iOS 8 standalone apps screen lock detector

ios-screenlock-detector.js

//
// Detect screenlock on standalone ios apps and replace content with message to reload the app
// When user locks screen on iOS 8 devices, everything dies except web workers
// This will definitely drain battery.
// Based on the idea by ajayel: http://stackoverflow.com/a/26091749/351305
//
(function () {
	var WORKER_URI = '/app/worker.js';
	var TIMEOUT = 2000;

	// run on iOS 8+ only
	if(!window.navigator.standalone || window.navigator.userAgent.toLowerCase().indexOf('os 8_') === -1) {
		return;
	}

	var t = {
		tok:  null,
		tik: null,
		pan: false,
		worker: null,
		interval: null
	};

	function stopWork() {
		t.worker.terminate();
		clearInterval(t.interval);

		t.worker = null;
		t.interval = null;
	}

	function startWork() {
		t.worker = new Worker(WORKER_URI);
		t.tik = t.tok = +new Date();
		t.worker.onmessage = function onmessage (e) {
			var d = JSON.parse(e.data);
			if(d.message === 'tiktok') {
				// ignored delayed messages (due to touchmove)
				if(d.result < t.tok) {
					return;
				}

				t.tok = d.result;
				var diff = Math.abs(t.tik - t.tok);

				if(diff > TIMEOUT * 2) {
					stopWork();
					document.write('Please restart app after locking the screen. Apple are working to prevent this issue.');
				}
			}
		}
		t.interval = setInterval(function () {
			t.tik = +new Date();
		}, TIMEOUT);
	}

	window.addEventListener('touchmove', function () {
		if(!t.pan) {
			t.tok = t.tik = +new Date();
			t.pan = true;
		}
	});

	window.addEventListener('touchend', function () {
		if(t.pan) {
			t.tok = t.tik = +new Date();
			t.pan = false;
		}
	});
	startWork();
})();

worker.js

(function () {
	setInterval(function () {
		postMessage(JSON.stringify({
			message: "tiktok",
			result: +new Date()
		}));
	}, 1000);
})();