createDiskStore.swift

import Foundation.NSFileManager

func createDiskStore(
    pathInDocs   p: String      = "state.json",
    fileManager fm: FileManager = .default
) -> Store
{
    var state = loadAppStateFromStore(pathInDocuments: p, fileManager: fm) { didSet { callbacks.forEach { $0() } } }
    var callbacks: [() -> ()] = []
    return (
        state   : { state },
        change  : { state     = change(state, $0); persistStore(pathInDocuments: p, state: state, fileManager: fm) },
        reset   : { state     = AppState()       ; persistStore(pathInDocuments: p, state: state, fileManager: fm) },
        updated : { callbacks = callbacks + [$0] },
        destroy : { destroyStore(pathInDocuments: p, fileManager: fm) }
    )
}

//MARK: -
private func persistStore(pathInDocuments: String, state: AppState, fileManager: FileManager ) {
    do {
        let encoder = JSONEncoder()
        #if DEBUG
        encoder.outputFormatting = .prettyPrinted
        #endif
        let data = try encoder.encode(state)
        try data.write(to: fileURL(pathInDocuments: pathInDocuments, fileManager: fileManager))
    } catch { print(error) }
}

private func loadAppStateFromStore(pathInDocuments: String, fileManager: FileManager) -> AppState {
    do {
        return try JSONDecoder().decode(AppState.self, from: try Data(contentsOf: fileURL(pathInDocuments: pathInDocuments, fileManager: fileManager)))
    } catch {
        print(error)
        return AppState()
    }
}

private func destroyStore(pathInDocuments: String, fileManager: FileManager) {
    try? fileManager.removeItem(at: try! fileURL(pathInDocuments: pathInDocuments))
}

private func fileURL(pathInDocuments: String, fileManager: FileManager = .default) throws -> URL {
    try fileManager
        .url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: true)
        .appendingPathComponent(pathInDocuments)
}