TextRecognizer.swift

TextRecognizer.swift

import Foundation
import Vision

struct TextRecognizer {
 
  static func get(from image: CGImage, onComplete: @escaping (String) -> Void) {
    // We kick off a text recognition handler
    handle(VNRecognizeTextRequest { (request, error) in
      // First we need to make sure, that the request returned results
      guard let observations = request.results as? [VNRecognizedTextObservation] else {
        // MARK: - TODO, handle appropriately
        return
      }
      // We're going to iterate over all possible results                             
      observations.forEach { observation in
        // And for each result, we're only going to care for the top result
        // Since our recognizer has to return at max one letter,
        // this should be a fairly acceptable way of handling that.                    
        observation.topCandidates(1).forEach { text in
          // Lastly we're passing the String to our closure                                    
          onComplete(text.string)
        }
      }
    }, for: image)
  }

  // MARK: - Private
  // We have extracted the handler into its own class to clean up the code.
  // Without it we've had a whole bunch of nesting.
  private static func handle(_ request: VNRecognizeTextRequest, for image: CGImage) {
    // Since we only need one letter, we can go with the highest accuracy
    request.recognitionLevel = .accurate
    // The language is en_GB 
    request.recognitionLanguages = ["en_GB"]
    // I found, that the letter Q wasn't always recognized well.
    // Adding it to custom words solved the issue for good.
    request.customWords = ["Q"]

    do {
      try VNImageRequestHandler(cgImage: image, options: [:]).perform([request])
    } catch {
      fatalError(error.localizedDescription)
    }
  }
}