Universe.swift

class A {
    /// A `Bool` indicating if we have the answer to the universe.
    private(set) var hasAnswer = false
    /// The instance of B we are using to check if we have the answer.
    private let b: B
    
    init(b: B) {
        self.b = b
    }

    /// Checks if we have the answer to the universe and updates the `hasAnswer` property.
    func checkAnswer() {
        hasAnswer = b.validateAnswer()
    }
}

class ConcreteB: B {
    /// The instance of `C` used to get the number.
    private let c: C
    
    init(c: C) {
        self.c = c
    }
    
    /// Validates if `C` has the answer.
    func validateAnswer() -> Bool {
        c.number == 42
    }
}

class ConcreteC: C {
    /// The shared singleton instance of `ConcreteC`.
    static let shared = ConcreteC()
    
    /// The answer to the universe.
    var number: Int { 42 }
}