Apollo Client：更新缓存时是 writeFragment 还是 readFragment？

Question

在useMutation 的update 钩子中，Apollo 的文档推荐使用writeFragment 获取对新添加对象的内部引用。我觉得这很奇怪，因为该对象已经存在于缓存中。所以我用 readFragment 测试了它，果然，它运行良好。在此用例中，是否优先使用 writeFragment 而不是 readFragment？

示例 1：

https://www.apollographql.com/docs/react/data/mutations/#making-all-other-cache-updates

const [addTodo] = useMutation(ADD_TODO, {
    update(cache, { data: { addTodo } }) {
      cache.modify({
        fields: {
          todos(existingTodos = []) {
            const newTodoRef = cache.writeFragment({
              data: addTodo,
              fragment: gql`
                fragment NewTodo on Todo {
                  id
                  type
                }
              `
            });
            return [...existingTodos, newTodoRef];
          }
        }
      });

该页面的摘录：

With the help of cache.writeFragment we get an internal reference to the added todo, then store that reference in the ROOT_QUERY.todos array.

示例 2：

https://www.apollographql.com/docs/react/caching/cache-interaction/#example-updating-the-cache-after-a-mutation

const [addComment] = useMutation(ADD_COMMENT, {
  update(cache, { data: { addComment } }) {
    cache.modify({
      fields: {
        comments(existingCommentRefs = [], { readField }) {
          const newCommentRef = cache.writeFragment({
            data: addComment,
            fragment: gql`
              fragment NewComment on Comment {
                id
                text
              }
            `
          });
          return [...existingCommentRefs, newCommentRef];
        }
      }
    });
  }
});

该页面的摘录：

the comment was already added to the cache by useMutation. Consequently, cache.writeFragment returns a reference to the existing object.

我也在 Apollo Client 的讨论区 (https://github.com/apollographql/apollo-client/discussions/7515) 上发布了这个问题，但没有得到回复。

Answer 1

从缓存中获取项目时使用 writeFragment 优于 readFragment 的好处在此处解释（摘自 https://www.apollographql.com/docs/react/caching/cache-interaction/#example-updating-the-cache-after-a-mutation）：

If you call writeFragment with an options.data object that the cache is able to identify, based on its __typename and primary key fields, you can avoid passing options.id to writeFragment.

Whether you provide options.id explicitly or let writeFragment figure it out using options.data, writeFragment returns a Reference to the identified object.

This behavior makes writeFragment a good tool for obtaining a Reference to an existing object in the cache, which can come in handy when writing an update function for useMutation

这是违反直觉的，因为名称 writeFragment 暗示它用于写入缓存而不是从中读取，但这似乎是推荐的最佳做法。