ios - 如何解决 Swift 4 URLSession.downloadTask 中的内存循环问题？

Question

我有一个基本的 viewController，它带有一个按钮，当点击该按钮时，它会调用一个方法开始从给定的有效 url 下载图像。

我非常小心地将任何强指针传递给 View Controller 。而且我能够关闭 View Controller 并返回到呈现 Controller 而不会出现任何问题。然而，由 View Controller 创建的 Web() 的对象实例不会被释放，即使 View Controller 的 deinit 被调用。

我做错了什么？

顺便说一句:文件下载开始，报告进度和文件位置。但是，一旦文件下载完成，该对象就永远不会被释放。

viewController.swift

@IBAction func buttonTapped(_ sender: UIButton) {
   //first create an instance of Web class (download helper)
   let web = Web(url: "https://www.google.com.sa/images/branding/googlelogo/2x/googlelogo_color_120x44dp.png")

   // all call back closures have no reference to self    
   web.downloadFile(
       finishedHandler: {fileLocation in print("file stored to \(fileLocation)")},
       progressHandler: {bytes,total in print("written \(bytes) out of \(total)")},
       errorHandler: {msg in print("error: \(msg)")}
  )

}

并且我将 Web() 定义为 NSObject，它符合 URLSessionDownloadDelegate 以利用委托(delegate)事件方法 (didFinishDownload/didWriteBytes 等)。

网络.swift

import UIKit

class Web : NSObject {
    var urlToDownload : String?

   // the following variables are references to closures passed to the object.
    var progressCallback : ((_ bytesWritten:Int64, _ totalExpectedBytes: Int64)->())?
    var finishedCallback : ((_ fileLocation: String)->())?


    static var instanceCount = 0 // keep track of number of instances created

    init(url: String) {

       Web.instanceCount += 1
       urlToDownload = url
       print(" new instance of Web created. Total : \(Web.instanceCount)")
   }

   deinit {
      Web.instanceCount -= 1
      print("Web instance deallocated. Remaining: \(Web.instanceCount)")

   }
}

然后我将文件下载逻辑添加为同一文件的扩展:

extension Web : URLSessionDownloadDelegate {

    func downloadFile(
        finishedHandler: @escaping (_ fileLocation:String)->(),
        progressHandler: @escaping (_ bytesWritten:Int64, _ totalBytes: Int64)->(),
        errorHandler: @escaping (_ errorMsg:String)->()) {

        // we need to capture the closure because, these will
        // be called once the delegate methods are triggered
        self.progressCallback = progressHandler
        self.finishedCallback = finishedHandler


        if let url = URL(string: self.urlToDownload!) {
             let session = URLSession(
                configuration: .default,
                delegate: self,
                delegateQueue: nil)

            let task = session.downloadTask(with: url)

            task.resume()

        }

    }

    // MARK :- Delegate methods
    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
        // call the closure if it still exists
        self.finishedCallback?(location.absoluteString)
    }

    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
        // report progress by calling captured closure, if exists
        self.progressCallback?(totalBytesWritten,totalBytesExpectedToWrite)
    }
}

Answer 1

在我跟踪问题之后(使用Memory Graph Debugger)，我弄清楚了这段代码(Web 类 - 下载文件方法)中的问题原因:

if let url = URL(string: self.urlToDownload!) {
     let session = URLSession(
        configuration: .default,
        delegate: self,
        delegateQueue: nil)

    let task = session.downloadTask(with: url)

    task.resume()
}

实际上有一个与实例化URLSession相关的问题没有摆脱它；因此，你必须做一种 - 手动处理来解决它，调用 finishTasksAndInvalidate()适合这样的问题:

if let url = URL(string: self.urlToDownload!) {
    let session = URLSession(
        configuration: .default,
        delegate: self,
        delegateQueue: nil)

    let task = session.downloadTask(with: url)

    task.resume()
    // here we go:
    session.finishTasksAndInvalidate()
}

为了确保它按预期工作，我建议在 Web 类中的委托(delegate)方法和 deinit 添加断点，您应该会看到它按预期工作(委托(delegate)方法可以工作并且然后将调用 deinit。

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此外:

如果您想了解更多有关如何使用内存图调试器的信息，您可以查看:How to debug memory leaks when Leaks instrument does not show them?