visibility change and visibility state to drive web worker processing, closeand restart.

visibility-close-restart-worker.js

// 31 May 2024
// visibility change and visibility state to drive web worker processing, close
// and restart.

// previous version, pause and resume, at
// https://gist.github.com/dfkaye/3a9d6752301733fba81c740cf4fb68b2

var source = `
  var count = 0;
  var interval;

  self.actions = {
    init() {
      interval = setInterval(function () {
        count += 1;
        postMessage({ action: "advanced", value: count });
      }, 1000);
    },
    stop() {
      clearInterval(interval);
      self.close();

      // this post will deliver even though we've requested the current worker
      // be terminated (closed).

      postMessage({ action: "stopped", value: count });
    },
    start() {
      actions.init();
      postMessage({ action: "started", value: count });
    }
  };

  self.onmessage = function (request) {
    var { action, value } = Object(request.data);
    var fn = self.actions[action];
    typeof fn == "function"
      ? fn()
      : console.error("no action for", action);
  };

  actions.start();
`;
var blob = new Blob([source], { type: "text/javascript" });
var url = URL.createObjectURL(blob);
//   var worker = new Worker(url);

//   worker.onmessage = function (response) {
//     var { action, value } = Object(response.data);

//     console.log(action, value);
//   };

~(function restart() {
  var worker = new Worker(url);

  worker.onmessage = function (response) {
    var { action, value } = Object(response.data);

    console.log(action, value);
  };

  document.onvisibilitychange = function (e) {
    var { visibilityState } = e.target;
    if (visibilityState == 'hidden') {
      worker.postMessage({ action: "stop" });
    }
    else {
      console.log("should restart");

      // use a timeout to make the start log more obvious
      setTimeout(restart, 1000);
    }
  };
})();