AstroPhotoFetcher+Finished.swift

import Foundation
import libspacekit

public struct AstroPhoto: Equatable {
    public let title: String
    public let url: URL
}

private struct AstroPhotoContext {
    let fetcher: AstroPhotoFetcher
    let completion: AstroPhotoFetcher.PhotoCompletion
}

/// Utility for fetching the NASA astronomy photo of the day
public final class AstroPhotoFetcher {
    public typealias PhotoCompletion = (Result<AstroPhoto, PhotoError>) -> Void

    public enum PhotoError: Error {
        case noPhotoFound
        case invalidPhotoURLs
    }

    private let nasaAPIKey: String

    public init(nasaAPIKey: String) {
        self.nasaAPIKey = nasaAPIKey
    }

    public func fetchPhoto(with completion: @escaping PhotoCompletion) {
        let context = AstroPhotoContext(fetcher: self, completion: completion)
        let contextPointer = UnsafeMutablePointer<AstroPhotoContext>.allocate(capacity: 1)
        contextPointer.initialize(to: context)

        nasaAPIKey.withCString { apiKeyPointer in
            fetch_photo_of_the_day(
                apiKeyPointer,
                photoCallback(info:rawContext:),
                UnsafeMutableRawPointer(contextPointer)
            )
        }
    }

    // MARK: - Completion handlers

    fileprivate func handleDidReceivePhoto(_ info: PhotoInfo?, completion: PhotoCompletion) {
        guard let info = info else {
            return completion(.failure(.noPhotoFound))
        }

        let title = String(cString: info.title)
        let urlString = String(cString: info.url)
        guard let url = URL(string: urlString) else {
            return completion(.failure(.invalidPhotoURLs))
        }

        let photo = AstroPhoto(title: title, url: url)
        completion(.success(photo))
    }
}

// MARK: - Callback

private func photoCallback(info: UnsafeMutablePointer<PhotoInfo>?, rawContext: UnsafeMutableRawPointer?) {
    guard let rawContext = rawContext else { return }

    let contextPointer = rawContext.assumingMemoryBound(to: AstroPhotoContext.self)
    let context = contextPointer.pointee

    defer {
        contextPointer.deinitialize(count: 1)
        contextPointer.deallocate()
    }

    context.fetcher.handleDidReceivePhoto(info?.pointee, completion: context.completion)
}

AstroPhotoFetcher+Template.swift

import Foundation
import libspacekit

public struct AstroPhoto: Equatable {
    public let title: String
    public let url: URL
}

/// Utility for fetching the NASA astronomy photo of the day
public final class AstroPhotoFetcher {
    public typealias PhotoCompletion = (Result<AstroPhoto, PhotoError>) -> Void

    public enum PhotoError: Error {
        case noPhotoFound
        case invalidPhotoURLs
    }

    private let nasaAPIKey: String

    public init(nasaAPIKey: String) {
        self.nasaAPIKey = nasaAPIKey
    }

    public func fetchPhoto(with completion: @escaping PhotoCompletion) {
        // Todo: implement
    }

    // MARK: - Completion handlers

    fileprivate func handleDidReceivePhoto(_ info: PhotoInfo?, completion: PhotoCompletion) {
        guard let info = info else {
            return completion(.failure(.noPhotoFound))
        }

        let title = String(cString: info.title)
        let urlString = String(cString: info.url)
        guard let url = URL(string: urlString) else {
            return completion(.failure(.invalidPhotoURLs))
        }

        let photo = AstroPhoto(title: title, url: url)
        completion(.success(photo))
    }
}