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/
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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
};
}
}