How to use TouchID using Swift?

Here is my view controller that does these checks in Swift. While working on this I found the completion block/closure syntax in Swift to be very confusing.

Notice that some of the options changed in Beta 2 in order to give you more control over the Touch ID dialog such as disabling the fallback option or the cancel button.

// Imports
import UIKit
import LocalAuthentication

// Class Implementation
class FirstViewController: UIViewController {

// Class Properties
@IBOutlet var statusLabel : UILabel
@IBOutlet var headerString: UILabel
var authError : NSError?
var authContext = LAContext()
var statusText = ""
var alert = UIAlertController(title: "", message: "", preferredStyle: UIAlertControllerStyle.Alert)


// Class Methods
@IBAction func swiftButtonPress(sender : AnyObject) {

    statusLabel.text = "Authenticating"

    //Can we use local auth?
    if authContext.canEvaluatePolicy(LAPolicy.DeviceOwnerAuthenticationWithBiometrics, error: &authError) {

        authContext.evaluatePolicy(LAPolicy.DeviceOwnerAuthenticationWithBiometrics,
            localizedReason: "I need to see if it's really you",
            reply: {(success: Bool, error: NSError!) -> Void in

                if success {
                    self.statusText = "Touch ID success!"
                    self.alert.title = "Success"
                    self.alert.message = "I knew it was you!"
                } else {
                    self.statusText = "Touch ID failed!"
                    self.alert.title = "Failure"

                    switch error!.code {
                    case LAError.AuthenticationFailed.toRaw():
                        self.alert.message = "Authentication Failed"
                    case LAError.UserCancel.toRaw():
                        self.alert.message = "User canceled!"
                    case LAError.SystemCancel.toRaw():
                        self.alert.message = "The system canceled!"
                    case LAError.UserFallback.toRaw():
                        self.alert.message = "User request to enter passcode"
                    default:
                        self.alert.message = "Something else went wrong"
                    }
                }
                self.presentViewController(self.alert, animated: true, completion:{self.statusLabel.text = self.statusText})
            })
    } else {
        self.statusText = "No local authentication"
        alert.title = "Uh oh!"

        switch authError!.code {
        case LAError.TouchIDNotAvailable.toRaw():
            alert.message = "No Touch ID on device"
        case LAError.TouchIDNotEnrolled.toRaw():
            alert.message = "No fingers enrolled"
        case LAError.PasscodeNotSet.toRaw():
            alert.message = "No passcode set"
        default:
            alert.message = "Something went wrong getting local auth"
        }
        self.presentViewController(self.alert, animated: true, completion: {self.statusLabel.text = self.statusText})
    }
    resetTouchID()
}

// Reset the system so we can go again
func resetTouchID() {
    authContext = LAContext()
    alert = UIAlertController(title: "", message: "", preferredStyle: UIAlertControllerStyle.Alert)
    alert.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.Cancel, handler: nil))
    let passcodeDetector = SwiftPasscodeDetector()
    if passcodeDetector.checkForPasscode() {
        headerString.text = "Passcode Set on Device"
    } else {
        headerString.text = "No Passcode Set"
    }

}

// Inherited Methods
override func viewDidLoad() {
    super.viewDidLoad()
    resetTouchID()
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

}

The API name is LAContext, and it's in the docs right here. It's pretty sparse, but it does its job. The method you probably want is

evaluatePolicy(_ policy: LAPolicy,
    localizedReason localizedReason: String!, 
    reply reply: ((Bool, NSError!) -> Void)!)

The string argument is a subheader to display to the user, the reply is simply a callback block, and the policy currently has to be LAPolicy.DeviceOwnerAuthenticationWithBiometrics, but it appears the framework is there for other types of authentication in the future. Interesting...

Hope that helps! I tried on my phone because I was curious and it works wonderfully. Just make sure to query for ability to evaluate policy before you try to use it, in case it's on an older device.

The LAContext reference has method signatures in both Obj-C and Swift. Furthermore, if you ⌘-click on the LAContext class in your Swift code, you should be able to view the generated "header" for it in Swift.

Updated to Swift 3

static func authorizeWithTouchIDIfPossible(){
        let authContext = LAContext()
        var authError : NSError?
        if authContext.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &authError) {
            authContext.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "I need to see this", reply: { (success, error) in
                if success {
                    print("Touch ID success!")
                    DispatchQueue.main.async {
                        //Do stuff here
                    }
                } else {
                    print("Touch ID failed!")
                }}
            );
        } else {
            print("No local authentication")
        }
    }

Swift 3.0 in:

import UIKit
import LocalAuthentication

class ViewController: UIViewController
{
    @IBAction func TouchBtn(_ sender: AnyObject)
    {
        let context:LAContext = LAContext()

        if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error:nil)
        {
            context.evaluatePolicy(LAPolicy.deviceOwnerAuthenticationWithBiometrics, localizedReason:"We Need Your Id", reply:{
                (wasSuccessful,Error) in
                if wasSuccessful
                {
                    print("Was a Sucess")
                }
                else
                {
                    print("Not Logged In")
                }

            })
        }

    }
}

Swift 5 :

import LocalAuthentication

func authenticateUser() {
    let context = LAContext()
    var error: NSError?

    if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) {
        let reason = "Identify yourself!"

        context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: reason) {
            [unowned self] success, authenticationError in

            DispatchQueue.main.async {
                if success {
                    print("Success!")
                } else {
                    let ac = UIAlertController(title: "Authentication failed", message: "Sorry!", preferredStyle: .alert)
                    ac.addAction(UIAlertAction(title: "OK", style: .default))
                    self.present(ac, animated: true)
                }
            }
        }
    } else {
        let ac = UIAlertController(title: "Touch ID not available", message: "Your device is not configured for Touch ID.", preferredStyle: .alert)
        ac.addAction(UIAlertAction(title: "OK", style: .default))
        present(ac, animated: true)
    }
}