添加下面的代码就可以让取景框全屏.
CGSize screenBounds = [UIScreen mainScreen].bounds.size;
CGFloat cameraAspectRatio = 4.0f/3.0f;
CGFloat camViewHeight = screenBounds.width * cameraAspectRatio;
CGFloat scale = screenBounds.height / camViewHeight;
pickerController.cameraViewTransform = CGAffineTransformMakeTranslation(0, (screenBounds.height - camViewHeight) / 2.0);
pickerController.cameraViewTransform = CGAffineTransformScale(pickerController.cameraViewTransform, scale, scale);
