Swift 4预览图，然后选择

Question

我有图像选择器（下面的源代码）。我想做的是...从图库中选择图像，并通过取消和接受按钮将其显示为预览。.我该怎么做？

func showPhotoSourceSelection() {

    let alertController = UIAlertController.init(title: nil,
                                                 message: "You can pick image from camera or gallery",
                                                 preferredStyle: .actionSheet)

    let cancelAction = UIAlertAction(title: "Cancel", style: .cancel, handler: nil)
    let photoLibraryAction = UIAlertAction(title: "Photo from gallery", style: .default) { (action) in

        self.showImagePickerController(sourceType: .photoLibrary)
    }

    alertController.addAction(cancelAction)
    alertController.addAction(photoLibraryAction)

    if UIImagePickerController.isFlashAvailable(for: .rear) {

        let cameraAction = UIAlertAction.init(title: "Photo from Camera", style: .default, handler: { (action) in

            self.showImagePickerController(sourceType: .camera)
        })

        alertController.addAction(cameraAction)
    }

    viewController.present(alertController, animated: true, completion: nil)
}