解决移动图形越界的问题

<!--边界问题研究 2018.3.11-->

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <title></title>
    <link rel="stylesheet" type="text/css" href="css/core.css" />
    <script src="js/jquery-1.9.1.min.js"></script>
    <script src="js/fabric1.1.15.js"></script>
</head>
<body>
    <canvas id="c" width="880" height="520"></canvas>

    <script>
        ///////////////
        var canvas = new fabric.Canvas("c");

        var boundingBox = new fabric.Rect({
            fill: "yellow",
            left: 0,
            top: 0,
            originX: 'left',
            originY: 'top',
             600,
            height: 400,
            hasBorders: false,
            hasControls: false,
            lockMovementX: true,
            lockMovementY: true,
            evented: false
        });

        var movingBox = new fabric.Rect({
            fill: "red",
            left: 0,
            top: 0,
            originX: 'left',
            originY: 'top',
             100,
            height: 100,
            hasBorders: false
        });


        canvas.on("object:moving", function () {

            var top = movingBox.top;
            var bottom = top + movingBox.height;

            var left = movingBox.left;
            var right = left + movingBox.width;

            //固定的盒子边界

            var topBound = 0;
            var bottomBound = canvas.height;
            var leftBound = 0;
            var rightBound = canvas.width;


            var limitX = Math.min(Math.max(left, leftBound), rightBound - movingBox.currentWidth);
            var limitY = Math.min(Math.max(top, topBound), bottomBound - movingBox.currentHeight);

            movingBox.setLeft(limitX <= 0 ? 0 : limitX);//防止产生负值
            movingBox.setTop(limitY <= 0 ? 0 : limitY);
            //console.log("x=" + movingBox.left + " " + "Y=" + movingBox.top);
        });

        canvas.add(boundingBox);
        canvas.add(movingBox);
    </script>
</body>
</html>