iOS 检测前置或后置摄像头是否可用

// UIImagePickerControllerCameraDeviceRear 后置摄像头
// UIImagePickerControllerCameraDeviceFront 前置摄像头
BOOL isRearCamera = [UIImagePickerController isCameraDeviceAvailable:UIImagePickerControllerCameraDeviceRear];
BOOL isFrontCamera = [UIImagePickerController isCameraDeviceAvailable:UIImagePickerControllerCameraDeviceFront];