A simple example of reactive programming

Observable.swift

// Automatically responds to changes in `value` property
class Observable<Value, Return> {
    
    // Contents of observable
    var value: Value
    
    // Run whenever value changes
    let onChange: Value -> Return
    
    init(withInitial value: Value, onChange change: Value -> Return) {
        self.value = value
        self.onChange = change
    }
    
    // Returns current value
    func getValue() -> Value {
        return self.value;
    }
    
    // Updates value
    func update(value: Value) {
        self.value = value
        self.onChange(self.value)
    }
    
    // Applies transform to value
    func flatmap(transform: Value -> Value) {
        self.update(transform(self.value));
    }
    
    
}

// Example 1: Logging integers

func logInt(i: Int) {
    print("logging int: ", i)
}

let intObserver = Observable(withInitial: 0, onChange: logInt)

for i in 1...10 {
    intObserver.flatmap { i in i + 1 };
}

// Example 2: Saving user automatically on update

class User {
    let id: Int
    let username: String
    var password: String
    
    init(id: Int, username: String, password: String) {
        self.id = id
        self.username = username
        self.password = password
    }
}

func saveUser(user: User) {
    print("Saving ", user.username);
}

let user = User(id: 1, username: "wdavenport", password: "testing")
let userObserver = Observable(withInitial: user, onChange: saveUser)

// Change password
userObserver.flatmap { user in
    user.password = "newPassword"
    return user
}