What is `withCheckedContinuation` and why is it needed?

Answer: withCheckedContinuation bridges old callback-based APIs into the async/await world. Use it to wrap any completion-handler function so it can be awaited.

Code Example:

// Old callback-based API
func fetchData(completion: @escaping (Data?, Error?) -> Void) { ... }

// Wrap it for async/await use
func fetchData() async throws -> Data {
    try await withCheckedThrowingContinuation { continuation in
        fetchData { data, error in
            if let error = error {
                continuation.resume(throwing: error)
            } else if let data = data {
                continuation.resume(returning: data)
            }
        }
    }
}

// Now you can await it
let data = try await fetchData()

Key Rules:

• continuation.resume() must be called exactly once — never zero, never twice
• Use withCheckedThrowingContinuation when the operation can fail
• withUnsafeContinuation skips the "called once" check — only use if performance is critical