_getObserverOptions() {
    const defaultOptions = this._getDefaultRootMargin();
    let rootMargin = Object.assign(defaultOptions, get(this, 'rootMargin') || {});
    rootMargin = `${rootMargin.top}px ${rootMargin.right}px ${rootMargin.bottom}px ${rootMargin.left}px`;
    return { root: null, rootMargin, threshold: 0 };