Xamarin.iOS Touch ID Add Touch ID to your App

Example

First, establish if the device is capable of accepting Touch ID input.

if (context.CanEvaluatePolicy(LAPolicy.DeviceOwnerAuthenticationWithBiometrics, out AuthError))

If it does then we can display the Touch ID UI by using:

context.EvaluatePolicy(LAPolicy.DeviceOwnerAuthenticationWithBiometrics, myReason, replyHandler);

There are three pieces of information we have to pass into EvaluatePolicy – the policy itself, a string explaining why authentication is necessary, and a reply handler. The reply handler tells the application what it should do in the case of a successful, or unsuccessful, authentication.

One of the caveats of Local Authentication is that it must be run on the foreground, so make sure to use InvokeOnMainThread for the reply handler:

var replyHandler = new LAContextReplyHandler((success, error) =>
{
    this.InvokeOnMainThread(() =>
    {
        if (success)
        {
            Console.WriteLine("You logged in!");
            PerformSegue("AuthenticationSegue", this);
        }
        else {
            //Show fallback mechanism here
        }
    });
});

To determine whether the database of authorized fingerprints has been modified you can check the opaque structure (NSData) returned by context.EvaluatedPolicyDomainState. Your app will need to store and compare the policy state to detect changes. One thing to note which Apple states:

However, the nature of the change cannot be determined from this data.

if (context.CanEvaluatePolicy(LAPolicy.DeviceOwnerAuthenticationWithBiometrics, out AuthError))
{
    var policyState = context.EvaluatedPolicyDomainState;

    var replyHandler = new LAContextReplyHandler((success, error) =>
    {

        this.InvokeOnMainThread(() =>
        {
            if (success)
            {
                Console.WriteLine("You logged in!");
                PerformSegue("AuthenticationSegue", this);
            }
            else {
                //Show fallback mechanism here
            }
        });

    });
    context.EvaluatePolicy(LAPolicy.DeviceOwnerAuthenticationWithBiometrics, myReason, replyHandler);
};

Button Example

partial void AuthenticateMe(UIButton sender)
{
    var context = new LAContext();
    //Describes an authentication context 
    //that allows apps to request user authentication using Touch ID.
    NSError AuthError;
    //create the reference for error should it occur during the authentication.
    var myReason = new NSString("To add a new chore");
    //this is the string displayed at the window for touch id

    if (context.CanEvaluatePolicy(LAPolicy.DeviceOwnerAuthenticationWithBiometrics, out AuthError)) 
    // check if the device have touchId capabilities.
    {
        var replyHandler = new LAContextReplyHandler((success, error) =>
        {

            this.InvokeOnMainThread(() =>
            {
                if (success)
                {
                    Console.WriteLine("You logged in!");
                    PerformSegue("AuthenticationSegue", this);
                }
                else {
                    //Show fallback mechanism here
                }
            });

        });
        context.EvaluatePolicy(LAPolicy.DeviceOwnerAuthenticationWithBiometrics, myReason, replyHandler);//send touch id request
    };
}