//BlockOperation////  ViewController.swift import UIKitclass ViewController: UIViewController {    @IBOutlet weak var image1: UIImageView!    @IBOutlet weak var image2: UIImageView!    @IBOutlet weak var image3: UIImageView!    @IBOutlet weak var image4: UIImageView!        let imageUrls = [        "https://dn-boxueio.qbox.me/image1-big.jpg",        "https://dn-boxueio.qbox.me/image2-big.jpg",        "https://dn-boxueio.qbox.me/image3-big.jpg",        "https://dn-boxueio.qbox.me/image4-big.jpg"    ]        override func viewDidLoad() {        super.viewDidLoad()      //  present(<#T##viewControllerToPresent: UIViewController##UIViewController#>, animated: <#T##Bool#>, completion: nil)        // Do any additional setup after loading the view, typically from a nib.    }    override func didReceiveMemoryWarning() {        super.didReceiveMemoryWarning()        // Dispose of any resources that can be recreated.    }    @IBAction func clearDownload(_ sender: UIButton) {              //  self.image1.image=UIImage(named: <#T##String#>)                self.image1.image = nil        self.image2.image = nil        self.image3.image = nil        self.image4.image = nil                URLCache.shared.removeAllCachedResponses()    }    let queue = OperationQueue()        @IBAction func cancelDownload(_ sender: AnyObject) {        self.queue.cancelAllOperations()    }        @IBAction func downloadImages(_ sender: UIButton) {               //        queue.addOperation({//            //            //            let img1 = Downloader.downloadImageWithURL(self.imageUrls[0])//            //            OperationQueue.main.addOperation({//                self.image1.image = img1//                self.image1.clipsToBounds = true//            })//        })//        //                let op1 = BlockOperation(block: {            let img1 = Downloader.downloadImageWithURL(self.imageUrls[0])                        OperationQueue.main.addOperation({                self.image1.image = img1                self.image1.clipsToBounds = true            })        })        op1.completionBlock = { print("image1 downloaded") }                let op2 = BlockOperation(block: {            let img2 = Downloader.downloadImageWithURL(self.imageUrls[1])                        OperationQueue.main.addOperation({                self.image2.image = img2                self.image2.clipsToBounds = true            })        })        op2.completionBlock = { print("image2 downloaded") }                                let op3 = BlockOperation(block: {            let img3 = Downloader.downloadImageWithURL(self.imageUrls[2])                        OperationQueue.main.addOperation({                self.image3.image = img3                self.image3.clipsToBounds = true            })        })        op3.completionBlock = { print("image3 downloaded") }                                let op4 = BlockOperation(block: {            let img4 = Downloader.downloadImageWithURL(self.imageUrls[3])                        OperationQueue.main.addOperation({                self.image4.image = img4                self.image4.clipsToBounds = true            })        })        op4.completionBlock = { print("image4 downloaded") }           //     op3.addDependency(op4)       // op2.addDependency(op3)                queue.addOperation (op1)        queue.addOperation(op4)        queue.addOperation(op3)        queue.addOperation(op2)    }}//GCD实用代码块import UIKitclass ViewController: UIViewController {    @IBOutlet weak var image1: UIImageView!    @IBOutlet weak var image2: UIImageView!    @IBOutlet weak var image3: UIImageView!    @IBOutlet weak var image4: UIImageView!        let imageUrls = [        "https://dn-boxueio.qbox.me/image1-big.jpg",        "https://dn-boxueio.qbox.me/image2-big.jpg",        "https://dn-boxueio.qbox.me/image3-big.jpg",        "https://dn-boxueio.qbox.me/image4-big.jpg"    ]        override func viewDidLoad() {        super.viewDidLoad()        // Do any additional setup after loading the view, typically from a nib.    }    override func didReceiveMemoryWarning() {        super.didReceiveMemoryWarning()        // Dispose of any resources that can be recreated.    }    @IBAction func clearDownload(_ sender: UIButton) {        self.image1.image = nil        self.image2.image = nil        self.image3.image = nil        self.image4.image = nil                URLCache.shared.removeAllCachedResponses()    }     func loadImageWithURL(_ url:String) -> UIImage! {        let data = try? Data(contentsOf: URL(string: url)!)        return UIImage(data: data!)                 }        @IBAction func downloadImages(_ sender: UIButton) {                      var currQueue = DispatchQueue.global(qos: .default)      //  currQueue = DispatchQueue(label: "com.boxueio.images", attributes: .concurrent)        //并行        currQueue=DispatchQueue(label: "currqueue1", qos: .default, attributes: .concurrent)                // dispatch_async        currQueue.async(execute: {            let img1 = self.loadImageWithURL(self.imageUrls[0])            DispatchQueue.main.async(execute: {                self.image1.image = img1                self.image1.clipsToBounds = true            })        })                currQueue.async(execute: {            let img2 = self.loadImageWithURL(self.imageUrls[1])            DispatchQueue.main.async(execute: {                self.image2.image = img2                self.image2.clipsToBounds = true            })        })                currQueue.async(execute: {            let img3 = Downloader.downloadImageWithURL(self.imageUrls[2])            DispatchQueue.main.async(execute: {                self.image3.image = img3                self.image3.clipsToBounds = true            })        })                currQueue.async(execute: {            let img4 = Downloader.downloadImageWithURL(self.imageUrls[3])            DispatchQueue.main.async(execute: {                self.image4.image = img4                self.image4.clipsToBounds = true            })        })    }}//class图片基类import UIKitclass Downloader {        class func downloadImageWithURL(_ url:String) -> UIImage! {        let data = try? Data(contentsOf: URL(string: url)!)        return UIImage(data: data!)    }}