javascript - Angular 2 : Promise return

Question

我想从我的 API 获取评论。那么，函数应该 promise 返回吗？什么是更好的？经典还是 promise 回归？

我还有一个问题， promise 返回未定义。

comments.component.ts

import { Component, OnInit } from '@angular/core';
import { CommentService } from '../services/comment.service';
import { Comment } from '../class/Comment';

@Component({
  template: 'dadada',
  providers: [CommentService]
})

export class CommentsComponent implements OnInit {
    coms: Comment[];

    constructor(private commentService: CommentService) {
    }

    ngOnInit() { 
        console.log( this.commentService.testfunction() ); 

        this.commentService.get_all_comments().then((data) => {
            this.coms = data;
          });
        console.log ( this.commentService.get_all_comments2() );
        console.log ( this.coms );
    }
}

comment.service.ts

import { Injectable } from '@angular/core';
import { Comment, Comments } from '../class/Comment';

@Injectable()
export class CommentService {
    testfunction() {
        return 'valoare';
    }
    get_all_comments() {
        return Promise.resolve(Comments);
    }
    get_all_comments2() {
        return Comments;
    }    
}

评论.ts

export class Comment {
  id: number;
  text: string;
  author: string;
  created_at: number;
  updated_at: number;
}

export const Comments: Comment[] = [
  {id: 1, text: 'Look I am a test comment.', author: 'Chris Sevilleja', created_at: 0, updated_at: 0}
];

我在控制台中得到这些:

valoare

Array [ Object ]

undefined

Answer 1

您需要将代码移至 then(...) 内(对于具有 subscribe(...) 的可观察量也是如此

ngOnInit() { 
    console.log( this.commentService.testfunction() ); 

    this.commentService.get_all_comments().then((data) => {
        this.coms = data;
        console.log ( this.commentService.get_all_comments2() );
        console.log ( this.coms );
      });
}

Promise 和 then(...) 的目的是启用链接调用，以便在前一个调用完成时执行后续调用。

异步执行意味着调用将被排入事件队列，然后执行同步代码(您的 console.log())。传递给 .then(...) 的代码最终会在 Promise 解析时执行(通常是在服务器响应到达时)。