swift - 无法将 '()?' 类型的值转换为指定的 Bool 类型

Question

我尝试编写更改文件属性的代码，但出现编译器错误。

这是代码:

func addSkipBackupAttributeToItemAtURL(URL: NSURL) -> Bool{

    let fileManager = NSFileManager.defaultManager()
    assert(fileManager.fileExistsAtPath(URL.absoluteString))

    var error:NSError?
    let success:Bool = try? URL.setResourceValue(NSNumber(bool: true),forKey: NSURLIsExcludedFromBackupKey)

    if !success {
        print("Error excluding \(URL.lastPathComponent) from backup \(error)")
    } else {
        print("File at path \(URL) was succesfully updated")
    }

    return success
}

错误是:

Cannot convert value of type '()?' to specified type 'Bool'

如何修复它？

Answer 1

调用 url?.setResourceValue(NSNumber(bool: true),forKey: NSURLIsExcludedFromBackupKey) 不会返回 Bool。您期望得到 Bool 作为响应。这就是错误的原因。

Apple 文档中有关 setResourceValue 方法的说明

In Swift, this method returns Void and is marked with the throws keyword to indicate that it throws an error in cases of failure.

do {
    try url?.setResourceValue(NSNumber(bool: true), forKey: NSURLIsExcludedFromBackupKey)

    //...
}
catch let error as NSError {
     //...
}