डेटा कैशिंग के लिए SWR और React Query

import useSWR from 'swr';

const fetcher = (url) => fetch(url).then(r => r.json());

function Profile() {
  const { data, error, isLoading } = useSWR('/api/me', fetcher);

  if (error) return <ErrorMessage error={error} />;
  if (isLoading) return <Spinner />;
  return <h1>{data.name}</h1>;
}

import useSWR, { mutate } from 'swr';

async function updateProfile(updates) {
  // Optimistic update — UI reflects change immediately
  mutate('/api/me', { ...currentData, ...updates }, false);
  await fetch('/api/me', { method: 'PUT', body: JSON.stringify(updates) });
  // Revalidate to get the server's canonical state
  mutate('/api/me');
}

import { QueryClient, QueryClientProvider } from '@tanstack/react-query';

const client = new QueryClient({
  defaultOptions: {
    queries: { staleTime: 30000, retry: 2 }
  }
});

function App() {
  return (
    <QueryClientProvider client={client}>
      <Routes />
    </QueryClientProvider>
  );
}

import { useQuery } from '@tanstack/react-query';

function UserList() {
  const { data, isLoading, error } = useQuery({
    queryKey: ['users'],
    queryFn: () => fetch('/api/users').then(r => r.json()),
    staleTime: 60_000 // fresh for 1 minute
  });

  if (isLoading) return <Spinner />;
  if (error) return <ErrorMessage error={error} />;
  return <ul>{data.map(u => <li key={u.id}>{u.name}</li>)}</ul>;
}

import { useMutation, useQueryClient } from '@tanstack/react-query';

function CreateUserForm() {
  const qc = useQueryClient();
  const m = useMutation({
    mutationFn: (user) => fetch('/api/users', { method: 'POST', body: JSON.stringify(user) }),
    onSuccess: () => qc.invalidateQueries({ queryKey: ['users'] })
  });

  return <button onClick={() => m.mutate({ name: 'Alice' })} disabled={m.isPending}>Create</button>;
}

useMutation({
  mutationFn: updateUser,
  onMutate: async (newUser) => {
    await qc.cancelQueries({ queryKey: ['user', newUser.id] });
    const previous = qc.getQueryData(['user', newUser.id]);
    qc.setQueryData(['user', newUser.id], newUser);
    return { previous };
  },
  onError: (_, newUser, ctx) => {
    qc.setQueryData(['user', newUser.id], ctx.previous); // rollback
  },
  onSettled: (_, __, newUser) => {
    qc.invalidateQueries({ queryKey: ['user', newUser.id] });
  }
});