错误缩小MKMapView

Question

这样可以正常工作，但是当我不断按下缩小按钮时，它会在某种程度上给我异常。我该如何预防？

//MARK: MKMapView
extension MKMapView {

    // delta is the zoom factor
    // 2 will zoom out x2
    // .5 will zoom in by x2

    func setZoomByDelta(delta: Double, animated: Bool) {
        var _region = region
        var _span = region.span
        _span.latitudeDelta *= delta
        _span.longitudeDelta *= delta
        _region.span = _span

        setRegion(_region, animated: animated)
    }
}