// http://ryanmorr.com/using-mutation-observers-to-watch-for-element-availability/
// Expose event when element is added to the dom
(function (win) {
  'use strict';

  let listeners = [],
    doc = win.document,
    MutationObserver = win.MutationObserver || win.WebKitMutationObserver,
    observer;

  function ready(selector, fn, rand) {
    // Store the selector and callback to be monitored
    listeners.push({
      selector: selector,
      fn: fn,
      rand: rand
    });
    if (!observer) {
      // Watch for changes in the document
      observer = new MutationObserver(check);
      observer.observe(doc.documentElement, {
        childList: true,
        subtree: true
      });
    }
    // Check if the element is currently in the DOM
    check();
  }

  function check() {
    // Check the DOM for elements matching a stored selector
    for (let i = 0, len = listeners.length, listener, elements; i < len; i++) {
      listener = listeners[i];
      // Query for elements matching the specified selector
      elements = doc.querySelectorAll(listener.selector);
      for (let j = 0, jLen = elements.length, element; j < jLen; j++) {
        element = elements[j];
        // Make sure the callback isn't invoked with the
        // same element more than once
        if (listener.rand) {
          if (!element[listener.rand]) {
            element[listener.rand] = true;
            // Invoke the callback with the element
            listener.fn.call(element, element);
          }
        } else {
          if (!element.ready) {
            element.ready = true;
            // Invoke the callback with the element
            listener.fn.call(element, element);
          }
        }
      }
    }
  }

  // Expose `ready`
  win.ready = ready;

})(this);