joint.all.js
function(evt, x, y) {

        if ( // target is a valid magnet start linking
            evt.target.getAttribute('magnet') &&
            this.paper.options.validateMagnet.call(this.paper, this, evt.target)
        ) {
                this.model.trigger('batch:start');

                var link = this.paper.getDefaultLink(this, evt.target);
                link.set({
                    source: {
                        id: this.model.id,
                        selector: this.getSelector(evt.target),
                        port: $(evt.target).attr('port')
                    },
                    target: { x: x, y: y }
                });

                this.paper.model.addCell(link);

	        this._linkView = this.paper.findViewByModel(link);
                this._linkView.startArrowheadMove('target');

        } else {

            this._dx = x;
            this._dy = y;

            joint.dia.CellView.prototype.pointerdown.apply(this, arguments);
        }
    }
function(evt, x, y) {

        if ( // target is a valid magnet start linking
            evt.target.getAttribute('magnet') &&
            this.paper.options.validateMagnet.call(this.paper, this, evt.target)
        ) {
                this.model.trigger('batch:start');

                var link = this.paper.getDefaultLink(this, evt.target);
                link.set({
                    source: {
                        id: this.model.id,
                        selector: this.getSelector(evt.target),
                        port: $(evt.target).attr('port')
                    },
                    target: { x: x, y: y }
                });

                this.paper.model.addCell(link);

	        this._linkView = this.paper.findViewByModel(link);
                this._linkView.startArrowheadMove('target');

        } else {

            this._dx = x;
            this._dy = y;

            joint.dia.CellView.prototype.pointerdown.apply(this, arguments);
        }
    }
Interaction. The controller part. ---------------------------------