Angular 7 api没有被调用

我正在尝试调用 API，但我认为出了问题，

import { map } from "rxjs/operators";
import { Injectable } from "@angular/core";
import { HttpClient } from "@angular/common/http";

export class Message {
    constructor(public text: any, public sentBy: any) { }
}

@Injectable()
export class ChatService {
    constructor(public http: HttpClient) {
    }

    public sendMessage(message) {
        const usertext = new Message(message, 'user');
        console.log("message send",usertext);
       return this.http
            .post<any>(`http://locahost:3000/api/text_query`, usertext)
            .pipe(
                map(response => {
                    return response;
                })
            );
    }

}

没有获得任何登录信息Network tab铬。 我正在使用 Angular 7.0.1 和 Typescript: 3.1.3 这是我的应用程序组件文件

import {Component, OnInit} from '@angular/core';
import {ChatService} fom './chat.service';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss']
})
export class AppComponent implements OnInit {
  message: string;
  constructor(private chatService: ChatService) {}

  sendMessage() {
    this.chatService.sendMessage(this.message).subscribe(res=>{
    })
  }
  ngOnInit() {
  }
}

该服务已正确添加到 app.module.ts 文件中

暴露的方法HttpClient通常返回一个 Cold Observable。这本质上意味着这些方法不会进行任何 API 调用，除非它们返回的 Observables 是subscribed to.

要解决您的问题：

import { map } from "rxjs/operators";
import { Injectable } from "@angular/core";
import { HttpClient } from "@angular/common/http";

export interface Message {
  public text: any;
  public sentBy: any;
}

@Injectable()
export class ChatService {
  constructor(public http: HttpClient) {}

  public sendMessage(message) {
    const usertext: Message = { text: message, sentBy: 'user' };
    return this.http
      .post<any>(`http://locahost:3000/api/text_query`, usertext);
  }

}

在你的组件中：

...
import { ChatService } from "path-to-chat-service";

@Component({...})
export class ChatComponent {
  constructor(private chatService: ChatService) {}

  sendMessage(message) {
    this.chatService.sendMessage(message)
      .subscribe(res => console.log(res));
  }

  ...

}

有用的资源：

1. 热观测值与冷观测值 https://medium.com/@benlesh/hot-vs-cold-observables-f8094ed53339- Ben Lesh（RXJS 负责人）。
2. 冷与热的观测结果 https://blog.thoughtram.io/angular/2016/06/16/cold-vs-hot-observables.html- 思想传输。