W3CHINA.ORG讨论区--[求助]关于鼠标拖拽svg图像

下面这段代码是我从网上找的，是实现鼠标拖拽svg图像，但是我运行时，会报错。
TrueCoords = SVGRoot.createSVGPoint();
GrabPoint = SVGRoot.createSVGPoint();
就是上面这两行代码报错，说没有该createSVGPoint（）属性。
请大家看看如何解决！

var SVGDocument = null;
var SVGRoot = null;
var TrueCoords = null;
var GrabPoint = null;
var BackDrop = null;
var DragTarget = null;

function Init(evt) {

    SVGDocument = evt.target.ownerDocument;
    SVGRoot = SVGDocument.documentElement;

    // these svg points hold x and y values...
    //    very handy, but they do not display on the screen (just so you know)
    TrueCoords = SVGRoot.createSVGPoint();
    GrabPoint = SVGRoot.createSVGPoint();

    // this will serve as the canvas over which items are dragged.
    //    having the drag events occur on the mousemove over a backdrop
    //    (instead of the dragged element) prevents the dragged element
    //    from being inadvertantly dropped when the mouse is moved rapidly
    BackDrop = SVGDocument.getElementById(svgmain);
}

function Grab(evt) {
// find out which element we moused down on
var targetElement = evt.target;

    // you cannot drag the background itself, so ignore any attempts to mouse down on it
    if (BackDrop != targetElement) {
        //set the item moused down on as the element to be dragged
        DragTarget = targetElement;

        // move this element to the "top" of the display, so it is (almost)
        //    always over other elements (exception: in this case, elements that are
        //    "in the folder" (children of the folder group) with only maintain
        //    hierarchy within that group
        DragTarget.parentNode.appendChild(DragTarget);

        // turn off all pointer events to the dragged element, this does 2 things:
        //    1) allows us to drag text elements without selecting the text
        //    2) allows us to find out where the dragged element is dropped (see Drop)
        DragTarget.setAttributeNS(null, 'pointer-events', 'none');

        // we need to find the current position and translation of the grabbed element,
        //    so that we only apply the differential between the current location
        //    and the new location
        var transMatrix = DragTarget.getCTM();
        GrabPoint.x = TrueCoords.x - Number(transMatrix.e);
        GrabPoint.y = TrueCoords.y - Number(transMatrix.f);

}
};

function Drag(evt) {
// account for zooming and panning
GetTrueCoords(evt);

    // if we don't currently have an element in tow, don't do anything
    if (DragTarget) {
        // account for the offset between the element's origin and the
        //    exact place we grabbed it... this way, the drag will look more natural
        var newX = TrueCoords.x - GrabPoint.x;
        var newY = TrueCoords.y - GrabPoint.y;

        // apply a new tranform translation to the dragged element, to display
        //    it in its new location
        DragTarget.setAttributeNS(null, 'transform', 'translate(' + newX + ',' + newY + ')');
    }
};

function Drop(evt) {
    // if we aren't currently dragging an element, don't do anything
    if (DragTarget) {
        // since the element currently being dragged has its pointer-events turned off,
        //    we are afforded the opportunity to find out the element it's being dropped on
        var targetElement = evt.target;

        // turn the pointer-events back on, so we can grab this item later
        DragTarget.setAttributeNS(null, 'pointer-events', 'all');
        if ('Folder' == targetElement.parentNode.id) {
            // if the dragged element is dropped on an element that is a child
            //    of the folder group, it is inserted as a child of that group
            targetElement.parentNode.appendChild(DragTarget);
            alert(DragTarget.id + ' has been dropped into a folder, and has been inserted as a child of the containing group.');
        }
        else {
            // for this example, you cannot drag an item out of the folder once it's in there;
            //    however, you could just as easily do so here
            alert(DragTarget.id + ' has been dropped on top of ' + targetElement.id);
        }

        // set the global variable to null, so nothing will be dragged until we
        //    grab the next element
        DragTarget = null;
    }
};

function GetTrueCoords(evt) {
    // find the current zoom level and pan setting, and adjust the reported
    //    mouse position accordingly
    var newScale = SVGRoot.currentScale;
    var translation = SVGRoot.currentTranslate;
    TrueCoords.x = (evt.clientX - translation.x) / newScale;
    TrueCoords.y = (evt.clientY - translation.y) / newScale;
};


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