ios - Swift:作为 NSErrorPointer 传递的 NSError 始终为 nil

Question

我正在尝试按照 Apple 的 Swift 文档 (https://developer.apple.com/library/mac/documentation/swift/conceptual/BuildingCocoaApps/AdoptingCocoaDesignPatterns.html) 的“错误报告”部分中的说明实现错误处理，但我一定遗漏了一些东西。

我有下面的代码(简化)，即使网络请求明显失败(我与互联网断开连接，并且打印“ERROR”)我作为指针传递的错误变量在完成 block 内始终为 nil ...

我做错了什么？谢谢!

override func viewDidLoad() {

    super.viewDidLoad()

    var error: NSError?

    // THIS IS ACTUALLY IN ANOTHER CLASS,
    // BUT EVEN IF PLACED IN THE SAME CLASS I STILL GET THE SAME PROBLEM
    doStuff({ (result) -> Void in

        println("\(error)") // THIS PRINTS NIL

    }, errorPointer: &error)        
}

func doStuff(completion: (result:SomeType) -> Void, errorPointer: NSErrorPointer) {

    // SETUP CODE NOT INCLUDED

    let downloadTask = manager.downloadTaskWithRequest(req, progress: nil, destination: { (url, response) -> NSURL! in

        return destinationURL

    }) { (response, url, error) -> Void in

        // CREATE RESULT

        if(error != nil && errorPointer != nil) {
            println(“ERROR”) // THIS GETS PRINTED
            errorPointer.memory = error // ALSO TRIED NSError(...)
        }
        completion(result)
    }
    downloadTask.resume()        
}

Answer 1

解决方案

您可以在 Apple 的免费电子书 Using Swift with Cocoa and Objective-C(第 51 页)中找到 Swift 错误处理的清晰示例。

var writeError: NSError?
let written = myString.writeToFile(path, atomically: false,
    encoding: NSUTF8StringEncoding,
    error: &writeError)
if !written {
    if let error = writeError {
        println("write failure: \(error.localizedDescription)")
    }
}

Excerpt From: Apple Inc. “Using Swift with Cocoa and Objective-C.” iBooks. https://itun.es/us/1u3-0.l

基本上您想要查看错误的内存地址，然后测试该值以处理错误。尝试从中推断并重写您的代码以获得有效的结果。

此外，如果您必须使用 NSErrorPointer，请参阅以下内容 SO question and answer .