dia.LinkView: snap arrowhead to the anchor and vertices

[kumilingus]

As user I want to the link's arrowhead (as being dragged) to be automatically aligned with the other end of the link or the existing vertices.

Here's an example of possible implementation.

const paper = new joint.dia.Paper({
    /* ... */
    linkView: joint.dia.LinkView.extend({
        snapDistance: 20,
        snapToAnchor(x, y, endType) {
            const { snapDistance } = this;
            const anchor = this.getEndAnchor(endType === 'source' ? 'target' : 'source');
            if (Math.abs(anchor.x - x) < snapDistance) {
                x = anchor.x;
            }
            if (Math.abs(anchor.y - y) < snapDistance) {
                y = anchor.y;
            }
            return { x, y };
        },
        dragArrowhead: function(evt, x0, y0) {
            const { x, y } = this.snapToAnchor(x0, y0, this.eventData(evt).arrowhead);
            return joint.dia.LinkView.prototype.dragArrowhead.call(this, evt, x, y);
        },
        dragArrowheadEnd: function(evt, x0, y0) {
            const { x, y } = this.snapToAnchor(x0, y0, this.eventData(evt).arrowhead);
            return joint.dia.LinkView.prototype.dragArrowheadEnd.call(this, evt, x, y);
        }
    })
});