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Prevent Fabric js Objects from scaling out of the canvas boundary

I have been trying to keep an object (constructed in fabric js over a canvas) inside the boundaries at all the times. It has been achieved at moving and rotating it. I took help from Move object within canvas boundary limit for achieving this. But when I start to scale the object, it simply keeps on going out of boundary. I do not understand what has to be done to keep it inside the boundary only, even while scaling. Please help me with a code to prevent this behavior. It would be great if you can attach a demo too.

    <html>
<head>
    <title>Basic usage</title>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/1.7.3/fabric.min.js"></script>

</head>
<body>
<canvas id="canvas" style= "border: 1px solid black" height= 480 width = 360></canvas>
<script>
 var canvas = new fabric.Canvas('canvas');
  canvas.add(new fabric.Circle({ radius: 30, fill: '#f55', top: 100, left: 100 }));

  canvas.item(0).set({
    borderColor: 'gray',
    cornerColor: 'black',
    cornerSize: 12,
    transparentCorners: true
  });
  canvas.setActiveObject(canvas.item(0));
  canvas.renderAll();


  canvas.on('object:moving', function (e) {
        var obj = e.target;
         // if object is too big ignore
        if(obj.currentHeight > obj.canvas.height || obj.currentWidth > obj.canvas.width){
            return;
        }        
        obj.setCoords();        
        // top-left  corner
        if(obj.getBoundingRect().top < 0 || obj.getBoundingRect().left < 0){
            obj.top = Math.max(obj.top, obj.top-obj.getBoundingRect().top);
            obj.left = Math.max(obj.left, obj.left-obj.getBoundingRect().left);
        }
        // bot-right corner
        if(obj.getBoundingRect().top+obj.getBoundingRect().height  > obj.canvas.height || obj.getBoundingRect().left+obj.getBoundingRect().width  > obj.canvas.width){
            obj.top = Math.min(obj.top, obj.canvas.height-obj.getBoundingRect().height+obj.top-obj.getBoundingRect().top);
            obj.left = Math.min(obj.left, obj.canvas.width-obj.getBoundingRect().width+obj.left-obj.getBoundingRect().left);
        }
});

</script>
</body>
</html>

My demo is attached here. : https://jsfiddle.net/3v0cLaLk/

Answer

If you want to perform a real time prevention, you should use object:scaling event, as object:modified is only triggered at the end of the transformation.

1) Add event handler to canvas:

this.canvas.on('object:scaling', (e) => this._handleScaling(e));

2) In the handler function, get the old and the new object’s bounding rect:

_handleScaling(e) {
  var obj = e.target;
  var brOld = obj.getBoundingRect();
  obj.setCoords();
  var brNew = obj.getBoundingRect();

3) For each border, check if object has scaled beyond the canvas boundaries and compute its left, top and scale properties:

  // left border
  // 1. compute the scale that sets obj.left equal 0
  // 2. compute height if the same scale is applied to Y (we do not allow non-uniform scaling)
  // 3. compute obj.top based on new height
  if(brOld.left >= 0 && brNew.left < 0) {
    let scale = (brOld.width + brOld.left) / obj.width;
    let height = obj.height * scale;
    let top = ((brNew.top - brOld.top) / (brNew.height - brOld.height) *
      (height - brOld.height)) + brOld.top;
    this._setScalingProperties(0, top, scale);
  } 

4) Similar code for the other borders:

  // top border
  if(brOld.top >= 0 && brNew.top < 0) {
    let scale = (brOld.height + brOld.top) / obj.height;
    let width = obj.width * scale;
    let left = ((brNew.left - brOld.left) / (brNew.width - brOld.width) * 
      (width - brOld.width)) + brOld.left;
    this._setScalingProperties(left, 0, scale);
  }
  // right border
  if(brOld.left + brOld.width <= obj.canvas.width 
  && brNew.left + brNew.width > obj.canvas.width) {
    let scale = (obj.canvas.width - brOld.left) / obj.width;
    let height = obj.height * scale;
    let top = ((brNew.top - brOld.top) / (brNew.height - brOld.height) * 
      (height - brOld.height)) + brOld.top;
    this._setScalingProperties(brNew.left, top, scale);
  }
  // bottom border
  if(brOld.top + brOld.height <= obj.canvas.height 
  && brNew.top + brNew.height > obj.canvas.height) {
    let scale = (obj.canvas.height - brOld.top) / obj.height;
    let width = obj.width * scale;
    let left = ((brNew.left - brOld.left) / (brNew.width - brOld.width) * 
      (width - brOld.width)) + brOld.left;
    this._setScalingProperties(left, brNew.top, scale);
  }

5) If object’s BoundingRect has crossed canvas boundaries, fix its position and scale:

  if(brNew.left < 0
  || brNew.top < 0
  || brNew.left + brNew.width > obj.canvas.width
  || brNew.top + brNew.height > obj.canvas.height) {
    obj.left = this.scalingProperties['left'];
    obj.top = this.scalingProperties['top'];
    obj.scaleX = this.scalingProperties['scale'];
    obj.scaleY = this.scalingProperties['scale'];
    obj.setCoords();
  } else {
    this.scalingProperties = null;
  }
}

6) Finally, when setting the scaling properties, we have to stick with the smallest scale in case the object has crossed more than one border:

_setScalingProperties(left, top, scale) {
  if(this.scalingProperties == null 
  || this.scalingProperties['scale'] > scale) {
    this.scalingProperties = {
      'left': left,
      'top': top,
      'scale': scale
    };
  }
}