退出应用后按需资源真的会持续存在吗？

Question

我已创建 NSBundleResourceRequest 对象作为该类的实例成员。如果我退出应用程序，请求对象将被释放。根据苹果文档，如果请求对象被释放，则下载的资源的控制将由系统处理，并且数据可能会被永久删除。我的疑问是，如果游戏对每个级别的元数据使用 ODR，我们不会每次重新打开应用程序时都下载资源。下载资源后是否需要手动保存 ODR？

class ViewController: UIViewController {
    var request = NSBundleResourceRequest(tags: Set(arrayLiteral: "prefetch"))
    
    lazy var imageView1:UIImageView = {
        let imageView  = UIImageView(image: UIImage(named: "car"))
        imageView.layer.borderColor = UIColor.red.cgColor
        imageView.layer.borderWidth = 2
        return imageView
    }()
    let label = UILabel()
    lazy var  imageView2:UIImageView = {
        let imageView  = UIImageView(image:UIImage(named: "hd1"))
        imageView.layer.borderColor = UIColor.red.cgColor
        imageView.layer.borderWidth = 2
        return imageView
    }()
    
    lazy var imageView3:UIImageView = {
        let imageView  = UIImageView(image:UIImage(named: "hd2"))
        imageView.layer.borderColor = UIColor.red.cgColor
        imageView.layer.borderWidth = 2
        return imageView
    }()
    
    let button:UIButton = {
//        let config = UIButton.Configuration.bordered()
        let button = UIButton()
        button.setTitleColor(.white, for: .normal)
        button.backgroundColor = .green
        button.setTitle("Load HD image", for: .normal)
        return button
    }()
    
    override func loadView() {
        super.loadView()
        loadImage()
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        configureImageViews()
        fetchImage()
    }

    func configureImageViews(){
        view.addSubview(imageView1)
        view.addSubview(imageView2)
        view.addSubview(imageView3)
        view.addSubview(button)
        view.addSubview(label)
        
        label.translatesAutoresizingMaskIntoConstraints = false
        imageView1.translatesAutoresizingMaskIntoConstraints = false
        imageView2.translatesAutoresizingMaskIntoConstraints = false
        imageView3.translatesAutoresizingMaskIntoConstraints = false
        button.translatesAutoresizingMaskIntoConstraints = false

        label.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor).isActive = true
        label.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        label.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -10).isActive = true
        label.heightAnchor.constraint(equalToConstant: 40).isActive = true
        
        imageView1.topAnchor.constraint(equalTo: label.safeAreaLayoutGuide.bottomAnchor).isActive = true
        imageView1.heightAnchor.constraint(equalToConstant: 200).isActive = true
        imageView1.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
        imageView1.widthAnchor.constraint(equalToConstant: 200).isActive = true
        
        imageView2.topAnchor.constraint(equalTo: imageView1.safeAreaLayoutGuide.bottomAnchor,constant: 20).isActive = true
        imageView2.heightAnchor.constraint(equalToConstant: 200).isActive = true
        imageView2.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
        imageView2.widthAnchor.constraint(equalToConstant: 200).isActive = true
        
        imageView3.topAnchor.constraint(equalTo: imageView2.safeAreaLayoutGuide.bottomAnchor,constant: 20).isActive = true
        imageView3.heightAnchor.constraint(equalToConstant: 200).isActive = true
        imageView3.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
        imageView3.widthAnchor.constraint(equalToConstant: 200).isActive = true
        
        button.topAnchor.constraint(equalTo: imageView3.safeAreaLayoutGuide.bottomAnchor,constant: 20).isActive = true
        button.heightAnchor.constraint(equalToConstant: 40).isActive = true
        button.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
        button.widthAnchor.constraint(equalToConstant: 200).isActive = true
        
        button.addTarget(self, action: #selector(buttonDidTapped), for: .touchUpInside)
    }
    
    func fetchImage(){
        request.endAccessingResources()
        self.request.conditionallyBeginAccessingResources(completionHandler: {isImageAvailable in
            if isImageAvailable == true{
                //image already available
                print("--> Local data available")
                self.loadImage()
                DispatchQueue.main.async {
                    self.label.text = "Local data available"
                }
            }else{
                //download from app store
                self.request.beginAccessingResources(completionHandler: {error in
                    if let error{
                        print("--> something went wrong")
                        self.label.text = "something went wrong"
                        return
                    }
                    DispatchQueue.main.async {
                        self.label.text = "fetching remote"
                    }
                    self.loadImage()
                })
            }
            
        })
    }

    
    func loadImage(){
        DispatchQueue.main.async {
            self.imageView1.image = UIImage(named: "car")
            self.imageView2.image = UIImage(named: "hd1")
            self.imageView3.image = UIImage(named: "hd2")
        }
    }
    
   @objc func buttonDidTapped(){
        request = NSBundleResourceRequest(tags: Set(arrayLiteral: "hd1"))
       fetchImage()
    }
}

我希望我们如何保证ODR数据不会被再次下载？

Answer 1

您对特定 ODR 资源的使用必须通过调用

beginAccessingResources

和

endAccessingResources

括起来，在它们之间保留 NSBundleResourceRequest。这就是您所知道的以及您需要知道的全部。

如果您的应用程序在调用

endAccessingResources

之前终止，那么在重新启动时您显然没有 NSBundleResourceRequest 实例；但这并不重要，因为您只需继续遵循有关如何访问资源的规则即可。启动后立即创建一个 NSBundleResourceRequest 并调用

beginAccessingResources

；您的资源可能仍然存在，因此您将立即获得访问权限，并且保留 NSBundleResourceRequest 可以使资源继续存在，直到您最终通过说

endAccessingResources

释放它们。