ui/src/app/services/chat.service.ts

import { Injectable } from '@angular/core';
import { catchError, from, map, merge, mergeMap, Observable, throwError } from 'rxjs';
import { webSocket } from 'rxjs/webSocket';
import { Channel } from '../models/channel';
import { Message } from '../models/message';
import { HttpClient } from '@angular/common/http';
import { environment } from '../../environment/environment';
import { GetMessagesResponse, ListAvailableChannelsResponse, SendMessageResponse } from './responses/chat';

@Injectable({
  providedIn: 'root'
})
export class ChatService {

  private readonly CHANNEL: string = "channel_id";
  private readonly CONTENT: string = "content";

  constructor(private http: HttpClient) { }

  public ListAvailableChannels(): Observable<Channel[]> {
    let url = `${environment.apiBase}/chat/channels`
    return this.http.get<ListAvailableChannelsResponse>(url, { withCredentials: true })
      .pipe(
        map(response => {
          if (response.error) {
            throw new Error(response.error);
          }

          if (response.channels) {
            return response.channels;
          }

          throw new Error("bad API response, missing channels with no error");
        }),
        catchError(error => throwError(() => new Error(error.error.message)))
      );
  }

  public GetMessages(channelID: number): Observable<Message> {
    let url = `${environment.apiBase}/chat/messages/${channelID}`;
    let messages: Observable<Message[]> =
      this.http.get<GetMessagesResponse>(url, { withCredentials: true })
        .pipe(
          map(response => {
            if (response.error) {
              throw new Error(response.error);
            }

            if (response.messages) {
              return response.messages;
            }

            throw new Error("bad API response, missing messages with no error");
          }),
          catchError(error => throwError(() => new Error(error.error.message)))
        );

    url = `${environment.apiBase}/chat/subscribe/${channelID}`;
    let socket = webSocket<Message>(url);

    return merge(
      messages.pipe(mergeMap(msgArray => from(msgArray))),
      socket.asObservable());
  }

  public SendMessage(channel: number, content: string): Observable<SendMessageResponse> {
    let url = `${environment.apiBase}/chat/send`;

    let data = new FormData();
    data.append(this.CHANNEL, channel.toString());
    data.append(this.CONTENT, content);

    return this.http.post<SendMessageResponse>(url, data, { withCredentials: true })
      .pipe(
        map(response => {
          if (response.error) {
            throw new Error(response.error);
          }

          return response;
        }),
        catchError(error => throwError(() => new Error(error.error.message)))
      );
  }
}