import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let downloadA = createOperation("thread1", "url1")
        let downloadB = createOperation("thread2", "url2")

        let queue = OperationQueue.init()
        queue.maxConcurrentOperationCount = 1
        queue.addOperation(downloadA)
        queue.addOperation(downloadB)

    }

    func createOperation(_ name: String, _ url: String) -> BlockOperation {
        let download = BlockOperation.init {
            //下载图片
            print("\(name) start download \(url)")

            // 下载完成后，回主线程显示图片
            self.perform(#selector(self.showImage(_:)), on: Thread.main, with: nil, waitUntilDone: true)
        }
        download.name = name

        return download
    }

    @objc func showImage(_ image: UIImage?) {

    }

}