process-listener.js

const EventEmitter = require('events'),
 ps = require('xps');

class ProcessListener extends EventEmitter {
  constructor() {
    super();
    let listenTimeout = 1000;
    let listenTimeoutId = false;
    let running = false;
    let previousProcesses = [];
    let previousProcessesById = {};

    const executeListenIntervalFn = () => {
      getProcessListFn((processes, newProcesses, closedProcesses) => {
        if (running) {
          // notify the listeners about the new & closed processes
          newProcesses.forEach(process => this.emit('start', process));
          closedProcesses.forEach(process => this.emit('stop', process));
          this.emit('list', processes);
          // schedule the next call
          listenTimeoutId = setTimeout(executeListenIntervalFn, listenTimeout);
        }
      });
    };

    const getProcessListFn = cb => {
      ps.list().fork(
        error => {
          throw error
        },
        processes => {
          const processesById = {};
          processes.forEach(process => processesById[process.pid] = process);
          // find the new processes
          const newProcesses = processes.filter(process => previousProcessesById[process.pid] == null);
          const closedProcesses = previousProcesses.filter(process => processesById[process.pid] == null);
          previousProcesses = processes;
          previousProcessesById = processesById;
          if (cb) {
            cb(processes, newProcesses, closedProcesses);
          }
        }
      );
    };

    this.getRunningProcesses = () => {
      return new Promise(resolve => {
        if (previousProcesses.length > 0) {
          return resolve(previousProcesses);
        }
        if (running) {
          // wait for the first 'list', at that point we have the processes
          this.once('list', () => resolve(previousProcesses));
        } else {
          getProcessListFn((processes, newProcesses, closedProcesses) => {
            resolve(processes);
          });
        }
      });
    };

    this.setListenTimeout = value => {
      listenTimeout = value;
    };

    this.stop = () => {
      running = false;
      clearTimeout(listenTimeoutId);
    };

    this.start = (listenTimeoutValue = 1000) => {
      if (running) {
        return;
      }
      listenTimeout = listenTimeoutValue;
      running = true;
      executeListenIntervalFn();
    };
  }
};

module.exports = new ProcessListener();