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如何使用self-in-closure防止内存泄漏

swift ios

Evgeniy Kleban · 技术社区 · 7 年前

我有下载文件的课程:

class FileDownloader {

    private let downloadsSession = URLSession(configuration: .default)
    private var task: URLSessionDownloadTask?
    private let url: URL

    init(url: URL) {
        self.url = url
    }

    public func startDownload(){
        download()
    }

    private func download(){

        task = downloadsSession.downloadTask(with: url) {[weak self] (location, response, error) in
            guard let weakSelf = self else {
                assertionFailure("self was deallocated")
                return }
            weakSelf.saveDownload(sourceUrl: weakSelf.url, location: location, response: response, error: error)
        }

        task!.resume()
    }

    private func saveDownload(sourceUrl : URL, location : URL?, response : URLResponse?, error : Error?) {
        if error != nil {
            assertionFailure("error \(String(describing: error?.localizedDescription))")
            return }

        let destinationURL = localFilePath(for: sourceUrl)

        let fileManager = FileManager.default
        try? fileManager.removeItem(at: destinationURL)
        do {
            try fileManager.copyItem(at: location!, to: destinationURL)
            print("save was completed at \(destinationURL) from \(String(describing: location))")
        } catch let error {
            print("Could not copy file to disk: \(error.localizedDescription)")
        }
    }

    private func localFilePath(for url: URL) -> URL {
        let documentsPath = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
        return documentsPath.appendingPathComponent(url.lastPathComponent)
    }
}

当我打电话时 startDownload() 我在第行调试时出错:

assertionFailure("self was deallocated")

当我将下载功能更改为:

private func download(){

        task = downloadsSession.downloadTask(with: url) {(location, response, error) in
            self.saveDownload(sourceUrl: self.url, location: location, response: response, error: error)
        }

        task!.resume()
    }

这一切都很好,但我担心这可能会导致内存中未正确释放的对象出现问题。如何避免这种情况?我做得对吗?

2 回复 | 直到 7 年前

Rob Md Fahim Faez Abir 7 年前

FileDownloader 实例不在范围内。您尚未分享如何调用此函数,但可能将其用作局部变量。如果你解决了这个问题,你的问题就会消失。

第二,当您更改实现以删除 [weak self] 直到下载完成。如果这是你想要的行为,那就好了。这是一种完全可以接受的模式,即在异步任务完成之前,保持对自身的引用。事实上,这正是我想要的 URLSessionTask [虚弱的自我] 模式,在某些情况下,它可以引入一个强引用循环,但在本例中不是这样。

强引用循环仅在两个对象之间具有持久的强引用时发生(或者有时可能涉及两个以上的对象)。就 URLSession downloadTask

例如,考虑这个例子:

class Foo {
    func performAfterFiveSeconds(block: @escaping () -> Void) {
        DispatchQueue.main.asyncAfter(deadline: .now() + 5.0) {
            self.doSomething()

            block()
        }
    }

    func doSomething() { ... }
}

asyncAfter 运行闭包时释放闭包。但是考虑这个例子,我们在自己的IVAR中保存闭包:

class BarBad {
    private var handler: (() -> Void)?

    func performAfterFiveSeconds(block: @escaping () -> Void) {
        handler = block

        DispatchQueue.main.asyncAfter(deadline: .now() + 5.0) {
            self.calledWhenDone()
        }
    }

    func calledWhenDone() {
        // do some stuff

        doSomething()

        // when done, call handler

        handler?()
    }

    func doSomething() { ... }
}

但幸运的是,这很容易纠正:

class BarGood {
    private var handler: (() -> Void)?

    func performAfterFiveSeconds(block: @escaping () -> Void) {
        handler = block

        DispatchQueue.main.asyncAfter(deadline: .now() + 5.0) {
            self.calledWhenDone()
        }
    }

    func calledWhenDone() {
        // do some stuff

        doSomething()

        // when done, call handler

        handler?()

        // make sure to release handler when done with it to prevent strong reference cycle

        handler = nil
    }

    func doSomething() { ... }
}

这将在设置时解析强引用循环 handler nil . 这实际上是什么 URLSession (和GCD方法,如 async 异步后 )做。他们保存闭包直到调用它,然后释放它。

Camilo Lopez 7 年前

不要使用这个:

task = downloadsSession.downloadTask(with: url) {(location, response, error) in
            self.saveDownload(sourceUrl: self.url, location: location, response: response, error: error)
        }

class FileDownloader:URLSessionTaskDelegate, URLSessionDownloadDelegate

    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
        if totalBytesExpectedToWrite > 0 {
            let progress = Float(totalBytesWritten) / Float(totalBytesExpectedToWrite)
            debugPrint("Progress \(downloadTask) \(progress)")
        }
    }

    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
        debugPrint("Download finished: \(location)")
        try? FileManager.default.removeItem(at: location)
    }

    func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
        debugPrint("Task completed: \(task), error: \(error)")
    }

我知道该值不会为零,但请尽量避免强制展开:

task!.resume()

临时文件,将进度更新作为数据提供给应用程序从服务器到达。在后台使用下载任务时会话,即使您的应用程序被挂起或关闭,这些下载仍将继续

您可以暂停(取消)下载任务,稍后再继续(假设服务器支持这样做)。你也可以继续下载