Managing Loading State in React Query

React Query is a powerful library for fetching, caching, and updating data in React applications. It simplifies data-fetching processes and provides helpful tools to manage loading states, making your application more responsive and user-friendly.

What is isLoading in React Query?

When you initiate a data fetch using React Query, it provides several status flags to track the request's state. The isLoading flag is one of these, indicating whether the query is in the process of fetching data. It's a boolean value: true when the query is fetching for the first time and has no data, and false otherwise.

const { data, isLoading } = useQuery('todos', fetchTodos);

How to implement a loading indicator in React Query

Integrating a loading indicator is pretty straightforward. You can render different UI elements based on the isLoading flag. Here’s a simple example:

const TodoList = () => {
  const { data, isLoading } = useQuery('todos', fetchTodos);

  if (isLoading) {
    return <div>Loading...</div>;
  }

  return (
    <ul>
      {data.map(todo => (
        <li key={todo.id}>{todo.title}</li>
      ))}
    </ul>
  );
};

How to handle loading states with Suspense

React Query supports React's Suspense for better handling of loading states. By wrapping your components in Suspense, you can provide a fallback UI during data fetching:

import { Suspense } from 'react';

const App = () => (
  <Suspense fallback={<div>Loading...</div>}>
    <TodoList />
  </Suspense>
);

Advanced loading patterns in React Query

For more complex scenarios, such as dependent or parallel queries, React Query offers additional hooks like useQueries and useQueryClient. You can use these to orchestrate multiple loading states more efficiently.

const todoQuery = useQuery(['todos', todoId], fetchTodoById);
const userQuery = useQuery(['user', userId], fetchUserById);

if (todoQuery.isLoading || userQuery.isLoading) {
  return <div>Loading...</div>;
}

How to combine loading with error and data states in React Query

In real-world applications, you should handle loading, error, and data states together. React Query provides isError and error properties for error handling:

const { data, isLoading, isError, error } = useQuery('todos', fetchTodos);

if (isLoading) {
  return <div>Loading...</div>;
}

if (isError) {
  return <div>Error: {error.message}</div>;
}

// Render data

isLoading vs. isFetching in React query

isLoading

As previously discussed, isLoading is true when a query is fetching for the first time and does not have any data cached. This flag is particularly useful for initial data loading scenarios.

isFetching

On the other hand, isFetching indicates whether a query is fetching in any scenario, including background refetching. This means isFetching can be true in the following cases:

1. When the query is executing for the first time (similar to isLoading).
2. When the query is refetching in the background (e.g., due to window refocus or data stale state).
3. When a refetch is manually triggered.

Here's a quick example to illustrate their usage:

const { data, isLoading, isFetching } = useQuery('todos', fetchTodos);

return (
  <div>
    {isLoading ? (
      <div>Initial Loading...</div>
    ) : (
      <>
        {isFetching && <div>Updating...</div>}
        <ul>
          {data.map(todo => (
            <li key={todo.id}>{todo.title}</li>
          ))}
        </ul>
      </>
    )}
  </div>
);

In this example, isLoading is used to show a loading message during the initial data fetch. Once data is loaded, isFetching is used to indicate any background data updates without hindering user interaction with the already loaded data.

Choosing Between isLoading and isFetching

• Use isLoading when you need to display a loading state before any data is available.
• Use isFetching to indicate ongoing data operations after the initial load, like background updates or refetches.

By understanding and correctly implementing isLoading and isFetching, you can significantly improve the user experience in applications that handle dynamic and frequently updated data.

Conclusion

React Query's isLoading flag is a vital feature for enhancing user experience in data-driven applications. By efficiently handling loading states, you can build more responsive and intuitive interfaces. Keep in mind the importance of balancing loading indicators with error handling and data display to provide a complete solution for your data fetching needs.