Defaults/migration.md

# Migration guide from v4 to v5

**Warning: Test the migration thoroughly in your app. It might cause unintended data loss if you're not careful.**

## Summary

We have improved the stored representation of some types. Some types will require migration. Previously, all `Codable` types were serialized to a JSON string and stored as a `UserDefaults` string. `Defaults` is now able to store more types using the appropriate native `UserDefaults` type.

- Primitive types (`Int`, `Double`, `Bool`, `String`, etc) require no changes.
- Custom types (`struct`, `enum`, etc.) must now conform to `Defaults.Serializable` (in addition to `Codable`).
- `Array`, `Set`, and `Dictionary` will need to be manually migrated with `Defaults.migrate()`.

---

In v4, `Defaults` stored `Codable` types as a JSON string.\
In v5, `Defaults` stores many `Codable` types as native `UserDefaults` types.

```swift
// v4
let key = Defaults.Key<[String: Int]>("key", default: ["0": 0])

UserDefaults.standard.string(forKey: "key")
//=> "[\"0\": 0]"
```

```swift
// v5
let key = Defaults.Key<[String: Int]>("key", default: ["0": 0])

UserDefaults.standard.dictionary(forKey: "key")
//=> [0: 0]
```

## Issues

1. **The compiler complains that `Defaults.Key<Value>` does not conform to `Defaults.Serializable`.**
	Since we replaced `Codable` with `Defaults.Serializable`, `Key<Value>` will have to conform to `Value: Defaults.Serializable`.
	For this situation, please follow the guides below:
	- [From `Codable` struct in Defaults v4 to `Codable` struct in Defaults v5](#from-codable-struct-in-defaults-v4-to-codable-struct-in-defaults-v5)
	- [From `Codable` enum in Defaults v4 to `Codable` enum in Defaults v5](#from-codable-enum-in-defaults-v4-to-codable-enum-in-defaults-v5)

2. **The previous value in `UserDefaults` is not readable. (for example, `Defaults[.array]` returns `nil`).**
	In v5, `Defaults` reads value from `UserDefaults` as a natively supported type, but since `UserDefaults` only contains JSON string before migration for `Codable` types, `Defaults` will not be able to work with it. For this situation, `Defaults` provides the `Defaults.migrate()` method to automate the migration process.
	- [From `Codable` `Array/Dictionary/Set` in Defaults v4 to native `Array/Dictionary/Set`(with natively supported elements) in Defaults v5](#from-codable-arraydictionaryset-in-defaults-v4-to-native-arraydictionarysetwith-native-supported-elements-in-defaults-v5)
	- [From `Codable` `Array/Dictionary/Set` in Defaults v4 to native `Array/Dictionary/Set` (with codable elements) in Defaults v5](#from-codable-arraydictionaryset-in-defaults-v4-to-native-arraydictionarysetwith-codable-elements-in-defaults-v5)

## Testing

We recommend doing some manual testing after migrating.

For example, let's say you are trying to migrate an array of `Codable` string to a native array.

1. Get the previous value in `UserDefaults` (using `defaults` command or whatever you want).

```swift
let string = "[\"a\",\"b\",\"c\"]"
```

2. Insert the above value into `UserDefaults`.

```swift
UserDefaults.standard.set(string, forKey: "testKey")
```

3. Call `Defaults.migrate()` and then use `Defaults` to get its value.

```swift
let key = Defaults.Key<[String]>("testKey", default: [])
Defaults.migrate(key, to: .v5)

Defaults[key] //=> [a, b, c]
```

## Migrations

### From `Codable` struct in Defaults v4 to `Codable` struct in Defaults v5

In v4, `struct` had to conform to `Codable` to store it as a JSON string.

In v5, `struct` has to conform to `Codable` and `Defaults.Serializable` to store it as a JSON string.

#### Before migration

```swift
private struct TimeZone: Codable {
	var id: String
	var name: String
}

extension Defaults.Keys {
	static let timezone = Defaults.Key<TimeZone?>("TimeZone")
}
```

#### Migration steps

1. Make `TimeZone` conform to `Defaults.Serializable`.

```swift
private struct TimeZone: Codable, Defaults.Serializable {
	var id: String
	var name: String
}
```

2. Now `Defaults[.timezone]` should be readable.

### From `Codable` enum in Defaults v4 to `Codable` enum in Defaults v5

In v4, `enum` had to conform to `Codable` to store it as a JSON string.

In v5, `enum` has to conform to `Codable` and `Defaults.Serializable` to store it as a JSON string.

