如何检测AVCaptureDevice的闪光灯何时完成并且静止图像捕捉已开始？

I am making a pretty simple/standard camera app that uses a AVCaptureVideoPreviewLayer. When the user presses a button, I call captureStillImageAsynchronouslyFromConnection on the AVCaptureStillImageOutput to capture an image. At the time that I capture the image, I animate a simple white UIView over the preview layer as an indication to the user that a photo is being captured, and hide it again after the image capture is complete.I am making a pretty simple/standard camera app