Angular5中HttpClient回調(diào)方法使用注入對象方法無效 ？

1.使用angular HttpClient 請求后臺數(shù)據(jù)獲取標(biāo)題，通過@angular/platform/Title 服務(wù)的方法設(shè)置標(biāo)題后沒有效果，請指教，如何解決？

2.代碼片段如下，angular 版本：5.2.0 ，沒有報錯信息，就是方法調(diào)用后沒有達到預(yù)期

import {Title} from '@angular/platform-browser';
import {HttpClient, HttpErrorResponse, HttpResponse} from '@angular/common/http';

constructor(private titleService: Title) {

 }


 
  ngOnInit() {
    let that = this;
    that.titleService.setTitle("原標(biāo)題"); //生效
    this.http.get<any>('/api/dingtalk/get-file-title/'+1,{ observe: 'response'}).subscribe(
      (res: HttpResponse<any>) => {
        that.titleService.setTitle("新標(biāo)題"); //無效
        console.log("success!" ,that.titleService.getTitle())
      },
      (res: HttpErrorResponse) => console.log(res.error)
    );
  }

console.log("success!" ,that.titleService.getTitle()) 輸出：

但是頁面效果：