swift - Swift 中的 UTTypeCreatePreferredIdentifierForTag 和 CFStringRef

Question

import Foundation
import MobileCoreServices

func checkFileExtension(fileName: NSString){
    println(fileName)

    var fileExtension:CFStringRef = fileName.pathExtension

    println(fileExtension)

    var fileUTI:CFStringRef = UTTypeCreatePreferredIdentifierForTag(kUTTagClassFilenameExtension, fileExtension, nil)

    println(fileUTI)

    let testBool = UTTypeConformsTo(fileUTI, kUTTypeImage) != 0

    if  testBool{
        println("image")
    }
}

我收到这个错误

error : 'Unmanaged' is not convertible to 'CFStringRef'

在线

var fileUTI:CFStringRef = UTTypeCreatePreferredIdentifierForTag(kUTTagClassFilenameExtension, fileExtension, nil)

有什么想法吗？？谢谢

Answer 1

UTTypeCreatePreferredIdentifierForTag传回一个 Unmanaged<CFStringRef> ，因此您需要从 Unmanaged 中获取值对象之前你可以使用它:

var unmanagedFileUTI = UTTypeCreatePreferredIdentifierForTag(kUTTagClassFilenameExtension, fileExtension, nil)
var fileUTI = unmanagedFileUTI.takeRetainedValue()

请注意，我正在调用 takeRetainedValue()自 UTTypeCreatePreferredIdentifierForTag正在返回一个我们负责释放的对象。关于 takeRetainedValue() 的评论说:

Get the value of this unmanaged reference as a managed reference and consume an unbalanced retain of it.

This is useful when a function returns an unmanaged reference and you know that you're responsible for releasing the result.

如果你得到一个 Unmanaged从您确定您不负责释放该对象的函数返回对象，调用takeUnretainedValue()相反。