Swift中的UTTypeCreatePreferredIdentifierForTag和CFStringRef [英] UTTypeCreatePreferredIdentifierForTag and CFStringRef in Swift

问题描述

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")
    }
}

我收到此错误

错误:非托管"无法转换为"CFStringRef"

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

在线

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

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

有什么想法吗?谢谢

推荐答案

UTTypeCreatePreferredIdentifierForTag传回 Unmanaged<CFStringRef> ，因此您需要先从Unmanaged对象中获取值，然后才能使用它:

UTTypeCreatePreferredIdentifierForTag passes back an Unmanaged<CFStringRef>, so you need to get the value out of the Unmanaged object before you can use it:

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

请注意，由于UTTypeCreatePreferredIdentifierForTag返回的是我们负责释放的对象，因此我正在调用takeRetainedValue(). takeRetainedValue()上的评论说:

Note that I'm calling takeRetainedValue() since UTTypeCreatePreferredIdentifierForTag is returning an object that we are responsible for releasing. The comments on takeRetainedValue() say:

获取此非托管引用的值作为托管引用，然后 消耗了不平衡的保留.

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

当函数返回非托管引用并且您 知道您有责任发布结果.

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

如果从确定您没有负责释放该对象的函数中获取了Unmanaged对象，请调用takeUnretainedValue().

If you get an Unmanaged object back from a function where you are sure you aren't responsible for releasing that object, call takeUnretainedValue() instead.

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