Support dynamic default value (#121)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

Sources/Defaults/Defaults.swift

@@ -41,7 +41,15 @@ public enum Defaults {
 	}
 
 	public final class Key<Value: Serializable>: AnyKey {
-		public let defaultValue: Value
+		/**
+		It will be executed in these situations:
+
+		- `UserDefaults.object(forKey: string)` returns `nil`
+		- A `bridge` cannot deserialize `Value` from `UserDefaults`
+		*/
+		private let defaultValueGetter: () -> Value
+
+		public var defaultValue: Value { defaultValueGetter() }
 
 		/**
 		Create a defaults key.
@@ -51,7 +59,7 @@ public enum Defaults {
 		The `default` parameter should not be used if the `Value` type is an optional.
 		*/
 		public init(_ key: String, default defaultValue: Value, suite: UserDefaults = .standard) {
-			self.defaultValue = defaultValue
+			self.defaultValueGetter = { defaultValue }
 
 			super.init(name: key, suite: suite)
 
@@ -66,6 +74,25 @@ public enum Defaults {
 			// Sets the default value in the actual UserDefaults, so it can be used in other contexts, like binding.
 			suite.register(defaults: [name: serialized])
 		}
+
+		/**
+		Create a defaults key with a dynamic default value.
+		
+		This can be useful in cases where you cannot define a static default value as it may change during the lifetime of the app.
+
+		```swift
+		extension Defaults.Keys {
+			static let camera = Key<AVCaptureDevice?>("camera") { .default(for: .video) }
+		}
+		```
+
+		- Note: This initializer will not set the default value in the actual `UserDefaults`. This should not matter much though. It's only really useful if you use legacy KVO bindings.
+		*/
+		public init(_ key: String, suite: UserDefaults = .standard, default defaultValueGetter: @escaping () -> Value) {
+			self.defaultValueGetter = defaultValueGetter
+
+			super.init(name: key, suite: suite)
+		}
 	}
 
 	public static subscript<Value: Serializable>(key: Key<Value>) -> Value {

Tests/DefaultsTests/DefaultsTests.swift

@@ -15,6 +15,8 @@ extension Defaults.Keys {
 	static let data = Key<Data>("data", default: Data([]))
 	static let date = Key<Date>("date", default: fixtureDate)
 	static let uuid = Key<UUID?>("uuid")
+	static let defaultDynamicDate = Key<Date>("defaultDynamicOptionalDate") { Date(timeIntervalSince1970: 0) }
+	static let defaultDynamicOptionalDate = Key<Date?>("defaultDynamicOptionalDate") { Date(timeIntervalSince1970: 1) }
 }
 
 final class DefaultsTests: XCTestCase {
@@ -54,6 +56,17 @@ final class DefaultsTests: XCTestCase {
 		XCTAssertEqual(Defaults[url], fixtureURL2)
 	}
 
+	func testInitializeDynamicDateKey() {
+		_ = Defaults.Key<Date>("independentInitializeDynamicDateKey") {
+			XCTFail("Init dynamic key should not trigger getter")
+			return Date()
+		}
+		_ = Defaults.Key<Date?>("independentInitializeDynamicOptionalDateKey") {
+			XCTFail("Init dynamic optional key should not trigger getter")
+			return Date()
+		}
+	}
+
 	func testKeyRegistersDefault() {
 		let keyName = "registersDefault"
 		XCTAssertFalse(UserDefaults.standard.bool(forKey: keyName))
@@ -100,6 +113,27 @@ final class DefaultsTests: XCTestCase {
 		XCTAssertEqual(Defaults[.date], newDate)
 	}
 
