Segue 和使用 instantiateViewController 有什么区别？

Question

segue 和 instanceViewController 有什么区别？

我一直在试图弄清楚如何使用 segues 将图像从一个视图控制器发送到另一个视图控制器和 2 个答案（Passing Image to another View Controller (Swift) 和 How do I segue an image to another ViewController and display it within an ImageView?）都说要使用 segues，但是在尝试使用 segues 时我遇到了一些问题，例如照片库关闭后第二个视图控制器未显示或图像未显示。

转义示例

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.destination is XferImageViewController {
        print("Test: ", pickedImage.image)
        let xferVC = segue.destination as? XferImageViewController
        xferVC?.storedImage = pickedImage.image
    }
    print("WHAT IS GOING ON")
//        if segue.destination is XferImageViewController {
//            let xferVC = segue.destination as? XferImageViewController
//            print(pickedImage.image)
//            //xferVC?.storedImage = pickedImage.image
//            xferVC?.storedImage = pickedImageVar
//        }

}

func imagePickerController(_ picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [String : Any]) {
    if let image = info[UIImagePickerControllerOriginalImage] as? UIImage {
        pickedImage.image = image
    } else {
        print("Something went wrong")
    }
    let image = info[UIImagePickerControllerOriginalImage] as? UIImage
    dismiss(animated:true, completion: nil)
    performSegue(withIdentifier: "xferImage", sender: self)
}

InstantiateViewController 示例

func imagePickerController(_ picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [String : Any]) {
    if let image = info[UIImagePickerControllerOriginalImage] as? UIImage {
        pickedImage.image = image
    } else {
        print("Something went wrong")
    }
    let image = info[UIImagePickerControllerOriginalImage] as? UIImage
    dismiss(animated:true, completion: nil)
    let storyboard = UIStoryboard(name: "Main", bundle: nil)
    let controller = storyboard.instantiateViewController(withIdentifier: "xferImage") as! XferImageViewController
    controller.storedImage = image
    present(controller, animated: true, completion: nil)
}

所以在使用 InstantiateViewController 而不是 segue 之后，我得到了我想要的结果。两者有什么区别？ （我确定了那个 segue 标识符，尝试了 segue.destination 和 storyboard ID，但仍然没有得到我需要的东西）在照片库关闭呼叫后我可能不知道如何使用 segue，但仍然想知道区别。

Answer 1

问题在于，当您启动 segue 时，segue 会特别关注视图控制器层次结构的状态。您必须将performSegue(withIdentifier:sender:) 推迟到dismiss 完成，即将其放入dismiss 的完成句柄中：

func imagePickerController(_ picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [String : Any]) {
    if let image = info[UIImagePickerControllerOriginalImage] as? UIImage {
        pickedImage.image = image
    } else {
        print("Something went wrong")
    }

    dismiss(animated: true) {
        self.performSegue(withIdentifier: "xferImage", sender: self)
    }
}

以上对我来说很好。

顺便说一句，您可以简化您的 prepare(for:sender:) 实现：

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if let xferVC = segue.destination as? XferImageViewController {
        xferVC.storedImage = pickedImage.image
    }
}

---

两个有些无关的观察结果：

1. 如果pickedImage 是图像视图，我建议将其重命名（并更新故事板中的出口）为pickedImageView。避免混淆 UIImage 属性和 UIImageView 出口是一个很好的约定。

2. 这是一个更次要的观察，但在Model-View-Controller 设计中，您通常不想依赖UIKit 对象，如UIImageView 来保存模型对象，即选定的图像。它暗示了“视图”对象和“模型”对象之间的概念混淆。另外，如果当前视图控制器没有 UIImageView 怎么办？

   我个人建议将所选图像存储在单独的 UIImage? 属性中：

   private var selectedImage: UIImage?
   
   func imagePickerController(_ picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [String : Any]) {
       if let image = info[UIImagePickerControllerOriginalImage] as? UIImage {
           selectedImage = image
   
           // if you also want to update a `UIImageView` in the current
           // view controller, fine, do that, but it shouldn't be confused
           // with the "model".
           //
           // pickedImageView.image = image
       } else {
           print("Something went wrong")
       }
   
       dismiss(animated: true) {
           self.performSegue(withIdentifier: "xferImage", sender: self)
       }
   }
   

   还有：

   override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
       if let xferVC = segue.destination as? XferImageViewController {
           xferVC.storedImage = selectedImage
       }
   }
   

Answer 2

尝试使用下面的函数

override func prepare(for segue: UIStoryboardSegue, sender: Any?)
{
    if segue.identifier == "segue"
    {
        if let xferVC = segue.destination as? XferImageViewController {
            xferVC.storedImage = pickedImage.image

                //Why Optional here ?
                //xferVC?.storedImage = pickedImage.image
            }
        }  
  }