Angular2：承诺回报

Question

I want to get the comments from my API. So, the function should promise a return? What is better? Clasic or promise return?

And I have still a problem, the promise return return undefined.

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;
    }    
}

Comment.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}
];

And I get in console these:

valoare

Array [ Object ]

undefined

Answer 1

You need to move the code inside the then(...) (same for observables with subscribe(...)

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 );
      });
}

The purpose of Promise and then(...) is to enable to chain calls so that a subsequent call is executed when the previous one completes.

Async execution means that the call is enqueued into the event queue and sync code (your console.log() ) is executed next. The code passed to .then(...) is eventually executed when the Promise resolves (usually when the response from the server arrives).