What are the SOLID principles, and how do they apply in Swift?

A Singleton ensures only one instance of a class exists globally.
SOLID is a set of five design guidelines for maintainable OOP code: | Principle | Idea | Swift application | |--|--|--| | Single Responsibility | A type should have one reason to change | Split a…
A retain cycle occurs when two objects hold strong references to each other, preventing ARC from deallocating either.
Yes — passing an observed property as inout always calls both willSet and didSet, even if the function never modifies the value.

Answer: SOLID is a set of five design guidelines for maintainable OOP code:

Principle	Idea	Swift application
Single Responsibility	A type should have one reason to change	Split a "God ViewController" into a ViewModel + Service
Open/Closed	Open for extension, closed for modification	Add behavior via protocol conformance/extensions, not by editing existing types
Liskov Substitution	Subtypes must be usable wherever their base type is expected	A `Square: Rectangle` shouldn't break callers that resize rectangles independently
Interface Segregation	Prefer many small protocols over one large one	`Codable` is `Encodable & Decodable` — adopt only what you need
Dependency Inversion	Depend on abstractions, not concrete types	Inject a `NetworkClientProtocol`, not a concrete `URLSessionClient`

// Interface Segregation + Dependency Inversion together
protocol UserFetching {
    func fetchUser(id: String) async throws -> User
}

final class ProfileViewModel {
    private let fetcher: UserFetching   // depends on an abstraction

    init(fetcher: UserFetching) {
        self.fetcher = fetcher
    }
}