What is Combine and how does it relate to networking?

Combine is Apple's reactive framework for processing asynchronous events over time.
TSCellSeal(key:) uses the bytes you supply directly as the symmetric key, while TSCellSeal(passphrase:) runs a human-typed string through a deliberately slow key derivation function first.
An iOS app moves through states managed by UIApplicationDelegate (UIKit) or @main + scene lifecycle (SwiftUI/iOS 13+).
A race condition occurs when two or more threads access shared mutable state simultaneously, producing unpredictable results depending on timing.

Answer: Combine is Apple's reactive framework for processing asynchronous events over time. URLSession has a built-in Combine publisher, making it natural to chain networking, decoding, and UI updates.

Code Example:

import Combine

class ArticleService {
    private var cancellables = Set<AnyCancellable>()

    func fetchArticles() -> AnyPublisher<[Article], Error> {
        let url = URL(string: "https://api.example.com/articles")!

        return URLSession.shared.dataTaskPublisher(for: url)
            .map(\.data)
            .decode(type: [Article].self, decoder: JSONDecoder())
            .receive(on: DispatchQueue.main)
            .eraseToAnyPublisher()
    }
}

// In ViewModel
service.fetchArticles()
    .sink(
        receiveCompletion: { completion in
            if case .failure(let error) = completion {
                print("Error: \(error)")
            }
        },
        receiveValue: { [weak self] articles in
            self?.articles = articles
        }
    )
    .store(in: &cancellables)

Note: async/await is now preferred for new code, but Combine is still widely used and worth knowing.