ios - 计算属性 - 如何仅在主线程上使用？

Question

我正在尝试使用 AVCaptureVideoPreviewLayer 来捕获视频，但是当我运行我的应用程序时，主线程检查器导致崩溃。

代码必须在什么时候在主线程上运行 - 我假设这将解决我遇到的问题。使用 DispatchQueue.main.async({}) 进行了多次尝试，但无济于事。

发生错误:

Main Thread Checker: UI API called on a background thread:

原文:

    var videoPreviewLayer: AVCaptureVideoPreviewLayer {

    let previewlayer = layer as! AVCaptureVideoPreviewLayer

    switch gravity {
      case .resize:
        previewlayer.videoGravity = AVLayerVideoGravity.resize
      case .resizeAspect:
        previewlayer.videoGravity = AVLayerVideoGravity.resizeAspect
      case .resizeAspectFill:
        previewlayer.videoGravity = AVLayerVideoGravity.resizeAspectFill
    }
    return previewlayer
   }

尝试修复:

var videoPreviewLayer: AVCaptureVideoPreviewLayer {

  getVideoPreviewLayer { (previewLayer) in
    return previewLayer
  }
}

func getVideoPreviewLayer( completion: @escaping (AVCaptureVideoPreviewLayer) -> ()) {

  DispatchQueue.main.async { [unowned self ] in

    let previewlayer = self.layer as! AVCaptureVideoPreviewLayer

    switch self.gravity {
    case .resize:
      previewlayer.videoGravity = AVLayerVideoGravity.resize
    case .resizeAspect:
      previewlayer.videoGravity = AVLayerVideoGravity.resizeAspect
    case .resizeAspectFill:
      previewlayer.videoGravity = AVLayerVideoGravity.resizeAspectFill
    }

    completion(previewlayer)
  }
}

---

原创 尝试修复

Answer 1

这里有一个解决方案可以确保无论使用哪个队列访问 videoPreviewLayer 都只获取主队列上的图层:

var videoPreviewLayer: AVCaptureVideoPreviewLayer {
    let previewlayer: AVCaptureVideoPreviewLayer = {
        if Thread.current.isMainThread {
            return layer as! AVCaptureVideoPreviewLayer
        } else {
            DispatchQueue.main.sync {
                return layer as! AVCaptureVideoPreviewLayer
            }
        }
    }()

    switch gravity {
    case .resize:
        previewlayer.videoGravity = AVLayerVideoGravity.resize
    case .resizeAspect:
        previewlayer.videoGravity = AVLayerVideoGravity.resizeAspect
    case .resizeAspectFill:
        previewlayer.videoGravity = AVLayerVideoGravity.resizeAspectFill
    }
    return previewlayer
}