calendar.service.ts

import { google } from 'googleapis';
import { Injectable, HttpService } from '@nestjs/common';
import { InjectSchedule, Schedule } from 'nest-schedule';
import { LoggerService } from '../logger/logger.service';

interface GoogleCalendarEvent {
  id: string;
  summary: string;
  description: string;
  // ...
  // more properties can be found here:
  // https://googleapis.dev/nodejs/googleapis/latest/calendar/interfaces/Schema$Event.html
}

/*
* CalendarService for polling a Google Calendar and looking for scheduled events to broadcast
* in a slack channel at the scheduled time. For each event, a message can be set explicitely.
*
* - A calendar must be selected with an id specified in the CALENDAR_ID environment variable
* - A service account json must be available through a location set in GOOGLE_APPLICATION_CREDENTIALS
* - Event summary (title) must be in the format of '[#channel] ...',  e.g. '[#general] simple reminder'
* - The broadcasted message must be placed in the event description
* - A processed event summary will be set to '[!channel]' (or '[?channel]' if no message was found)
*
* The Google NodeJS API can be found here:
* https://github.com/googleapis/google-api-nodejs-client
*
* The Google calendar API and resources can be found here:
* https://googleapis.dev/nodejs/googleapis/latest/calendar/index.html
*/
@Injectable()
export class CalendarService {
  calendar: ReturnType<typeof google.calendar>;
  POLLING_INTERVAL = 5 * 1000; // in milliseconds

  constructor(private loggerService: LoggerService,
              private httpService: HttpService,
              @InjectSchedule() private readonly schedule: Schedule) {
      // Connecting to google calendar is asynchronous, fired from constructor
      this.connectToCalendar();
  }

  async connectToCalendar() {
    try {
      // Create a new GoogleAuth instance with service-account credentials from a json file.
      // Note: credential file location must be set in GOOGLE_APPLICATION_CREDENTIALS env variable
      const auth = new google.auth.GoogleAuth({
        scopes: ['https://www.googleapis.com/auth/calendar'],
        clientOptions: {
          // Overwrite JWT subject to 'impersonate' calendar user
          subject: 'wilco@headfwd.com'
        }
      });

      this.calendar = google.calendar({
        version: 'v3',
        auth: await auth.getClient()
      });
    } catch (err) {
      this.loggerService.log(`[calendar] could not connect to calendar API: ${err}`);
      return;
    }

    this.loggerService.log('[calendar] successfully connected to calendar API');

    if (!process.env.EVENT_CALENDAR_ID) {
      this.loggerService.log('[calendar] no CALENDAR_ID found in env, poller not started');
      return;
    }

    this.loggerService.log('[calendar] starting poller');

    this.schedule.scheduleIntervalJob('poller', this.POLLING_INTERVAL, () => this.pollCalendarEvents());
  }

  async pollCalendarEvents() {
    this.loggerService.debug('[calendar] polling messages...');

    // Define a range of twice the polling interval (in case of polling delays)
    const timeNow = new Date();
    const timeMin = new Date(timeNow.getTime() - this.POLLING_INTERVAL * 2);

    try {
      // Request all events in the selected calendar in the selected range
      const response = await this.calendar.events.list({
        calendarId: process.env.EVENT_CALENDAR_ID,
        singleEvents: true, // Flatten any recurring events
        timeMin: timeMin.toISOString(),
        timeMax: timeNow.toISOString()
      });

      const events = response.data.items as GoogleCalendarEvent[];

      // Process events one by one (sequentially)
      for (const event of events) {
        this.loggerService.debug(`[calendar] found event: ${event.summary}`);
        await this.processEvent(event);
      }
    } catch (err) {
      this.loggerService.log(`[calendar] poller failed: ${err}`)
    }

    // Keep the interval job running
    return false;
  }

  async processEvent(event: GoogleCalendarEvent) {
    // Test event for a summary in the format of '[#channel] ...'
    const regex = /^\[(#[a-z0-9-_]+)\]/;
    const elements = regex.exec(event.summary);

    // This event does not match format (could have been processed before)
    if (elements === null) {
      return;
    }

    this.loggerService.log(`[calendar] processing event: ${event.summary}`)

    // Channel is extracted from regex, message is in event description
    const channel = elements[1]; // first captured group
    const message = event.description;

    if (message) {
      await this.postSlackMessage(channel, message);
    }

    // Set new summary to '[!channel] ...' (or '[?channel] ...' if no message was found)
    const newSymbol = message ? '!' : '?';
    const newSummary = '[' + newSymbol + event.summary.substring(2);

    // Patch calendar event with new summary
    await this.calendar.events.patch({
      calendarId: process.env.EVENT_CALENDAR_ID,
      eventId: event.id,
      requestBody: {
        summary: newSummary
      }
    });
  }

  async postSlackMessage(channel: string, text: string) {
    const data = { channel, text, link_names: true };
    const config = { headers: { Authorization: 'Bearer ' + process.env.SLACK_TOKEN }};
    const endpoint = 'https://slack.com/api/chat.postMessage';

    await this.httpService.post(endpoint, data, config).toPromise();
  }
}