如何快速将闭包用作函数的第二个参数？

Question

I have a method that should do two things. It should make a picture object, and return a closure thats called whenever a picture is finished downloaded locally. This following code snippet tries to draw out what im trying.

typealias localURLForDownloadedImage = () -> (NSURL)

func pictureForDate(#date: NSDate) -> (picture: Picture?,downloadedImageURL: urlForDownloadedImage?) {
    // start downloading picture from a url
    let picture = Picture()

    // start downloading url
    let closure = urlForDownloadedImage(){
        // async method that returns when image is downloaded
        return NSURL(string: "downloadedimageurl://")
    }

    return (picture,closure)
}

The error i get for the closure to attribute is "urlForDownloadedImage is not compatible with () -> () -> $TO"

As Xcode 6s autocomplete is broken still, how do you call this method? I am thinking somewhat like this:

pictureForDate(date: NSDate()) {
        // do stuff here after image was downloaded
    }

But this just gives a strange error.

Answer 1

The closure should be

let closure = { () -> NSURL in 
    // async method that returns when image is downloaded
    return NSURL(string: "downloadedimageurl://")
}

To call the method use:

let (pic, closure) = pictureForDate(date: NSDate())

now in pic and closure you have the 2 return values. The closure must be unwrapped before using:

closure?()