Swift 2到Swift 3:无法转换类型'(Data ?, NSError?)->的值无效"到期望的参数类型"GTMSessionFetcherCompletionHandler?" [英] Swift 2 to Swift 3: Cannot convert value of type '(Data?, NSError?) -> Void' to to expected argument type 'GTMSessionFetcherCompletionHandler?'

问题描述

我刚刚将工作正常的Swift 2更新为Swift 3程序，但出现了错误，

I just updated a working Swift 2 to Swift 3 program, and I am getting the error,

无法将类型'(Data ?, NSError?)-> Void'的值转换为预期的参数类型'GTMSessionFetcherCompletionHandler?'

Cannot convert value of type '(Data?, NSError?) -> Void' to expected argument type 'GTMSessionFetcherCompletionHandler?'

以下是相关详细信息(希望如此):

Here are the relevant details (I hope):

let fetcher = GTMSessionFetcher(urlString:url)
fetcher.authorizer = parentController.service.authorizer
fetcher.beginFetch(completionHandler: handleDownload(studentNum))
                                      ^^^^ causing the error

completionHandler的函数:

The function for the completionHandler:

func handleDownload(_ studentNum:Int) -> (Data?, NSError?) -> Void {
    return { (data: Data?, error: NSError?) -> Void in
        // code for function
    }
}

GTMSessionFetcherCompletionHandler在Objective-C标头中定义，如下所示:

GTMSessionFetcherCompletionHandler is defined in an Objective-C header, as follows:

#define GTM_NULLABLE_TYPE __nullable
typedef void (^GTMSessionFetcherCompletionHandler)(NSData * GTM_NULLABLE_TYPE data,
                                               NSError * GTM_NULLABLE_TYPE error);

我尝试将handleDownload()更改为以下内容:

I have tried changing handleDownload() to the following:

func handleDownload(_ studentNum:Int) -> (GTMSessionFetcherCompletionHandler?) {
    return { (data: Data?, error: NSError?) -> Void in
       // code for function
    }
}

但是将错误移至该函数:无法将类型'(Data ?, NSError?)-> Void'的返回表达式转换为'GTMSessionFetcherCompletionHandler吗?'"

but that moves the error down to this function: "Cannot convert return expression of type '(Data?, NSError?) -> Void' to return type 'GTMSessionFetcherCompletionHandler?'"

我不知道如何保留已处理(?)的数据和错误变量，并进行编译.

I can't figure out how to keep the curried (?) data and error variables, and have it compile.

推荐答案

根据 Error 桥接到Swift.协议.实际上，如果您⌥ +单击Swift中的GTMSessionFetcherCompletionHandler类型，您将确切看到它是如何桥接的:

As per SE-0112, NSError is now bridged to Swift as the Error protocol. In fact, if you ⌥ + click on the GTMSessionFetcherCompletionHandler type in Swift, you'll see exactly how it's bridged:

typealias GTMSessionFetcherCompletionHandler = (Data?, Error?) -> Void

因此，您只需要更改handleDownload(_:)的签名即可反映这一点:

Therefore you simply need to change your handleDownload(_:)'s signature to reflect this:

func handleDownload(_ studentNum:Int) -> (Data?, Error?) -> Void {
    return { (data: Data?, error: Error?) -> Void in
        // code for function
    }
}

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