使用immer库的produce函数，确保修改了状态。

示例代码：

import { createAsyncThunk, createSlice } from '@reduxjs/toolkit';
import produce from 'immer';

const initialState = {};

export const fetchUser = createAsyncThunk(
  'user/fetch',
  async (_, { rejectWithValue }) => {
    try {
      const response = await fetch('/api/user');
      const data = await response.json();
      return data;
    } catch (error) {
      return rejectWithValue(error.message);
    }
  }
);

const slice = createSlice({
  name: 'user',
  initialState,
  reducers: {},
  extraReducers: (builder) => {
    builder
      .addCase(fetchUser.pending, (state) => {
        return produce(state, (draftState) => {
          draftState.loading = true;
        });
      })
      .addCase(fetchUser.fulfilled, (state, action) => {
        return produce(state, (draftState) => {
          draftState.loading = false;
          draftState.user = action.payload;
        });
      })
      .addCase(fetchUser.rejected, (state) => {
        return produce(state, (draftState) => {
          draftState.loading = false;
          draftState.error = true;
        });
      });
  },
});

export default slice.reducer;