如何解决异步函数中的承诺？

Question

我有一个生命周期方法 componentDidMount 调用递归异步方法，我希望递归函数在获取所有数据后返回一个承诺。

  async componentDidMount() {
    let response = await fetch(`${STORY_URL}${this.props.match.params.id}.json`);
    let result = await response.json();

    totalComments = result.descendants;

    await this.fetchComments(result.kids, MARGIN);

    this.setState({
      by: result.by,
      time: calculateTimeDifference(result.time)
    });
  }

被调用的函数是

async fetchComments(comment, margin) {
    return new Promise((resolve, reject) => {

      comment.map(async commentId => {
        let response = await fetch(`${STORY_URL}${commentId}.json`);
        let result = await response.json();
        comments.push({
          by: result.by,
          margin: margin
        });

        if (comments.length === totalComments + 1)  resolve();

        if (result.kids !== undefined) this.fetchComments(result.kids, margin * 2);
      });
    });
  }

但是 resolve 方法没有返回到 setState 之前的 componentDidMount。我通过控制台日志检查。我不知道我做错了什么

Answer 1

你把事情复杂化了。使用Promise.all:

 async fetchComments(comments, margin) {
   // Collect all the results here, otgerwise we would have to flatMap the promises which is more complicated
   const result = [];

   // Make sure that all comments were processed before returning
   await Promise.all( comments.map(async commentId => {
    const response = await fetch(`${STORY_URL}${commentId}.json`);
    const { kids, by } = await response.json();

    if(kids) {
       // Get all children and append them to the results, right after the children
       const children = await fetchComments(kids, margin * 2);
       result.push({ by, margin }, ...children);
    } else {
       // Otherwise just append this node
      result.push({ by, margin });    
    }   
  }));

  return result;
}

如果顺序很重要，您必须将Promise.all 的结果展平：

  async fetchComments(comments, margin) {
    const result = await Promise.all(comments.map( async commentID => {
      const response = await fetch(STORY_URL + commentID + ".json");
      const { by, kids } = await response.json();

      const result = [{ by, margin }];
      if(kids) result.push(... await fetchComments(kids, margin * 2));

      return result;
   }));

   // Flatten the results
   return [].concat(...result);
 }