angular2 ng在ngOnInit()从api获取数据时不起作用

import { Component,OnInit } from '@angular/core';
import { Router} from '@angular/router'
import { Comment } from 'comment entity path'
import {CommentService} from 'comment service path'
import { Observable } from 'rxjs/Observable';
@Component({
    template: ` <ul><li *ngFor="let comment of comments|async"> {{comment.Name}}</li></ul>`
})
export class CommentComponent implements OnInit {
    comments: Observable<comment[]>;  

    constructor(private router: Router,private commentService: CommentService) {
    }

    ngOnInit() {
        this.comments = this.getComments();
    }

    getComments() {
        return this.commentService.getComments();
    }

}

comment.service.ts

import { Injectable } from '@angular/core';
import { Headers,Http,Response } from '@angular/http';
import { Comment } from 'comment path here';
import { Observable } from 'rxjs/Rx';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/catch';

@Injectable()
export class CommentService {
    private commentUrl = 'api path';  // URL to web api

    constructor(private http: Http) {
    }

    getComments(): Observable<Comment[]> {
        return this.http.get(this.commentUrl).map(
            (response) => {
                let data = response.text() ? response.json():[{}];
                if (data) {
                    console.log(data);
                    return data;
                }
                return data;
            });
        }
    }

在ngOnInit方法中,我能够获得注释列表,但问题是列表没有使用HTML上的ngFor绑定.这是因为HTML在响应之前呈现.但是刷新页面数据会自动绑定.我错过了什么吗？