KeychainAccess+PropertyWrapper.swift

import Foundation
import KeychainAccess

@propertyWrapper
struct KeychainStorage<Value: Codable> {
    
    let key: String
    let service: String
    let initialValue: Value
    
    private let keychain: KeychainAccess.Keychain
    private let encoder = JSONEncoder()
    private let decoder = JSONDecoder()

    init(wrappedValue initialValue: Value, _ key: String, service: String? = nil) {
        self.initialValue = initialValue
        self.key = key
        self.service = service ?? Bundle.main.bundleIdentifier ?? "com.kishikawakatsumi.KeychainAccess"
        self.keychain = KeychainAccess.Keychain(service: self.service)
    }

    var wrappedValue: Value {
        get {
            guard let data = try? keychain.getData(key),
                let value = try? decoder.decode(Value.self, from: data) else {
                return initialValue
            }
            return value
        }
        set {
            guard let newData = try? encoder.encode(newValue) else {
                return
            }
            try? keychain.set(newData, key: key)
        }
    }
}

Adapted to optional data:

@propertyWrapper
struct KeychainStorage<Value: Codable> {
	
	let key: String
	
	private let keychain: Keychain
	private let encoder = JSONEncoder()
	private let decoder = JSONDecoder()
	
	
	init(wrappedValue initialValue: Value? = nil, _ key: String, service: String? = nil) {
		self.key = key
		self.keychain = Keychain(service: service)
	}
	
	var wrappedValue: Value? {
		get {
			do {
				guard let data = try? keychain.getData(key) else {
					return nil
				}

				let value = try decoder.decode(Value.self, from: data)
				
				return value
			} catch {
				print(error)
			}
			
			return nil
		}
		
		set {
			guard let newData = try? encoder.encode(newValue) else {
				return
			}
			do {
				try keychain.set(newData, key: key)
			} catch {
				print(error)
			}
		}
	}
}