I want to execute some Javascript when a YouTube iframe changes its state (paused, stopped, played etc). You need to include the YouTube player API and make sure you enable it in your iframe embed code. This code works for multiple iframes on the same page.

youtube-trigger.js

$(".latest_work2 .swiper").on("change-class", function(){
  // if the swiper div has the class 'pause' (which it receives when a YT video is not playing) it stops the cycle
  if (slideshow2.slides.length > 3){
    if ($(".latest_work2 .swiper").hasClass("pause")){
      slideshow2.stopAutoplay()
    } else {
      slideshow2.startAutoplay()
    }
  }
});

youtube_api.js

var player;
function onYouTubeIframeAPIReady() {
  $(".swiper iframe").each(function(){
    var frameId = $(this).attr("id");
    player = new YT.Player(frameId, {
      events: {
        'onReady': onPlayerReady,
        'onStateChange': onPlayerStateChange
      }
    });
  });
}
var playerReady = false;
function onPlayerReady(event) {
  playerReady = true;
}
function onPlayerStateChange(event) {
  if (event.data == YT.PlayerState.PLAYING ) {
    $(".swiper").addClass("pause").trigger('change-class');
  } else {
    $(".swiper").removeClass("pause").trigger('change-class');
  }
}