检测到矩形时自动拍照以继续回线？ iOS，Swift

I am working on a project at the moment and wanted to find out what I need to be looking at to automatically take a picture when a rectangle is detected. I have seen this in action on an app called car spotter but wanted to know how it could be done. On car spotter it detects the rectangle and blurs the number plate automatically.I am working on a project at the moment and wan