Returns a function observing changes on an end of the link. If a change happens and new end is a new model, it stops listening on the previous one and starts listening to the new one.
function(endType) {

        function watchEnd(link, end) {

            end = end || {};

            var previousEnd = link.previous(endType) || {};

            // Pick updateMethod this._sourceBboxUpdate or this._targetBboxUpdate.
            var updateEndFunction = this['_' + endType + 'BBoxUpdate'];

            if (this._isModel(previousEnd)) {
                this.stopListening(this.paper.getModelById(previousEnd.id), 'change', updateEndFunction);
            }

            if (this._isModel(end)) {
                // If the observed model changes, it caches a new bbox and do the link update.
                this.listenTo(this.paper.getModelById(end.id), 'change', updateEndFunction);
            }

            _.bind(updateEndFunction, this)({ cacheOnly: true });

            return this;
        }

        return watchEnd;
    }