frontend/src/app/service/api.service.ts

import {Injectable} from '@angular/core';
import {environment} from '../../environments/environment';
import {HttpClient} from '@angular/common/http';
import {BehaviorSubject, Observable} from 'rxjs';
import ApiResponse from '../structures/ApiResponse';
import {MenuItem} from './db/MenuItem';
import {DatabaseService} from './db/database.service';
import {ConnectionService} from 'ng-connection-service';

@Injectable({
  providedIn: 'root'
})
export class ApiService {
  protected baseEndpoint: string = environment.backendBase;
  protected statUrl: string = environment.statUrl;
  protected versionUrl: string = environment.versionUrl;
  protected offline = false;
  public readonly offline$: BehaviorSubject<boolean> = new BehaviorSubject<boolean>(false);

  get isOffline() {
    return this.offline;
  }

  constructor(
      protected http: HttpClient,
      protected db: DatabaseService,
      protected connection: ConnectionService,
  ) {
    connection.monitor().subscribe(isConnected => {
      if ( !isConnected ) {
        this.makeOffline();
      } else {
        this.makeOnline(); // TODO add checks for server.
      }
    });
  }

  public makeOffline() {
    this.offline = true;
    this.offline$.next(true);
  }

  public makeOnline() {
    this.offline = false;
    this.offline$.next(false);
  }

  public get(endpoint, params = {}): Observable<ApiResponse> {
    return this.request(endpoint, params, 'get');
  }

  public post(endpoint, body = {}): Observable<ApiResponse> {
    return this.request(endpoint, body, 'post');
  }

  public request(endpoint, params = {}, method: 'get'|'post' = 'get'): Observable<ApiResponse> {
    return this._request(this._build_url(endpoint), params, method);
  }

  public stat(): Observable<ApiResponse> {
    return new Observable<ApiResponse>(sub => {
      (async () => {
        const statKV = await this.db.getKeyValue('host_stat');

        // If offline, look up the last stored stat for information
        if ( this.isOffline ) {
          if ( typeof statKV !== 'object' ) {
            throw new Error('No locally stored host stat found.');
          }

          sub.next(new ApiResponse(statKV.data));
          sub.complete();
        }

        // Otherwise, fetch the stat and cache it locally
        this._request(this.statUrl).subscribe(apiResponse => {
          statKV.data = {status: apiResponse.status, message: apiResponse.message, data: apiResponse.data};
          statKV.save().then(() => {
            sub.next(statKV.data);
            sub.complete();
          });
        });
      })();

    });
  }

  public version(): Promise<string> {
    return new Promise(async (res, rej) => {
      const versionKV = await this.db.getKeyValue('app_version');

      // If offline, look up the local app version.
      if ( this.isOffline ) {
        if ( versionKV ) {
          return res(versionKV.data);
        } else {
          return rej(new Error('No local app version found.'));
        }
      }

      // Otherwise, look up the app version and store it locally
      this._request(this.versionUrl).subscribe({
        next: async result => {
          const version = result.data.text.trim();

          versionKV.data = version;
          await versionKV.save();

          res(version);
        },
        error: rej,
      });
    });
  }

  protected _request(endpoint, params = {}, method: 'get'|'post' = 'get'): Observable<ApiResponse> {
    return new Observable<ApiResponse>(sub => {
      let data: any = {}
      if ( method === 'get' ) {
        data.params = params;
      } else {
        data = params;
      }

      this.http[method](endpoint, data).subscribe({
        next: (response: any) => {
          sub.next(new ApiResponse(response));
        },
        error: (err) => {
          const response = {
            status: err.status,
            message: err.message,
            data: err.error,
          };

          sub.next(new ApiResponse(response));
        },
      });
    });
  }

  public _build_url(endpoint) {
    if ( !endpoint.startsWith('/') ) {
      endpoint = `/${endpoint}`;
    }

    return `${this.baseEndpoint.endsWith('/') ? this.baseEndpoint.slice(0, -1) : this.baseEndpoint}${endpoint}`;
  }

  public getMenuItems(): Promise<any[]> {
    return new Promise(async (res, rej) => {
      await this.db.createSchemata();

      // If offline, fetch the menu from the database
      if ( this.isOffline ) {
        const items = await this.db.menuItems.toArray();
        const nodes = MenuItem.inflateTree(items as MenuItem[]);
        return res(nodes);
      }

      // Download the latest menu items
      const tree: any[] = await new Promise(res2 => {
        this.get('/menu/items').subscribe({
          next: async result => {
            const nodes = result.data as any[];
            const items = MenuItem.deflateTree(nodes);

            // Update the locally stored nodes
            await this.db.menuItems.clear();
            await Promise.all(items.map(item => item.save()));

            res2(nodes);
          },
          error: rej,
        });
      });

      res(tree);
    });
  }

  public getSessionData(): Promise<any> {
    return new Promise(async (res, rej) => {
      const sessionKV = await this.db.getKeyValue('session_data');

      // If offline, just return the locally cached session data
      if ( this.isOffline ) {
        if ( typeof sessionKV.data !== 'object' ) {
          return rej(new Error('No locally cached session data found.'));
        }

        return res(sessionKV.data);
      }

      // Otherwise, fetch the session data from the server and cache it locally
      this.get('/session').subscribe(async result => {
        sessionKV.data = result.data;
        await sessionKV.save();
        res(sessionKV.data);
      });
    });
  }

  public saveSessionData(data: any): Promise<void> {
    return new Promise(async (res, rej) => {
      // Update the local session data
      const sessionKV = await this.db.getKeyValue('session_data');
      sessionKV.data = data;
      await sessionKV.save();

      // If we're not offline, then update the data on the server
      if ( !this.isOffline ) {
        await new Promise(res2 => {
          this.post('/session', data || {}).subscribe({
            next: res2,
            error: rej,
          });
        });
      }

      res();
    });
  }
}