iOS 设置默认吊起前置摄像头

UIImagePickerController *controller = [[UIImagePickerController alloc] init];
                controller.sourceType = UIImagePickerControllerSourceTypeCamera;
                NSMutableArray *mediaTypes = [[NSMutableArray alloc] init];
                [mediaTypes addObject:(__bridge NSString *)kUTTypeImage];
                controller.mediaTypes = mediaTypes;
                controller.cameraDevice = UIImagePickerControllerCameraDeviceFront;// 此处是关键。