乐观更新

React Query 提供了两种在 mutation 完成之前乐观地更新 UI 的方法。你可以使用 onMutate 选项直接更新你的缓存，或者利用返回的 variables 从 useMutation 结果中更新你的 UI。

通过 UI

这是更简单的变体，因为它不直接与缓存交互。

tsx

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'] }),
})

const { isPending, submittedAt, variables, mutate, isError } = addTodoMutation

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'] }),
})

const { isPending, submittedAt, variables, mutate, isError } = addTodoMutation

然后你将可以访问 addTodoMutation.variables，其中包含添加的 todo。在你的 UI 列表中，query 在此处渲染，你可以在 mutation isPending 时向列表追加另一个项目

tsx

<ul>
  {todoQuery.items.map((todo) => (
    <li key={todo.id}>{todo.text}</li>
  ))}
  {isPending && <li style={{ opacity: 0.5 }}>{variables}</li>}
</ul>

<ul>
  {todoQuery.items.map((todo) => (
    <li key={todo.id}>{todo.text}</li>
  ))}
  {isPending && <li style={{ opacity: 0.5 }}>{variables}</li>}
</ul>

只要 mutation 处于 pending 状态，我们就会渲染一个临时的、具有不同 opacity 的项目。一旦它完成，该项目将自动不再渲染。鉴于 refetch 成功，我们应该在列表中看到该项目作为“正常项目”。

如果 mutation 出错，该项目也会消失。但是如果我们愿意，我们可以通过检查 mutation 的 isError 状态来继续显示它。variables 在 mutation 出错时不会被清除，因此我们仍然可以访问它们，甚至可以显示重试按钮

tsx

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

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

如果 mutation 和 query 不在同一个组件中

如果 mutation 和 query 位于同一组件中，则此方法非常有效。但是，你还可以通过专用的 useMutationState hook 访问其他组件中的所有 mutation。它最好与 mutationKey 结合使用

tsx

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

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

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

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

variables 将是一个 Array，因为可能同时运行多个 mutation。如果我们需要项目的唯一键，我们还可以选择 mutation.state.submittedAt。这将甚至使显示并发乐观更新变得轻而易举。

通过缓存

当你在执行 mutation 之前乐观地更新你的状态时，mutation 有可能会失败。在大多数这些失败情况下，你可以只需为你的乐观 query 触发 refetch，以将它们恢复到其真实的服务器状态。但在某些情况下，refetch 可能无法正常工作，并且 mutation 错误可能表示某种服务器问题，导致无法进行 refetch。在这种情况下，你可以选择回滚你的更新。

为此，useMutation 的 onMutate 处理函数选项允许你返回一个值，该值稍后将作为最后一个参数传递给 onError 和 onSettled 处理函数。在大多数情况下，传递回滚函数是最有用的。

添加新 todo 时更新 todo 列表

tsx

const queryClient = useQueryClient()

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

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

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

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

const queryClient = useQueryClient()

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

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

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

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

更新单个 todo

tsx

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

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

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

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

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

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

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

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

如果你愿意，你也可以使用 onSettled 函数来代替单独的 onError 和 onSuccess 处理函数

tsx

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

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

何时使用哪个

如果你只有一个地方需要显示乐观结果，则使用 variables 并直接更新 UI 是一种需要较少代码且通常更容易理解的方法。例如，你根本不需要处理回滚。

但是，如果屏幕上有多个地方需要了解更新，则直接操作缓存将为你自动处理此问题。