page.js

'use strict';

var atoa = require('atoa');
var serialization = require('./serialization');
var emitter = require('contra/emitter');

module.exports = createChannel;

function createChannel () {
  var channel = at(navigator.serviceWorker.controller);
  return channel;

  function at (worker) {
    var internalEmitter = emitter();
    var api = {
      on: selfed('on'),
      once: selfed('once'),
      off: selfed('off'),
      emit: postToWorker,
      at: at
    };
    var postFromWorker = serialization.emission(internalEmitter, { broadcast: false });
    navigator.serviceWorker.addEventListener('message', broadcastHandler);
    return api;

    function selfed (method) {
      return function selfish () {
        internalEmitter[method].apply(null, arguments);
        return api;
      };
    }

    function postToWorker () {
      if (!worker) {
        return Promise.reject(new Error('ServiceWorker not found.'));
      }
      var payload = serialization.parsePayload(atoa(arguments));
      var messageChannel = new MessageChannel();
      messageChannel.port1.addEventListener('message', postFromWorker);
      messageChannel.port1.start();
      return worker.postMessage(payload, [messageChannel.port2]);
    }

    function broadcastHandler (e) {
      if (e.source !== worker) {
        return; // ignore broadcast messages from other workers than the one we're talking to.
      }
      var data = e.data;
      if (data && data.__broadcast) {
        serialization.emission(internalEmitter, { broadcast: true })(e);
      }
    }
  }
}