如何在 Swift 6 中将 Vision 文本识别结果标记为 Sendable

Question

我有以下代码。它只需运行 Vision API 即可从图像中获取文本。我使用非常简单的 GCD 将繁重的 Vision 操作分派到后台队列，然后将其分派回主队列

completion

:

public struct TextRecognitionResult {
  let observation: VNRecognizedTextObservation
  let text: VNRecognizedText
  let rect: CGRect
}

public enum TextRecognitionUtil {
  
  private static let queue = DispatchQueue(label: "text_recognition", qos: .userInitiated)

  public static func process(
    image: UIImage,
    recognitionLevel: VNRequestTextRecognitionLevel,
    completion: @Sendable @escaping ([TextRecognitionResult]) -> Void)
  {
    guard let cgImage = image.cgImage else {
      completion([])
      return
    }
    let request = VNRecognizeTextRequest { (request, error) in
      guard
        error == nil,
        let observations = request.results as? [VNRecognizedTextObservation]
      else {
        DispatchQueue.main.async {
          completion([])
        }
        return
      }
      
      var results = [TextRecognitionResult]()
      
      // Vision's origin is on bottom left
      let transform = CGAffineTransform.identity
        .scaledBy(x: 1, y: -1)
        .translatedBy(x: 0, y: -image.size.height)
        .scaledBy(x: image.size.width, y: image.size.height)

      for observation in observations {
        guard let text = observation.topCandidates(1).first else { continue }
        let rect = observation.boundingBox.applying(transform)
        results += [TextRecognitionResult(observation: observation, text: text, rect: rect)]
      }
      DispatchQueue.main.async {
        completion(results)
      }
    }
    
    request.recognitionLevel = recognitionLevel
    self.queue.async {
      let handler = VNImageRequestHandler(cgImage: cgImage, options: [:])
      try? handler.perform([request])
    }
  }
}

此代码违反了 Swift 6 并发性，因为

TextRecognitionResult 

不是

Sendable

:

Sending 'results' risks causing data races; this is an error in the Swift 6 language mode

但是，我的

TextRecognitionResult

不能直接标记为Sendable，因为

VNRecognizedTextObservation

和

VNRecognizedText

不是Sendable，而且它们都是Vision中定义的类型，我无法更改。这是 GCD 中非常常见的做法。我不知道在这里做什么。

Answer 1

一些观察：

1. 我会将您的

   struct

   声明为

   Sendable

   :

   public struct TextRecognitionResult: Sendable {
       let observation: VNRecognizedTextObservation
       let text: VNRecognizedText
       let rect: CGRect
   }
   

2. 我会将

   Vision

   导入为

   @preconcurrency

   :

   @preconcurrency import Vision
   

3. 我会让

   results

   不可变（使用

   compactMap

   而不是

   for

   循环）：

   public enum TextRecognitionUtil {
       enum TextRecognitionUtilError: Error {
           case noCgImage
           case notVNRecognizedTextObservation
       }
   
       private static let queue = DispatchQueue(label: "text_recognition", qos: .userInitiated)
   
       public static func process(
           image: UIImage,
           recognitionLevel: VNRequestTextRecognitionLevel = .accurate,
           completion: @Sendable @escaping (Result<[TextRecognitionResult], Error>) -> Void)
       {
           guard let cgImage = image.cgImage else {
               completion(.failure(TextRecognitionUtilError.noCgImage))
               return
           }
   
           let request = VNRecognizeTextRequest { request, error in
               guard
                   error == nil,
                   let observations = request.results as? [VNRecognizedTextObservation]
               else {
                   DispatchQueue.main.async {
                       completion(.failure(error ?? TextRecognitionUtilError.notVNRecognizedTextObservation))
                   }
                   return
               }
   
               // Vision's origin is on bottom left
               let transform = CGAffineTransform.identity
                   .scaledBy(x: 1, y: -1)
                   .translatedBy(x: 0, y: -image.size.height)
                   .scaledBy(x: image.size.width, y: image.size.height)
   
               let results = observations.compactMap { (observation) -> TextRecognitionResult? in
                   guard let text = observation.topCandidates(1).first else { return nil }
                   let rect = observation.boundingBox.applying(transform)
                   return TextRecognitionResult(observation: observation, text: text, rect: rect)
               }
               DispatchQueue.main.async {
                   completion(.success(results))
               }
           }
   
           request.recognitionLevel = recognitionLevel
           self.queue.async {
               let handler = VNImageRequestHandler(cgImage: cgImage)
           }
       }
   }
   

   我还将闭包返回类型更改为

   Result

   ，以便调用者可以消除错误和无识别结果之间的歧义。

Swift 6 可能会稍微简化一下，但以上内容适用于 Xcode 15.4。