Optimistic Updates

const addTodoMutation = useMutation(() => ({
  mutationFn: (newTodo: string) => axios.post('/api/data', { text: newTodo }),
  // make sure to _return_ the Promise from the query invalidation
  // so that the mutation stays in `pending` state until the refetch is finished
  onSettled: () => queryClient.invalidateQueries({ queryKey: ['todos'] }),
}))

<ul>
  <For each={todoQuery.data}>{(todo) => <li>{todo.text}</li>}</For>
  <Show when={addTodoMutation.isPending}>
    <li style={{ opacity: 0.5 }}>{addTodoMutation.variables}</li>
  </Show>
</ul>

<Show when={addTodoMutation.isError}>
  <li style={{ color: 'red' }}>
    {addTodoMutation.variables}
    <button onClick={() => addTodoMutation.mutate(addTodoMutation.variables)}>
      Retry
    </button>
  </li>
</Show>

// somewhere in your app
const mutation = useMutation(() => ({
  mutationFn: (newTodo: string) => axios.post('/api/data', { text: newTodo }),
  onSettled: () => queryClient.invalidateQueries({ queryKey: ['todos'] }),
  mutationKey: ['addTodo'],
}))

// access variables somewhere else
const variables = useMutationState(() => ({
  filters: { mutationKey: ['addTodo'], status: 'pending' },
  select: (mutation) => mutation.state.variables,
}))

const queryClient = useQueryClient()

useMutation(() => ({
  mutationFn: updateTodo,
  // When mutate is called:
  onMutate: async (newTodo, context) => {
    // Cancel any outgoing refetches
    // (so they don't overwrite our optimistic update)
    await context.client.cancelQueries({ queryKey: ['todos'] })

    // Snapshot the previous value
    const previousTodos = context.client.getQueryData(['todos'])

    // Optimistically update to the new value
    context.client.setQueryData(['todos'], (old) => [...old, newTodo])

    // Return a result with the snapshotted value
    return { previousTodos }
  },
  // If the mutation fails,
  // use the result returned from onMutate to roll back
  onError: (err, newTodo, onMutateResult, context) => {
    context.client.setQueryData(['todos'], onMutateResult.previousTodos)
  },
  // Always refetch after error or success:
  onSettled: (data, error, variables, onMutateResult, context) =>
    context.client.invalidateQueries({ queryKey: ['todos'] }),
}))

useMutation(() => ({
  mutationFn: updateTodo,
  // When mutate is called:
  onMutate: async (newTodo, context) => {
    // Cancel any outgoing refetches
    // (so they don't overwrite our optimistic update)
    await context.client.cancelQueries({ queryKey: ['todos', newTodo.id] })

    // Snapshot the previous value
    const previousTodo = context.client.getQueryData(['todos', newTodo.id])

    // Optimistically update to the new value
    context.client.setQueryData(['todos', newTodo.id], newTodo)

    // Return a result with the previous and new todo
    return { previousTodo, newTodo }
  },
  // If the mutation fails, use the result we returned above
  onError: (err, newTodo, onMutateResult, context) => {
    context.client.setQueryData(
      ['todos', onMutateResult.newTodo.id],
      onMutateResult.previousTodo,
    )
  },
  // Always refetch after error or success:
  onSettled: (newTodo, error, variables, onMutateResult, context) =>
    context.client.invalidateQueries({ queryKey: ['todos', newTodo.id] }),
}))

useMutation(() => ({
  mutationFn: updateTodo,
  // ...
  onSettled: async (newTodo, error, variables, onMutateResult, context) => {
    if (error) {
      // do something
    }
  },
}))