Swift 5：等待全局队列在主队列上完成执行

Question

当最初的开发人员离开组织时，我们从另一个团队获得了一个项目。我需要修复一个缺陷。我有一些类似下面的代码，其中UIGraphicsBeginPDFContextToData 发生在Global 队列上。但这会给出如下内存警告UIView 扩展的renderToImage 不会发生在main 线程上。

我想将 renderToImage 包裹在 DispatchQueue.main.async 中，但它会破坏 someFunction 中的 for 循环。我不能在主线程上使用.sync。谁能告诉我如何完成renderToImage 等待Global 队列？

原始代码：

         DispatchQueue.global(qos: .userInitiated).async {
             let pdfData = NSMutableData()
             UIGraphicsBeginPDFContextToData(pdfData, CGRect.zero, nil)

             zip(self.pdfPages, self.pageViews).forEach { (page, view) in

                 let image = view.renderToImage()
                 guard let imageData = image.jpegData(compressionQuality: 0.5),
                     let compressedImage = UIImage(data: imageData) else { return }

                 UIGraphicsBeginPDFPageWithInfo(CGRect(origin: CGPoint.zero, size: page.originSize), nil)
                 compressedImage.draw(in: CGRect(origin: CGPoint.zero, size: page.originSize))

                 DispatchQueue.main.async {
                     self.progressLabel.text = "Processing PDF... \(page.pageNumber) / \(pageNumbers)"
                 }
             }
             UIGraphicsEndPDFContext()

             DispatchQueue.main.async {
                 self.progressIndicator.stopAnimating()
                 self.progressView.isHidden = true
                 completionBlock?(pdfData as Data)
             }
         }


extension UIView {
    func renderToImage() -> UIImage {
        UIGraphicsBeginImageContextWithOptions(bounds.size, isOpaque, 0.0)
        guard let context = UIGraphicsGetCurrentContext() else { return UIImage() }
        layer.render(in: context)
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        return image!
    }
}

应用@paulw11 回答后：

    fileprivate func generatePDF(_ completionBlock: ((_ pdfData: Data) -> Void)?) {

        let pageNumbers = pageViews.count

        progressView.isHidden = false
        progressIndicator.startAnimating()
        progressLabel.text = "Processing PDF... \(0) / \(pageNumbers)"

        let semaphore = DispatchSemaphore(value: 0)
        let pdfContextQueue = DispatchQueue(label: kMessagesFaxPDFConversionQueue, target: DispatchQueue.global(qos: .userInitiated))

        let pdfData = NSMutableData()
        UIGraphicsBeginPDFContextToData(pdfData, CGRect.zero, nil)

        pdfContextQueue.async {
            zip(self.pdfPages, self.pageViews).forEach { (page, view) in
                view.renderToImage { image in
                    guard let imageData = image.jpegData(compressionQuality: 0.5),
                    let compressedImage = UIImage(data: imageData) else { return }

                    UIGraphicsBeginPDFPageWithInfo(CGRect(origin: CGPoint.zero, size: page.originSize), nil)
                    compressedImage.draw(in: CGRect(origin: CGPoint.zero, size: page.originSize))

                    DispatchQueue.main.async {
                        self.progressLabel.text = "Processing PDF... \(page.pageNumber) / \(pageNumbers)"
                    }
                    semaphore.signal()
                }
                semaphore.wait()
            }
            UIGraphicsEndPDFContext()

            DispatchQueue.main.async {
                self.progressIndicator.stopAnimating()
                self.progressView.isHidden = true
                completionBlock?(pdfData as Data)
            }
        }
}


extension UIView {
    func renderToImage(completion: @escaping ((UIImage) -> Void)) {
         DispatchQueue.main.async {
             UIGraphicsBeginImageContextWithOptions(self.bounds.size, self.isOpaque, 0.0)

             guard let context = UIGraphicsGetCurrentContext() else {
                 completion(UIImage()); return
             }

             self.layer.render(in: context)
             let image = UIGraphicsGetImageFromCurrentImageContext()
             UIGraphicsEndImageContext()

             completion(image!)
         }
     }
}

Answer 1

我要检查的第一件事是您是否不能简单地在主队列上执行工作 - 真的需要那么长时间才能对 UI 产生明显影响吗？无论如何，在生成 PDF 时用户还会做什么？如果他们正在等待生成 PDF 以便共享或打印，那么只需显示一个微调器就足够了。

如果您确实想将工作保留在另一个队列中，那么我会将 renderToImage 转换为异步函数并使用 DispatchSemaphore 阻塞您的工作队列，直到循环迭代完成。

func someFunction() {
    let semaphore = DispatchSemaphore(value:0)
    let queue = DispatchQueue(label:"PDFQueue", target: DispatchQueue.global(qos: .userInitiated))
    queue.async {
        for view in PDFViews {
            UIGraphicsBeginPDFContextToData(pdfData, CGRect.zero, nil)
            zip(self.pdfPages, self.pageViews).forEach { (page, view) in
                view.renderToImage { image in 
                   if let image = image {
                      ...
                      ...
                   }
                   semaphore.signal()
                }
                semaphore.wait()
            }
        }
    }   
}

extension UIView {
    func renderToImage(completion: ((UIImage?)->Void)) {
        DispatchQueue.main.async {
            UIGraphicsBeginImageContextWithOptions(bounds.size, isOpaque, 0.0)
            guard let context = UIGraphicsGetCurrentContext() else {
                completion(nil)
            }

            layer.render(in: context)
            let image = UIGraphicsGetImageFromCurrentImageContext()
            UIGraphicsEndImageContext()

            completion(image)
        }
    }   
}