How to get a unique device ID in Swift?

You can use devicecheck (in Swift 4) Apple documentation

func sendEphemeralToken() {
        //check if DCDevice is available (iOS 11)

        //get the **ephemeral** token
        DCDevice.current.generateToken {
        (data, error) in
        guard let data = data else {
            return
        }

        //send **ephemeral** token to server to 
        let token = data.base64EncodedString()
        //Alamofire.request("https://myServer/deviceToken" ...
    }
}

Typical usage:

Typically, you use the DeviceCheck APIs to ensure that a new user has not already redeemed an offer under a different user name on the same device.

Server action needs:

See WWDC 2017 — Session 702 (24:06)

more from Santosh Botre article - Unique Identifier for the iOS Devices

Your associated server combines this token with an authentication key that you receive from Apple and uses the result to request access to the per-device bits.

You can use this (Swift 3):

UIDevice.current.identifierForVendor!.uuidString

For older versions:

UIDevice.currentDevice().identifierForVendor

or if you want a string:

UIDevice.currentDevice().identifierForVendor!.UUIDString

There is no longer a way to uniquely identify a device after the user uninstalled the app(s). The documentation says:

The value in this property remains the same while the app (or another app from the same vendor) is installed on the iOS device. The value changes when the user deletes all of that vendor’s apps from the device and subsequently reinstalls one or more of them.

You may also want to read this article by Mattt Thompson for more details:
http://nshipster.com/uuid-udid-unique-identifier/

Update for Swift 4.1, you will need to use:

UIDevice.current.identifierForVendor?.uuidString