When working with String enums that have a finite number of cases, it’s crucial to include an unknown case as a catch-all for potential future values. This practice helps future-proof our apps against updates or changes on the backend that may introduce new enum values.
To achieve this, we provide a custom initializer for the enum. This initializer tries to decode the received value and maps it to the corresponding enum case. If it encounters a value not defined in the enum, it defaults to the unknown case.
Here’s an example illustrating this approach:
enum UserStatus: String {
case pendingAuthorization, verified, unknown
init(from decoder: Decoder) throws {
guard let rawValue = try? decoder.singleValueContainer().decode(String.self) else {
Logger.log(.err, "Can't decode a String value from \(Self.self).")
self = .unknown
return
}
if let knownValue = UserStatus(rawValue: rawValue) {
self = knownValue
} else {
Logger.log(.err, "Unknown value (\(rawValue)) for \(Self.self).")
self = .unknown
}
}
}
extension UserStatus: Decodable {}
In this example, if the backend sends an unfamiliar value like userStatus = "blocked", the app won’t crash due to a failed decoding. Instead, the UserStatus for this particular instance will be set to unknown, allowing the app to continue operating normally.
Testing
You could add a simple unit test to make sure that the decoding will work correctly:
func test_userStatus_decoding() {
// Given
let decoder = JSONDecoder()
let dict: [String: UserStatus] = [
#" "pendingAuthorization" "#: .pendingAuthorization,
#" "verified" "#: .verified,
#" "blocked" "#: .unknown,
#" "manu" "#: .unknown
]
dict.forEach { json, expectedValue in
// When
let actualValue = try? decoder.decode(UserStatus.self, from: json.data(using: .utf8)!)
// Then
XCTAssertEqual(actualValue, expectedValue)
}
}