control Electron instances

Question

Wanted to check how many instances are running and control the number of instances running in one exe electron bundle. Let us say I wanted to allow only three instances running for the one exe bundle. I am not able to do this.

Current Behavior:

Only one and remaining can block. Or open for any number of instances. We need to control only three instances running, not more than that.

Example:

const { app } = require('electron')
let myWindow = null
    
const gotTheLock = app.requestSingleInstanceLock()
    
if (!gotTheLock) {
  app.quit()
} else {
  app.on('second-instance', (event, commandLine, workingDirectory) => {
    // Someone tried to run a second instance, we should focus our window.
    if (myWindow) {
      if (myWindow.isMinimized()) myWindow.restore()
      myWindow.focus()
    }
  })
    
  // Create myWindow, load the rest of the app, etc...
  app.on('ready', () => {
  })
}

user_3127812 · Answer

You can try with the following code to know how many windows have been opened.

const count = BrowserWindow.getAllWindows().length;

To check visible windows, you can try the following code

let count = BrowserWindow.getAllWindows()
  .filter(b => {
    return b.isVisible()
  }).length

Once you get the number of instances, based upon the condition for number of instance, ie. if it is more than 3, you can quit using app.quit().

user_9698583 · Answer

You can make each instance write to a file (increment a counter for example) when the instance starts and when it exits. (decrement the counter). You should check that file to see if the maximum number of instances are running

import { app } from "electron";
import path from "path";
import fs from "fs";

const MAX_APP_INSTANCES = 3;
const INSTANCE_COUNT_FILE_PATH = path.join(
  app.getPath("userData"),
  "numOfInstances"
);

// utils to read/write number of instances to a file
const instanceCountFileExists = () => fs.existsSync(INSTANCE_COUNT_FILE_PATH);
const readInstanceCountFile = () =>
  parseInt(fs.readFileSync(INSTANCE_COUNT_FILE_PATH, "utf-8"));
const writeInstanceCountFile = (value) =>
  fs.writeFileSync(INSTANCE_COUNT_FILE_PATH, value);
const incInstanceCountFile = () => {
  const value = readInstanceCountFile() + 1;
  writeInstanceCountFile(value.toString());
};
const decInstanceCountFile = () => {
  const value = readInstanceCountFile() - 1;
  writeInstanceCountFile(value.toString());
};

// logic needed to only allow a certain number of instances to be active
if (instanceCountFileExists() && readInstanceCountFile() >= MAX_APP_INSTANCES) {
  app.quit();
} else {
  if (!instanceCountFileExists()) {
    writeInstanceCountFile("1");
  } else {
    incInstanceCountFile();
  }
  app.on("quit", () => decInstanceCountFile());
}

Note: this is solution is somewhat hacky. For example, the quit event is not guaranteed to fire when the Electron app exits

control Electron instances

2 Answers