Swift 4：从 DispatchQueue.main 访问变量（作用域）

Question

我有一个 CoreML 图像分类任务，它从 iOS 设备的 [video] 摄像头获取 "live stream" 并在后台发生。一旦识别出对象，并发生其他应用程序逻辑，我想用一些数据更新 UI 的标签。

有人可以解释一下 DispatchQueue.main.asyc(execute: { }) 的标注是如何访问我一直在使用的变量的吗？我认为这本质上是一个范围界定问题？

我目前使用的代码：

func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {

    processCameraBuffer(sampleBuffer: sampleBuffer)

}

func processCameraBuffer(sampleBuffer: CMSampleBuffer) {

    let coreMLModel = Inceptionv3()

    if let model = try? VNCoreMLModel(for: coreMLModel.model) {
        let request = VNCoreMLRequest(model: model, completionHandler: { (request, error) in
            if let results = request.results as? [VNClassificationObservation] {

                var counter = 0
                var otherVar = 0

                for item in results[0...9] {

                    if item.identifier.contains("something") {
                        print("some app logic goes on here")
                        otherVar += 10 - counter
                    }
                    counter += 1

                }
                switch otherVar {
                case _ where otherVar >= 10:
                    DispatchQueue.main.async(execute: {
                        let displayVarFormatted = String(format: "%.2f", otherVar / 65 * 100)
                        self.labelPrediction.text = "\(counter): \(displayVarFormatted)%"
                    })
                default:
                    DispatchQueue.main.async(execute: {
                        self.labelPrediction.text = "No result!"
                    })
                }
            }
        })

            if let pixelBuffer: CVPixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer) {
                let handler = VNImageRequestHandler(cvPixelBuffer: pixelBuffer, options: [:])
                do {
                    try handler.perform([request])
                } catch {
                    print(error.localizedDescription)
                }
            }
    }
}

导致问题的是 switch 语句中的 self.labelPrediction.text = "" 行。此 var 当前始终为 0。

Answer 1

不是DispatchQueue的问题。从 processCameraBuffer(sampleBuffer:)，您的代码在获得结果之前更新了您的 UI。

要解决这个问题，您需要使用escaping closure。您的函数应如下所示。

func processCameraBuffer(sampleBuffer: CMSampleBuffer, completion: @escaping (Int, String) -> Void) {
    // 2.
    let request = VNCoreMLRequest(model: model, completionHandler: { (request, error) in

      DispatchQueue.main.async(execute: {
          // 3.
          let displayVarFormatted = String(format: "%.2f", otherVar / 65 * 100)
          completion(counter, displayVarFormatted)
      })
    }
}

func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) { 
    // 1.
    processCameraBuffer(sampleBuffer) { counter, displayVarFormatted in
        /*
         This Closure will be executed from 
         completion(counter, displayVarFormatted)
        */
        // 4.
        self.labelPrediction.text = "\(counter): \(displayVarFormatted)%"
    }
}

从这里开始，变量范围不是问题。您需要处理异步任务。

捕获发生。
processCameraBuffer 被调用并VNCoreMLRequest 被执行。
您将通过completion().

processCameraBuffer

更新标签。

Swift 4：从 DispatchQueue.main 访问变量（作用域）

Swift 4: Access Variables from DispatchQueue.main (Scope)

scope

swift

dispatch-queue