you use the context object returned from onMutate (which holds your snapshot of previousPost) to restore the cache to its original value.

Adding the onError handler to our useMutation setup:

Once the server successfully processes the mutation, the onSuccess callback is triggered. At this point, your optimistic UI update is correct, but you still want to ensure data consistency.

The best practice here is to invalidate relevant queries. This tells React Query to refetch the data in the background, ensuring your client-side cache is perfectly in sync with the server.

Here's the full useMutation with onSuccess to invalidate and refetch data after a successful server response:

Optimistic updates are fantastic for user experience, but they add complexity:

• Pro: Instant UI feedback, perceived performance boost.
• Pro: Reduces loading spinners and waiting times.
• Con: Requires careful rollback logic for errors.
• Con: Can be complex for operations that depend on server-generated IDs or complex data transformations.

Use them thoughtfully, especially for actions where immediate feedback is critical and conflicts are rare.

Consider an optimistic update for adding an item to a shopping cart. Which of the following actions are typically performed within the onMutate callback?

In this lesson, we explored optimistic updates, a powerful technique to enhance user experience by providing instant feedback.

• We learned how onMutate is used to optimistically update the UI and prepare for potential rollbacks.
• We saw how onError handles reverting the UI to its previous state if the server request fails.
• Finally, we covered how onSuccess ensures data consistency by invalidating and refetching data after a successful server response.

Mastering optimistic updates helps you build highly responsive and user-friendly tRPC applications!