NSOperation 和 NSOperationQueue 工作线程与主线程

在iOS开发中，我们经常会遇到需要在后台执行一些耗时操作的情况。为了避免阻塞主线程，提高用户体验，苹果推出了NSOperation和NSOperationQueue来管理线程的执行。NSOperation和NSOperationQueue提供了一种更高级的多线程编程方式，使得我们可以更方便地处理并发任务。

swift
class DownloadOperation: NSOperation {
    var imageURL: URL
    var image: UIImage?
    
    init(imageURL: URL) {
        self.imageURL = imageURL
    }
    
    override func main() {
        if self.cancelled {
            return
        }
        
        let imageData = try? Data(contentsOf: imageURL)
        
        if self.cancelled {
            return
        }
        
        if let data = imageData {
            self.image = UIImage(data: data)
        }
    }
}

swift
let imageURLs = [URL(string: "https://example.com/image1.jpg")!,
                 URL(string: "https://example.com/image2.jpg")!,
                 URL(string: "https://example.com/image3.jpg")!]
let operationQueue = NSOperationQueue()
operationQueue.maxConcurrentOperationCount = 3
for imageURL in imageURLs {
    let downloadOperation = DownloadOperation(imageURL: imageURL)
    
    downloadOperation.completionBlock = {
        DispatchQueue.main.async {
            // 更新UI
        }
    }
    
    operationQueue.addOperation(downloadOperation)
}