Maintaining State in Your Apps

Effectively managing state, the bits of data that keep track of how the app is being used at the moment, is an important part of a developing your app. Because enumerations define a finite number of states, and can bundle associated values with each individual state, you can use them to model the state of your app and its internal processes.

Consider an app that requires a user to log into an account. When the app is first opened, the user is unknown, so the state of the app could be called unregistered. After the user has registered or logged into an account, the state is logged in. After some time of inactivity, the user’s session may expire, leaving the app in a session expired state.

You can use an enumeration to specify the exact states needed for your app. This approach defines an App class with a nested State enumeration that includes only the specific states you need:

class App {
    enum State {
        case unregistered
        case loggedIn(User)
        case sessionExpired(User)
    }
  
    var state: State = .unregistered
  
    // ...
}

In this model, each state is represented by a case with a matching name. The loggedIn and sessionExpired cases include the user as an associated value, while the unregistered case doesn't include an associated value. When you update your app's state, there's a single variable, state, to modify, no matter what the transition.

It's also possible to model an app’s state by using individual variables in combination to hold the state and the required data, which is not recommended.

In this model, the app defines two variables: an optional user that stores user information, and a Boolean value named sessionExpired. The user variable is nil when the user not logged in and has a value once the user logs in. The sessionExpired variable begins as false and is set to true if the session expires. The three states are captured by different combinations of the two variables.

Using this approach is prone to mistakes for a few reasons, in ways that can lead to bugs and make it harder to reason about your code:

• For every change in state, you need to provides updates for both user and sessionExpired in tandem.

• If a future change to the app requires an additional state, you need to update an additional variable at every existing change in state.

• The two variables have an unused combination—it’s possible to set the user to nil and sessionExpired to true, even though that doesn’t have a corresponding state.