Xamarin: Win a $10k Mobile Developer Rig from Xamarin at Build 2015

Wear the new Xamarin t-shirt from the Xamarin Build 2015 Kick Off Party to Build on April 29th for your chance to win one of three Mobile Developer Rigs worth over $10,000!   How to Enter Attend the Xamarin Build 2015 Kick Off Party on 4/28, where you’ll receive the newly designed Xamarin t-shirt. Wear […]

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Ruben Macias: Xamarin.iOS Apps: Adding Login/Signup Capabilities

In today’s modern mobile apps it is very common for apps to require a user account in order to function.  Apps like Twitter and Instagram require a user account in order to be used.  So a very common User Experience is to show a Login Page when the app first starts up.  The user should […]

Wally McClure

Wallace B. (Wally) McClure: Data Binding with Xamarin and iOS (iPhone & iPad)

Url: http://visualstudiomagazine.com/articles/2015/04/01/a-simple-data-binding-trick.aspx

Applications and businesses live on data. Data is what makes a business a business, generates value for customers, and allows a company to generate income. Displaying data to a user is a common operation, especially in mobile. It might be immensely important for a salesperson to get the last bit of information before visiting a client or to display the location of a restaurant via a local consumer-oriented app. In this article, I’ll look at the UITableViewController class that allows you to get at and display that data in an iOS app.

Nic Wise: Authenticating with Touch ID and the iPhone pin

One thing I really love about the new iOS devices – iPhone 5S, 6, 6+ and the iPad Air 2 – is the Touch ID sensor. I’ve always had a PIN on my devices – there is too much important information on my phone to not have one – and TouchI ID takes the pain out of it1. It’s also core to the iOS experience: Apple Pay relies on it, and it’s used by iTunes and a lot of other Apple apps.

Touch ID promo shot

But one thing I want is a nice, easy way to use the Touch ID sensor in my own apps. iOS8 introduced the Local Authentication APIs. This makes it easy – trivial – to request a Touch ID authentication.

var context = new LAContext ();
context.EvaluatePolicy(LAPolicy.DeviceOwnerAuthenticationWithBiometrics, "Do Secret Stuff", (bool success, NSError error) => {
    if (success) {
    } else {
        switch (error.Code) {
        case LAStatus.AuthenticationFailed:
        case LAStatus.UserCancel:

This is all good and easy, but it gives the user an option I don’t like: they can use Touch ID, or they can enter a password (or cancel). And if there is no Touch ID, it’ll just not work – it’s finger prints or nothing. Worse, there is no way to turn off the password option (or change the text).

Touch ID shot using LAContext

One thing I didn’t know about until recently is that you can get the same – or similar – prompt, but allow it to fall back to the devices PIN, without a password option. This means it works on any iOS8 device.

For most of my uses – validate that the person on the phone is the owner or someone the owner has trusted – this is the best option.

It’s just not that obvious how to do it.

The general idea for this – and I’m assuming it’s a bit of a hack workaround – is to put a new item into the Keychain, but set it’s ACLs to require the user to authenticate in order to get it back. The magic ACLs are

new SecAccessControl (SecAccessible.WhenPasscodeSetThisDeviceOnly, SecAccessControlCreateFlags.UserPresence)

In Objective-C land, this is kSecAttrAccessibleWhenPasscodeSetThisDeviceOnly and kSecAccessControlUserPresence. For the life of me, I can’t find reference to it in Apple’s docs, but they do have a sample and also slides from a WWDC session on this. Maybe we get the docs in iOS9.

The full call to create the Keychain item is this:

var secret = NSData.FromString (UIDevice.CurrentDevice.IdentifierForVendor.ToString(), NSStringEncoding.Unicode);

var record = new SecRecord (SecKind.GenericPassword) {
    Service = NSBundle.MainBundle.BundleIdentifier,
    Account = "SecurityViewAccount",
    AccessControl = new SecAccessControl (SecAccessible.WhenPasscodeSetThisDeviceOnly, SecAccessControlCreateFlags.UserPresence),
    UseNoAuthenticationUI = true,
    ValueData = secret

var res = SecKeyChain.Add (record);

return (res == SecStatusCode.Success);

You can update it in a similar manner if needed, as the add will fail if the item already exists. To do the actual authentication:

var query = new SecRecord (SecKind.GenericPassword) {
    Service = NSBundle.MainBundle.BundleIdentifier,
    Account = "SecurityViewAccount",
    AccessControl = new SecAccessControl (SecAccessible.WhenPasscodeSetThisDeviceOnly, SecAccessControlCreateFlags.UserPresence),
    UseOperationPrompt = "Your message goes here", 
SecStatusCode status;

var res = SecKeyChain.QueryAsData (query, false, out status);
if (res != null) {
    return NSString.FromData(res, NSStringEncoding.Unicode).ToString ();

return null;

A non-null result means they authenticated – if you need it, the result is the secret that you stored when you created the item. This shows a subtley different UI to the user:

Touch ID UI with PIN

And better yet, on iOS8 devices without Touch ID, the user is prompted for the PIN if they press “Enter Passcode”.

There is still no customisation of the dialog. No option to ONLY use Touch ID (no PIN), which would be nice. And no fallback for devices which have no PIN set at all – you’d need to make your own PIN screen for that.

I’ve put together a very basic project which shows how it works. It’s iOS8 only, tho it works on iOS7 – it just will not let you in! When I get around to rewriting Trip Wallet, I think I’ll use this as the main authentication method.

  1. The only thing I don’t like is, sometimes, when I want to hit the media controls, the phone has already logged me in. First world problem, I know. ?