#### Before migration

```swift
private enum Period: String, Codable {
	case tenMinutes = "10 Minutes"
	case halfHour = "30 Minutes"
	case oneHour = "1 Hour"
}

extension Defaults.Keys {
	static let period = Defaults.Key<Period?>("period")
}
```

#### Migration steps

1. Make `Period` conform to `Defaults.Serializable`.

```swift
private enum Period: String, Defaults.Serializable, Codable {
	case tenMinutes = "10 Minutes"
	case halfHour = "30 Minutes"
	case oneHour = "1 Hour"
}
```

2. Now `Defaults[.period]` should be readable.

### From `Codable` `Array/Dictionary/Set` in Defaults v4 to native `Array/Dictionary/Set` (with natively supported elements) in Defaults v5

In v4, `Defaults` stored array/dictionary as a JSON string: `["a", "b", "c"]`.

In v5, `Defaults` stores it as a native array/dictionary with natively supported elements: `[a, b, c]`.

#### Before migration

```swift
extension Defaults.Keys {
	static let arrayString = Defaults.Key<[String]?>("arrayString")
	static let setString = Defaults.Key<Set<String>?>("setString")
	static let dictionaryStringInt = Defaults.Key<[String: Int]?>("dictionaryStringInt")
	static let dictionaryStringIntInArray = Defaults.Key<[[String: Int]]?>("dictionaryStringIntInArray")
}
```

#### Migration steps

1. **Call `Defaults.migrate(.arrayString, to: .v5)`, `Defaults.migrate(.setString, to: .v5)`, `Defaults.migrate(.dictionaryStringInt, to: .v5)`, `Defaults.migrate(.dictionaryStringIntInArray, to: .v5)`.**
2. Now `Defaults[.arrayString]`, `Defaults.[.setString]`, `Defaults[.dictionaryStringInt]`, `Defaults[.dictionaryStringIntInArray]` should be readable.

### From `Codable` `Array/Dictionary/Set` in Defaults v4 to native `Array/Dictionary/Set` (with `Codable` elements) in Defaults v5

In v4, `Defaults` would store array/dictionary as a single JSON string: `"{ "id": "0", "name": "Asia/Taipei" }"`, `"["10 Minutes", "30 Minutes"]"`.

In v5, `Defaults` will store it as a native array/dictionary with `Codable` elements: `{ id: 0, name: Asia/Taipei }`, `[10 Minutes, 30 Minutes]`.

#### Before migration

```swift
private struct TimeZone: Hashable, Codable {
	var id: String
	var name: String
}

private enum Period: String, Hashable, Codable {
	case tenMinutes = "10 Minutes"
	case halfHour = "30 Minutes"
	case oneHour = "1 Hour"
}

extension Defaults.Keys {
	static let arrayTimezone = Defaults.Key<[TimeZone]?>("arrayTimezone")
	static let setTimezone = Defaults.Key<[TimeZone]?>("setTimezone")
	static let arrayPeriod = Defaults.Key<[Period]?>("arrayPeriod")
	static let setPeriod = Defaults.Key<[Period]?>("setPeriod")
	static let dictionaryTimezone = Defaults.Key<[String: TimeZone]?>("dictionaryTimezone")
	static let dictionaryPeriod = Defaults.Key<[String: Period]?>("dictionaryPeriod")
}
```

#### Migration steps

1. Make `TimeZone` and `Period` conform to `Defaults.Serializable`.

```swift
private struct TimeZone: Hashable, Codable, Defaults.Serializable {
	var id: String
	var name: String
}

private enum Period: String, Hashable, Codable, Defaults.Serializable {
	case tenMinutes = "10 Minutes"
	case halfHour = "30 Minutes"
	case oneHour = "1 Hour"
}
```

2. **Call `Defaults.migrate(.arrayTimezone, to: .v5)`, `Defaults.migrate(.setTimezone, to: .v5)`, `Defaults.migrate(.dictionaryTimezone, to: .v5)`, `Defaults.migrate(.arrayPeriod, to: .v5)`, `Defaults.migrate(.setPeriod, to: .v5)` , `Defaults.migrate(.dictionaryPeriod, to: .v5)`.**
3. Now `Defaults[.arrayTimezone]`, `Defaults[.setTimezone]`, `Defaults[.dictionaryTimezone]`, `Defaults[.arrayPeriod]`, `Defaults[.setPeriod]` , `Defaults[.dictionaryPeriod]` should be readable.

---

## Optional migrations

### From `Codable` enum in Defaults v4 to `RawRepresentable` enum in Defaults v5 *(Optional)*

In v4, `Defaults` will store `enum` as a JSON string: `"10 Minutes"`.

In v5, `Defaults` can store `enum` as a native string: `10 Minutes`.

