AsyncThing.swift

import Foundation

enum StoredObject: Codable {
    case all(groups: [InterestGroup]?)
    case selected(group: InterestGroup?)
    case events(groupID: UUID, events: [Event]?)
    
    func url(root url: URL) -> URL {
        switch self {
        case .all:
            return url.appendingPathComponent("InteresteGroups.json")
        case .selected:
            return url.appendingPathComponent("SelectedInterestGroup.json")
        case .events(let uuid, _):
            return url.appendingPathComponent("Events-\(uuid.uuidString).json")
        }
    }
}

protocol CoffeeStore {
    associatedtype StoredType
    func save(_ stored: StoredObject) async throws
    func load(_ stored: StoredObject) async throws -> StoredObject
}

struct AsyncFileCoffeeStore<StoredType: Codable>: CoffeeStore {
    private let rootDirectoryURL: URL

    init(_ url: URL) {
        self.rootDirectoryURL = url
    }

    private func save<T: Codable>(_ object: T, at url: URL) async throws {
        Task(priority: .userInitiated) {
            try JSONEncoder().encode(object).write(to: url)
        }
    }
    
    private func load<T: Decodable>(_ type: T.Type, at url: URL) async throws -> T {
        return try await Task(priority: .userInitiated) {
            let data = try Data(contentsOf: url)
            return try JSONDecoder().decode(type, from: data)
        }
        .value
    }
    
    func load(_ stored: StoredObject) async throws -> StoredObject {
        switch stored {
        case .all:
            async let groups = load([InterestGroup].self,
                                    at: stored.url(root: rootDirectoryURL))
            return try await .all(groups: groups)
        case .selected:
            async let selected = load(InterestGroup.self,
                                      at: stored.url(root: rootDirectoryURL))
            return try await .selected(group: selected)
        case .events(let uuid, _):
            async let loadedEvents = load([Event].self,
                                          at: stored.url(root: rootDirectoryURL))
            return try await .events(groupID: uuid, events: loadedEvents)
        }
    }
    
    func save(_ stored: StoredObject) async throws {
        switch stored {
        case .all(groups: let groups):
            try await save(groups, at: stored.url(root: rootDirectoryURL))
        case .selected(group: let group):
            try await save(group, at: stored.url(root: rootDirectoryURL))
        case .events(_, events: let events):
            try await save(events, at: stored.url(root: rootDirectoryURL))
        }
    }
}