Swift 动画不'；不行？

我正在尝试旋转一个imageview，但当我从另一个屏幕加载该屏幕时，动画不起作用

class LoadingRequestViewController: UIViewController ,UICollectionViewDelegate,GMSMapViewDelegate, CLLocationManagerDelegate {
//UI OutletControls
@IBOutlet weak var loadingImg: UIImageView!
@IBOutlet var mapView: GMSMapView!
@IBOutlet weak var containerView: UIView!
//UI OutletControls


override func viewDidLoad() {
    super.viewDidLoad()

    loadingImg.startRotating()

}



func TimerStoppedTrue() {
    TimerStopped = true
    NSLog("TimerStopped\(TimerStopped)")
}

@IBAction func CancelHireAction(sender: AnyObject) {
    self.timer.invalidate()
    self.CancelHire(self.HireID)
}

}

}

当我调用LoadingRequestViewController时，就像下面的动画一样，它不起作用。但单独地说，它起作用了

let storyboard = UIStoryboard(name: "Main", bundle: nil)
    let vc = storyboard.instantiateViewControllerWithIdentifier("LoadingSTBID") as! LoadingRequestViewController

    self.presentViewController(vc, animated: true, completion: nil)

---

尝试将其添加到ViewWillDisplay中。它应该很好用

let storyboard = UIStoryboard(name: "Main", bundle: nil)
    let vc = storyboard.instantiateViewControllerWithIdentifier("LoadingSTBID") as! LoadingRequestViewController

    self.presentViewController(vc, animated: true, completion: nil)