javascript应用程序:试图使“wheel”事件侦听器不连续失败

var picArr = $('#pictures_list li');
// The index of the last element of the array is "maxIndex"
var maxIndex = picArr.length - 1;
// Initialize counter
var counter = 0;
var updateCounter = function(direction) {
  if (direction) {
    if (counter > 0) {
      counter--;
    } else {
      counter = maxIndex;
    }
  } else {
    if (counter < maxIndex) {
      counter++;
    } else {
      counter = 0;
    }
  }
}

var showCount = function(container_id) {
  var pageCount = counter + 1;
  document.getElementById(container_id).innerHTML = "Picture " + pageCount + " of " + picArr.length;
}

var showSlide = function() {
  var mTop = (200 * counter) * (-1) + 'px';
  $('#pictures_list').animate({
    'marginTop': mTop
  }, 400);
}

// Returns a function, that, as long as it continues to be invoked, will not
    // be triggered. The function will be called after it stops being called for
    // N milliseconds. If `immediate` is passed, trigger the function on the
    // leading edge, instead of the trailing.
    function debounce(func, wait, immediate) {
    	var timeout;
    	return function() {
    		var context = this, args = arguments;
    		var later = function() {
    			timeout = null;
    			if (!immediate) func.apply(context, args);
    		};
    		var callNow = immediate && !timeout;
    		clearTimeout(timeout);
    		timeout = setTimeout(later, wait);
    		if (callNow) func.apply(context, args);
    	};
    };
    
showCount('show_count');

$('#controls button').on('click', function() {
  updateCounter($(this).attr('data-direction') == 'left');
  showSlide();
  showCount('show_count');
});

var pictureFrame = document.getElementById("picture_frame");
var onScroll = debounce(function(direction) {
  updateCounter(direction);
  showSlide();
  showCount('show_count')
}, 100, true);

pictureFrame.addEventListener("wheel", function(e) {
  e.preventDefault();
  var delta;
  if (event.wheelDelta){
      delta = event.wheelDelta;
  }else{
      delta = -1 * event.deltaY;
  }
  
  onScroll(delta >= 0);
});

#picture_frame {
  width: 200px;
  height: 200px;
  margin: 5px auto;
  border: 1px solid #ccc;
  border-radius: 2px;
  overflow: hidden;
}

.controls>div {
  display: inline-block;
}

#picture_frame {
  height: 200px;
  overflow: hidden;
}

#pictures_list {
  flex-flow: row wrap;
  align-items: stretch;
  width: 200px;
}

#pictures_list li {
  display: flex;
  height: 200px;
  flex: 1 100%;
}

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css" rel="stylesheet" />
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="container">
  <div id="picture_frame">
    <ul id="pictures_list" class="list-unstyled d-flex">
      <li><img src="https://picsum.photos/200/200
" alt="First picture"></li>
      <li><img src="https://picsum.photos/200/200?gravity=east
" alt="Second picture"></li>
      <li><img src="https://picsum.photos/200/200?gravity=west
" alt="Third picture"></li>
      <li><img src="https://picsum.photos/200/200?gravity=north
" alt="Fourth picture"></li>
    </ul>
  </div>
  <div class="text-center mb-2">
    <span class="badge badge-primary" id="show_count"></span>
  </div>
  <div class="controls text-center">
    <div class="btn-group" id="controls">
      <button type="button" class="btn btn-primary btn-sm" data-direction="left">Prev</button>
      <button type="button" class="btn btn-primary btn-sm" data-direction="right">Next</button>
    </div>
  </div>
</div>