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Detailed explanation of how JS listens for resize events of div

  • 2021-10-11 17:36:56
  • OfStack

Demand

One of the most common problems in the development process is how to listen for size changes to an div.
For example, I drew an chart with canvas. When size of canvas changes, I need to redraw the contents inside. At this time, I need to listen to resize events for processing.

Although resize event monitoring can be added to window, it cannot meet our needs, because size of div has changed, but size of window has not changed.

However, we can indirectly use resize event monitoring of window to realize resize event monitoring of an div, please see the following specific implementation.

For div resize event monitoring, there are many ways to achieve, such as periodic inspection, through scroll events, etc. This paper mainly introduces the implementation of monitoring through object elements.

Concrete realization 


/**
 * Created by taozh on 2017/5/6.
 * taozh1982@gmail.com
 */
var EleResize = {
 _handleResize: function (e) {
  var ele = e.target || e.srcElement;
  var trigger = ele.__resizeTrigger__;
  if (trigger) {
   var handlers = trigger.__z_resizeListeners;
   if (handlers) {
    var size = handlers.length;
    for (var i = 0; i < size; i++) {
     var h = handlers[i];
     var handler = h.handler;
     var context = h.context;
     handler.apply(context, [e]);
    }
   }
  }
 },
 _removeHandler: function (ele, handler, context) {
  var handlers = ele.__z_resizeListeners;
  if (handlers) {
   var size = handlers.length;
   for (var i = 0; i < size; i++) {
    var h = handlers[i];
    if (h.handler === handler && h.context === context) {
     handlers.splice(i, 1);
     return;
    }
   }
  }
 },
 _createResizeTrigger: function (ele) {
  var obj = document.createElement('object');
  obj.setAttribute('style',
   'display: block; position: absolute; top: 0; left: 0; height: 100%; width: 100%; overflow: hidden;opacity: 0; pointer-events: none; z-index: -1;');
  obj.onload = EleResize._handleObjectLoad;
  obj.type = 'text/html';
  ele.appendChild(obj);
  obj.data = 'about:blank';
  return obj;
 },
 _handleObjectLoad: function (evt) {
  this.contentDocument.defaultView.__resizeTrigger__ = this.__resizeElement__;
  this.contentDocument.defaultView.addEventListener('resize', EleResize._handleResize);
 }
};
if (document.attachEvent) {//ie9-10
 EleResize.on = function (ele, handler, context) {
  var handlers = ele.__z_resizeListeners;
  if (!handlers) {
   handlers = [];
   ele.__z_resizeListeners = handlers;
   ele.__resizeTrigger__ = ele;
   ele.attachEvent('onresize', EleResize._handleResize);
  }
  handlers.push({
   handler: handler,
   context: context
  });
 };
 EleResize.off = function (ele, handler, context) {
  var handlers = ele.__z_resizeListeners;
  if (handlers) {
   EleResize._removeHandler(ele, handler, context);
   if (handlers.length === 0) {
    ele.detachEvent('onresize', EleResize._handleResize);
    delete ele.__z_resizeListeners;
   }
  }
 }
} else {
 EleResize.on = function (ele, handler, context) {
  var handlers = ele.__z_resizeListeners;
  if (!handlers) {
   handlers = [];
   ele.__z_resizeListeners = handlers;

   if (getComputedStyle(ele, null).position === 'static') {
    ele.style.position = 'relative';
   }
   var obj = EleResize._createResizeTrigger(ele);
   ele.__resizeTrigger__ = obj;
   obj.__resizeElement__ = ele;
  }
  handlers.push({
   handler: handler,
   context: context
  });
 };
 EleResize.off = function (ele, handler, context) {
  var handlers = ele.__z_resizeListeners;
  if (handlers) {
   EleResize._removeHandler(ele, handler, context);
   if (handlers.length === 0) {
    var trigger = ele.__resizeTrigger__;
    if (trigger) {
     trigger.contentDocument.defaultView.removeEventListener('resize', EleResize._handleResize);
     ele.removeChild(trigger);
     delete ele.__resizeTrigger__;
    }
    delete ele.__z_resizeListeners;
   }
  }
 }
}

Test code: 


<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>Resize</title>
 <script src="./EleResize.js"></script>
 <style>
  html, body {
   margin: 0;
   padding: 0;
   width: 100%;
   height: 100%;
  }

  #resizeDiv {
   width: 60%;
   height: 60%;
   border: 1px solid red;
   margin: 20px;
  }

  button {
   margin: 20px 20px 0;
  }
 </style>
</head>
<body>
<button onclick="addListener()">addListener</button>
<button onclick="removeListener()">removeListener</button>
<button onclick="resize()">resize</button>
<div id="resizeDiv"></div>
<script>
 var resizeDiv = document.getElementById('resizeDiv');
 function resize() {
  resizeDiv.style.width = "200px";
 }
 var listener = function () {
  console.log("resize");
 };
 function addListener() {
  EleResize.on(resizeDiv, listener);
 }
 function removeListener() {
  EleResize.off(resizeDiv, listener)
 }
</script>
</body>
</html>

Principle

The concrete implementation here can be divided into two categories,

ie9-10

resize events that support div by default can be directly passed through div. attachEvent ('onresize', handler); The way to realize

Other browsers

Listening is realized by adding a built-in object element to div.

Set the style of the object element so that it fills the div so that when the size of the div changes, the size of the object also changes.

Then listen for the resize event of contentDocument. defaultView (window object) of the object element.

Note: This article provides how to listen for resize events. In fact, when resize, it may be triggered continuously and quickly (such as dragging the browser). In order to improve efficiency, we can consider using batch processing mode.

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