如何在Swift中将String类型变量转换为CFString？

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 is not constructible with ‘@lvalue String’

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,0) == CFComparisonResult.CompareEqualTo) {
        editedImage = info.objectForKey(UIImagePickerControllerEditedImage) as UIImage
        originalImage = info.objectForKey(UIImagePickerControllerOriginalImage) as UIImage

它提醒我

Cannot convert the expression’s type ‘CFString’ to type ‘$T1’

解决方法

Swift.String和Nsstring自动桥接,Nsstring和CFString可以相互转换,但你不能(现在？)传递上从Swift字符串一直到CFString,而不需要额外的强制转换.

这是一个Swift字符串,一个接受CFString的函数,以及如何通过强制转换为Nsstring来调用它：

var str = "Hello,playground"

func takesCFString(s: CFString) {}

takesCFString(str as Nsstring)

注意,您不需要使用CFStringRefin Swift(而且大多数CFString API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.