Chris Eidhof's KeychainItem property wrapper made generic

KeychainItem.swift

//
//  KeychainItem.swift
//
//  Created by David Whetstone on 9/28/19.
//

// Original code by Chris Eidhof
// https://github.com/objcio/keychain-item
// As yet unlicensed

import Foundation
import Security

@propertyWrapper
final public class KeychainItem<T: Codable> {
    private let account: String

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

    private var baseDictionary: [String:AnyObject] {
        return [
            kSecClass as String: kSecClassGenericPassword,
            kSecAttrAccount as String: account as AnyObject
        ]
    }

    private var query: [String:AnyObject] {
        return baseDictionary.adding(key: kSecMatchLimit as String, value: kSecMatchLimitOne)
    }

    public var wrappedValue: T? {
        get {
            try! read()
        }
        set {
            if let v = newValue {
                if try! read() == nil {
                    try! add(v)
                } else {
                    try! update(v)
                }
            } else {
                try! delete()
            }
        }
    }

    private func delete() throws {
        // SecItemDelete seems to fail with errSecItemNotFound if the item does not exist in the keychain. Is this expected behavior?
        let status = SecItemDelete(baseDictionary as CFDictionary)
        guard status != errSecItemNotFound else { return }
        try throwIfNotZero(status)
    }

    private func read() throws -> T? {
        let query = self.query.adding(key: kSecReturnData as String, value: true as AnyObject)
        var result: AnyObject? = nil
        let status = SecItemCopyMatching(query as CFDictionary, &result)
        guard status != errSecItemNotFound else { return nil }
        try throwIfNotZero(status)
        guard let data = result as? Data else { throw KeychainError.invalidData }
        return try decode(data)
    }

    private func update(_ secret: T) throws {
        let dictionary: [String:AnyObject] = [
            kSecValueData as String: try encode(secret) as AnyObject
        ]
        try throwIfNotZero(SecItemUpdate(baseDictionary as CFDictionary, dictionary as CFDictionary))
    }

    private func add(_ secret: T) throws {
        let dictionary = baseDictionary.adding(key: kSecValueData as String, value: try encode(secret) as AnyObject)
        try throwIfNotZero(SecItemAdd(dictionary as CFDictionary, nil))
    }

    private func encode(_ secret: T) throws -> Data { try JSONEncoder().encode(secret) }
    private func decode(_ data: Data) throws -> T { try JSONDecoder().decode(T.self, from: data) }
}

private func throwIfNotZero(_ status: OSStatus) throws {
    guard status != 0 else { return }
    throw KeychainError.keychainError(status: status)
}


public enum KeychainError: Error {
    case invalidData
    case keychainError(status: OSStatus)
}

extension Dictionary {
    func adding(key: Key, value: Value) -> Dictionary {
        var copy = self
        copy[key] = value
        return copy
    }
}