swift - 如何让返回等待计算完成

Question

预先感谢您的帮助。

我正在尝试从 Dropbox 中的 CSV 下载数据。我能够成功下载数据，但现在我正在尝试从这些进程中封装/创建函数。也就是说，我想创建一个函数，在其中下载 CSV 并将数据作为字符串返回。

我的问题是这样的:我的函数在从 Dropbox 下载完成之前返回数据变量。因此它不会返回任何内容。即，我将首先看到注释“返回 allData 变量”，然后是“转换为 allData 变量”。我需要以某种方式扭转这一局面。

我看到 2 个选项:1)弄清楚如何工作这个异步位(我是 Swift 的新手，为了更好地理解它，我一直在网上搜索)，或者 2)以某种方式构建 return 调用仅在数据完全下载后执行。

对于如何继续前进，你们有什么建议吗？我在下面包含了我的代码。

func downloadFile () -> [[String:String]] {

var allData = [[String:String]]()

if let client = Dropbox.authorizedClient {

    let destination : (NSURL, NSHTTPURLResponse) -> NSURL = { temporaryURL, response in

        let fileManager = NSFileManager.defaultManager()
        let directoryURL = fileManager.URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)[0]

        // generate a unique name for this file in case we've seen it before

        let UUID = NSUUID().UUIDString

        let pathComponent = "\(UUID)-\(response.suggestedFilename!)"

        print ("The path component is \(pathComponent)")

        return directoryURL.URLByAppendingPathComponent(pathComponent)

    }

//Option 1: Create some bit of asynchronous code here?
    client.files.download(path: "/Master Test.csv", destination: destination).response { response, error in

        if let (metadata, url) = response {

            print("*** Download file ***")

            let data = NSData(contentsOfURL: url)
            let dlString = NSString(data: data!, encoding: NSUTF8StringEncoding)

            let a = CSwiftV(String: String(dlString!))

            allData = a.keyedRows!

            print ("converted to allData variable")

//Option 2: Somehow embed the return call here?

        } else {

            print (error!)

        }
    }
}

print ("returned allData variable")

return allData

}

Answer 1

我建议向您的函数添加一个completionHandler。您可以在 swift 文档中了解有关闭包的更多信息。 https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html

func downloadFile (completion: (data: [[String:String]]?, error: NSError?) -> ()) -> (){

var allData = [[String:String]]()

if let client = Dropbox.authorizedClient {

    let destination : (NSURL, NSHTTPURLResponse) -> NSURL = { temporaryURL, response in

        let fileManager = NSFileManager.defaultManager()
        let directoryURL = fileManager.URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)[0]

        // generate a unique name for this file in case we've seen it before

        let UUID = NSUUID().UUIDString

        let pathComponent = "\(UUID)-\(response.suggestedFilename!)"

        print ("The path component is \(pathComponent)")

        return directoryURL.URLByAppendingPathComponent(pathComponent)

    }

//Option 1: Create some bit of asynchronous code here?
    client.files.download(path: "/Master Test.csv", destination: destination).response { response, error in

        if let (metadata, url) = response {

            print("*** Download file ***")

            let data = NSData(contentsOfURL: url)
            let dlString = NSString(data: data!, encoding: NSUTF8StringEncoding)

            let a = CSwiftV(String: String(dlString!))

            allData = a.keyedRows!

            print ("converted to allData variable")

//Option 2: Somehow embed the return call here?
            completion(data: allData, error: nil)

        } else {

            print (error!)
            completion(data: nil, error: error)

        }
    }
}

print ("returned allData variable")

return allData

}

函数调用

 downloadFile(){
  //start your loader here
            dropboxData in
           //remove here
            if(dropboxData.error == nil){
                print(dropboxData.data)
            }else{
                print(dropboxData.error)
            }

        }