#### Before migration

```swift
private enum Period: String, Codable {
	case tenMinutes = "10 Minutes"
	case halfHour = "30 Minutes"
	case oneHour = "1 Hour"
}

extension Defaults.Keys {
	static let period = Defaults.Key<Period?>("period")
}
```

#### Migration steps

1. Create another enum called `CodablePeriod` and create an extension of it. Make the extension conform to `Defaults.CodableType` and its associated type `NativeForm` to `Period`.

```swift
private enum CodablePeriod: String {
	case tenMinutes = "10 Minutes"
	case halfHour = "30 Minutes"
	case oneHour = "1 Hour"
}

extension CodablePeriod: Defaults.CodableType {
	typealias NativeForm = Period
}
```

2. Remove `Codable` conformance so `Period` can be stored natively.

```swift
private enum Period: String {
	case tenMinutes = "10 Minutes"
	case halfHour = "30 Minutes"
	case oneHour = "1 Hour"
}
```

3. Create an extension of `Period` that conforms to `Defaults.NativeType`. Its `CodableForm` should be `CodablePeriod`.

```swift
extension Period: Defaults.NativeType {
	typealias CodableForm = CodablePeriod
}
```

4. **Call `Defaults.migrate(.period)`**
5. Now `Defaults[.period]` should be readable.

You can also instead implement the `toNative` function in `Defaults.CodableType` for flexibility:

```swift
extension CodablePeriod: Defaults.CodableType {
	typealias NativeForm = Period

	public func toNative() -> Period {
		switch self {
		case .tenMinutes:
			return .tenMinutes
		case .halfHour:
			return .halfHour
		case .oneHour:
			return .oneHour
		}
	}
}
```

### From `Codable` struct in Defaults v4 to `Dictionary` in Defaults v5 *(Optional)*

This happens when you have a struct which is stored as a `Codable` JSON string before, but now you want it to be stored as a native `UserDefaults` dictionary.

#### Before migration

```swift
private struct TimeZone: Codable {
	var id: String
	var name: String
}

extension Defaults.Keys {
	static let timezone = Defaults.Key<TimeZone?>("TimeZone")
	static let arrayTimezone = Defaults.Key<[TimeZone]?>("arrayTimezone")
	static let setTimezone = Defaults.Key<Set<TimeZone>?>("setTimezone")
	static let dictionaryTimezone = Defaults.Key<[String: TimeZone]?>("setTimezone")
}
```

#### Migration steps

1. Create a `TimeZoneBridge` which conforms to `Defaults.Bridge` and its `Value` is `TimeZone` and `Serializable` is `[String: String]`.

```swift
private struct TimeZoneBridge: Defaults.Bridge {
	typealias Value = TimeZone
	typealias Serializable = [String: String]

	func serialize(_ value: TimeZone?) -> Serializable? {
		guard let value = value else {
			return nil
		}

		return [
			"id": value.id,
			"name": value.name
		]
	}

	func deserialize(_ object: Serializable?) -> TimeZone? {
		guard
			let dictionary = object,
			let id = dictionary["id"],
			let name = dictionary["name"]
		else {
			return nil
		}

		return TimeZone(
			id: id,
			name: name
		)
	}
}
```

2. Create an extension of `TimeZone` that conforms to `Defaults.NativeType` and its static bridge is `TimeZoneBridge`. The compiler will complain that `TimeZone` does not conform to `Defaults.NativeType`. We will resolve that later.

```swift
private struct TimeZone: Hashable {
	var id: String
	var name: String
}

extension TimeZone: Defaults.NativeType {
	static let bridge = TimeZoneBridge()
}
```

3. Create an extension of `CodableTimeZone` that conforms to `Defaults.CodableType`.

```swift
private struct CodableTimeZone {
	var id: String
	var name: String
}

extension CodableTimeZone: Defaults.CodableType {
	/// Convert from `Codable` to native type.
	func toNative() -> TimeZone {
		TimeZone(id: id, name: name)
	}
}
```

4. Associate `TimeZone.CodableForm` to `CodableTimeZone`

```swift
extension TimeZone: Defaults.NativeType {
	typealias CodableForm = CodableTimeZone

	static let bridge = TimeZoneBridge()
}
```

5. **Call `Defaults.migrate(.timezone, to: .v5)`, `Defaults.migrate(.arrayTimezone, to: .v5)`, `Defaults.migrate(.setTimezone, to: .v5)`, `Defaults.migrate(.dictionaryTimezone, to: .v5)`**.
6. Now `Defaults[.timezone]`, `Defaults[.arrayTimezone]` , `Defaults[.setTimezone]`, `Defaults[.dictionaryTimezone]` should be readable.

**See [DefaultsMigrationTests.swift](./Tests/DefaultsTests/DefaultsMigrationTests.swift) for more example.**