3D Touch：在展开可选值时意外发现 nil

Posted 2023-03-14

【中文标题】3D Touch：在展开可选值时意外发现 nil

【英文标题】：3D Touch: unexpectedly found nil while unwrapping an Optional value

【发布时间】：2017-01-07 14:54:10

【问题描述】：

为什么imageRef.data 总是nil？我在 peek 函数中有imageRef.data 但它不属于那里？当我偷看时它会起作用（我假设图像开始下载）然后当我在 iPhone 上用力按压时，图像就会显示出来。但大多数时候我得到nil 错误。

现在，我已将该逻辑移到 pop 函数中，现在它始终是 fatal error: unexpectedly found nil while unwrapping an Optional value

import UIKit
import FirebaseStorage

extension ExpenseFeedVC: UIViewControllerPreviewingDelegate 

    // PEEK
    func previewingContext(_ previewingContext: UIViewControllerPreviewing, viewControllerForLocation location: CGPoint) -> UIViewController? 

        guard let indexPath = tableView.indexPathForRow(at: location),
            let cell = tableView.cellForRow(at: indexPath) else  return nil

        guard let expensePreview = storyboard?.instantiateViewController(withIdentifier: "expensePreviewVC") as? ExpensePreviewVC else return nil 
        expensePreview.selectedItem = expenses[indexPath.row].type
        quickActionString = expenses[indexPath.row].type

        let imageUrl: String = expenses[indexPath.row].receiptUrl
        urlToPass = imageUrl

        expensePreview.preferredContentSize = CGSize(width: 0, height: 150)

        previewingContext.sourceRect = cell.frame

        return expensePreview
    

    // POP
    func previewingContext(_ previewingContext: UIViewControllerPreviewing, commit viewControllerToCommit: UIViewController) 

        if let rvc = storyboard?.instantiateViewController(withIdentifier: "receiptImagePreviewVC") as? ReceiptImagePreviewVC 

            let imageRef = FIRStorage.storage().reference(forURL: urlToPass)
            imageRef.data(withMaxSize: 2*1024*1024, completion:  (data, error) in
                print(data!)

            )

            rvc.poppedImage = self.imageToPass

            show(rvc, sender: self)

        
    

我把它从“偷看”中去掉了，因为如果我快速强制触摸，我会得到一个错误。如果我偷看，等待几秒钟，然后用力按压，我看到了图像。 imageRef.data 最好放在哪里，为什么 data 是 nil（在 pop 函数内）？

urlToPass 总是返回正确的图片 url。

【参考方案1】：

异步编程的经典问题 - 下载并没有立即发生，这就是为什么它在您等待几秒钟时起作用的原因。你需要这样做：

let imageRef = FIRStorage.storage().reference(forURL: urlToPass)
imageRef.data(withMaxSize: 2*1024*1024, completion:  (data, error) in
  // Check for an error
  ...
  // Prepare to switch to the next VC once the file has downloaded
  rvc.poppedImage = UIImage(data: data!)
  show(rvc, sender: self)
)