watch.js

const chokidar = require('chokidar');
const cp = require('child_process');

let appProcesses;

const cwd = process.cwd();
const commands = process.argv[2].split(',');

void async function startWatch() {
  const reload = reloadAll();

  chokidar
    .watch(cwd, {
      ignoreInitial: true,
      ignored: /\.test-cache|node_modules|opencv*/
    })
    .on('all', reload)
    .on('ready', reload)
    .on('error', exit);
}();

function createAppProcesses() {
  const msg = appProcesses ? 'restarted' : 'started';
  appProcesses = new Map;

  for (let command of commands) {
    const process = cp.exec(`npm run ${ command }`, {
      stdio: "inherit"
    })
      .on('error', onAppProcessError(command))
      .on('exit', onAppProcessExit(command));

    console.log(`Robot ${ msg } (${ command })`);

    appProcesses.set(command, process);
  }
}

function exit(err) {
  console.log(err);
  process.exit(1);
}

async function killAppProcesses() {
  if (appProcesses && appProcesses.size) {
    return await Promise.all(
      [ ...appProcesses.values() ].map(process =>
        new Promise(resolve => {
          process.on('exit', resolve);
          process.kill();
        })
      )
    );
  }
}

function onAppProcessError(command) {
  return (error) => {
    appProcesses.delete(command);
    console.error(error);
    console.log(`Robot encountered an error (npm run ${ command })`);
  };
}

function onAppProcessExit(command) {
  return () => {
    appProcesses.delete(command);
    console.log(`Robot stopped (npm run ${ command })`);
  };
}

function reloadAll() {
  let reloading = false;
  let reloadAgain = false;

  const reload = async filepath => {
    if (filepath) {
      console.log(`Detected change (${ filepath })...`);
    } else {
      console.log('Initializing watch');
    }

    if (reloading) {
      reloadAgain = true;
    } else {
      reloading = true;
      reloadAgain = false;

      await killAppProcesses();
      createAppProcesses();

      reloading = false;

      if (reloadAgain) {
        await reload(filepath);
      }
    }
  };

  return reload;
}

process.on('unhandledRejection', err => {
  console.log(err);
  process.exit(1);
});