ios - Segue 和使用 instantiateViewController 有什么区别？

Question

segue 和 instantiateViewController 有什么区别？

我一直在尝试弄清楚如何使用 segues 将图像从一个 View Controller 发送到另一个 View Controller 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 View Controller 未显示或图像未显示。

继续示例

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)
}

实例化 View Controller 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？ (我确定那个 segue 标识符，尝试过 segue.destination 和 Storyboard ID 但仍然没有得到我需要的东西)有可能我不知道如何在照片库取消调用后使用 segue 但仍然想知道区别。

Answer 1

问题在于，当您启动转场时，转场对 View Controller 层次结构的状态有特殊要求。你必须推迟 performSegue(withIdentifier:sender:) 直到 dismiss 完成，即将它放入 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)
    }
}

以上对我来说效果很好。

顺便说一句，你可以简化你的prepare(for:sender:)实现:

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

---

两个有些不相关的观察结果:

1. 如果 pickedImage 是一个 ImageView ，我建议将它重命名(并更新 Storyboard中的导出)为 pickedImageView。避免混淆 UIImage 属性和 UIImageView 导出是一个很好的约定。

2. 这是一个更小的观察，但在 Model-View-Controller 中设计时，您通常不想依赖 UIKit 对象，如 UIImageView 来保存模型对象，即所选图像。它暗示了“ View ”对象和“模型”对象之间的概念混淆。另外，如果当前 View Controller 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)
       }
   }
   

   和:

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