如何在Swift中将String类型变量转换为CFString？

Question

我从iOS教程中学到了这一点,但它不起作用

func imagePickerController(picker: UIImagePickerController!, didFinishPickingMediaWithInfo info: NSDictionary!) {
    var mediaType = info.objectForKey(UIImagePickerControllerMediaType) as String
    var originalImage, editedImage, imageToUse: UIImage

    // Handle a still image picked from a photo album
    if (CFStringCompare(CFStringRef(mediaType), kUTTypeImage, 0) == CFComparisonResult.CompareEqualTo) {
        editedImage = info.objectForKey(UIImagePickerControllerEditedImage) as UIImage
        originalImage = info.objectForKey(UIImagePickerControllerOriginalImage) as UIImage

        if (editedImage) {
            imageToUse = editedImage
        } else {
            imageToUse = originalImage
        }

        // Do something with imageToUse
    }

它经常提醒我

CFStringRef不能用'@lvalue String'构造

所以我尝试了这个:

// Handle a still image picked from a photo album
    var temp = mediaType as CFString
    if (CFStringCompare(temp, kUTTypeImage, 0) == CFComparisonResult.CompareEqualTo) {
        editedImage = info.objectForKey(UIImagePickerControllerEditedImage) as UIImage
        originalImage = info.objectForKey(UIImagePickerControllerOriginalImage) as UIImage

它提醒我

无法将表达式的类型'CFString'转换为'$ T1'类型

Answer 1

Swift.String和NSString桥自动,并且NSString和CFString可以转换为彼此,但你不能(现在？)传递地从斯威夫特得到一路String到CFString不需要额外投或两个.

这是一个Swift字符串,一个带有a的函数CFString,以及如何通过强制转换为NSString:

var str = "Hello, playground"

func takesCFString(s: CFString) {}

takesCFString(str as NSString)

注意,您不需要CFStringRef在Swift中使用(并且大多数CFStringAPI 的Swift声明都没有).编译器利用Xcode 5引入的CF类型的自动ARC桥接,让您可以像Swift(或ObjC)对象那样处理这些类型.