Store User Token, If Invalid token - present Login page, - ios

Question

Learning iOS Swift programming and like to know how to implement user login process ?
The backend-iOS mechanism is this :
User login with email and Password,
The Server returns user token and user id
In subsequent requests, user token and user id is sent to fetch data/work with the App.
I have doubt in iOS implementation.
Will be storing User token and User id in Core Data. Will there be
any slowness if I get the user token on every screen from Core
Data?
If the login token expires or is invalid on any screen, how to fall back to login page? Should I check the JSON output and have code "present login VC" on every screen? Any streamlined way to have abstract the code to a swift or cocoa touch file?

Answer 1

Actually, there are many approaches. It's all depends on you, how you will manage it. I can point you two examples, how I manage it by myself.
Using NSOperation.
There was an awesome session on WWDC 2015, about advanced NSOperations. Here it is.
Basically, you create a subclass of NSOperaton and make other operations depend on it. In your case, you will have operation of user login, and all other operations will depend on user login (of course only ones, who needs it). If it succeed, then operation will execute. In user login operation, you will check, if user already logged in, and you have a token, if not, present logging screen.
There is also awesome library called Operations, based on that WWDC talk. Here it is.
Using PromiseKit.
Here is another one, using PromiseKit. It is not really much difference from operations, but, in my opinion, a little simpler. You create Promise that ensures that you did login. It is very simple to make a chain of promises, so you promise a user login and chain anything else from it. If login succeed, chain of promises continues executing.
It's all based on another awesome library, PromiseKit. Here it is
It is very powerful and very simple to use, once you understand the thing. It is very well documented, and has a bunch of tutorials here.
There are many other approaches, so you can choose any of it, and combine one with other however you like.
Asking on your first question, you make it asynchronous, so it is not so much matter about slowness of CoreData as you make a web request.

Answer 2

1 - well, yes, a bit. But I doubt you will notice any lag or anything: CoreData is really fast and it won't take any significant amount of time to fetch just one object. Alternatively, your CoreData object that holds this data (let's call it User) can be a property of your subclass of UIApplicationDelegate (let's call it MyAppDelegate). You will fetch this User in
- application:didFinishLaunchingWithOptions:, and update it on login/logout/expire/etc. In such way you can access it from everywhere, and you don't need to fetch it from CoreData anytime you need it.
Also, instead of CoreData you can use iOS Keychain. (Here is tutorial that might help with this).
2 - again, you can use MyAppDelegate. For example you can add method to it, that will save current navigation state, and change root controller of UIWindow to your LoginViewController. Or it will present LoginViewController on top of current controller. After successful login you will return navigation stack to previous state.
Also you can move this code to some kind of NavigationController - class that will handle this situation. It will have something like
func checkToken(Dictionary response, UIViewController currentController) -> Bool. If token is valid, it just returns true, and if not, it returns false and handles navigation to LoginViewController, and after login - back to currentController. This NavigationController can be property of MyAppDelegate, it can be singleton, created every time you need it, or you can pass it along to every UIViewController that you show.