+	func testDynamicDateType() {
+		XCTAssertEqual(Defaults[.defaultDynamicDate], Date(timeIntervalSince1970: 0))
+		let next = Date(timeIntervalSince1970: 1)
+		Defaults[.defaultDynamicDate] = next
+		XCTAssertEqual(Defaults[.defaultDynamicDate], next)
+		XCTAssertEqual(UserDefaults.standard.object(forKey: Defaults.Key<Date>.defaultDynamicDate.name) as! Date, next)
+		Defaults.Key<Date>.defaultDynamicDate.reset()
+		XCTAssertEqual(Defaults[.defaultDynamicDate], Date(timeIntervalSince1970: 0))
+	}
+
+	func testDynamicOptionalDateType() {
+		XCTAssertEqual(Defaults[.defaultDynamicOptionalDate], Date(timeIntervalSince1970: 1))
+		let next = Date(timeIntervalSince1970: 2)
+		Defaults[.defaultDynamicOptionalDate] = next
+		XCTAssertEqual(Defaults[.defaultDynamicOptionalDate], next)
+		XCTAssertEqual(UserDefaults.standard.object(forKey: Defaults.Key<Date>.defaultDynamicOptionalDate.name) as! Date, next)
+		Defaults[.defaultDynamicOptionalDate] = nil
+		XCTAssertEqual(Defaults[.defaultDynamicOptionalDate], Date(timeIntervalSince1970: 1))
+		XCTAssertNil(UserDefaults.standard.object(forKey: Defaults.Key<Date>.defaultDynamicOptionalDate.name))
+	}
+
 	func testFileURLType() {
 		XCTAssertEqual(Defaults[.file], fixtureFileURL)
 	}
@@ -188,6 +222,38 @@ final class DefaultsTests: XCTestCase {
 		waitForExpectations(timeout: 10)
 	}
 
+	@available(iOS 13.0, macOS 10.15, tvOS 13.0, watchOS 6.0, iOSApplicationExtension 13.0, macOSApplicationExtension 10.15, tvOSApplicationExtension 13.0, watchOSApplicationExtension 6.0, *)
+	func testDynamicOptionalDateTypeCombine() {
+		let first = Date(timeIntervalSince1970: 0)
+		let second = Date(timeIntervalSince1970: 1)
+		let third = Date(timeIntervalSince1970: 2)
+		let key = Defaults.Key<Date?>("combineDynamicOptionalDateKey") { first }
+		let expect = expectation(description: "Observation closure being called")
+
+		let publisher = Defaults
+			.publisher(key, options: [])
+			.map { ($0.oldValue, $0.newValue) }
+			.collect(3)
+
+		let expectedValues: [(Date?, Date?)] = [(first, second), (second, third), (third, first)]
+
+		let cancellable = publisher.sink { actualValues in
+			for (expected, actual) in zip(expectedValues, actualValues) {
+				XCTAssertEqual(expected.0, actual.0)
+				XCTAssertEqual(expected.1, actual.1)
+			}
+
+			expect.fulfill()
+		}
+
+		Defaults[key] = second
+		Defaults[key] = third
+		Defaults.reset(key)
+		cancellable.cancel()
+
+		waitForExpectations(timeout: 10)
+	}
+
 	@available(iOS 13.0, macOS 10.15, tvOS 13.0, watchOS 6.0, iOSApplicationExtension 13.0, macOSApplicationExtension 10.15, tvOSApplicationExtension 13.0, watchOSApplicationExtension 6.0, *)
 	func testObserveMultipleKeysCombine() {
 		let key1 = Defaults.Key<String>("observeKey1", default: "x")
@@ -321,6 +387,26 @@ final class DefaultsTests: XCTestCase {
 		waitForExpectations(timeout: 10)
 	}
 
+	func testObserveDynamicOptionalDateKey() {
+		let first = Date(timeIntervalSince1970: 0)
+		let second = Date(timeIntervalSince1970: 1)
+		let key = Defaults.Key<Date?>("observeDynamicOptionalDate") { first }
+
+		let expect = expectation(description: "Observation closure being called")
+
+		var observation: Defaults.Observation!
+		observation = Defaults.observe(key, options: []) { change in
+			XCTAssertEqual(change.oldValue, first)
+			XCTAssertEqual(change.newValue, second)
+			observation.invalidate()
+			expect.fulfill()
+		}
+
+		Defaults[key] = second
+
+		waitForExpectations(timeout: 10)
+	}
+
 	func testObservePreventPropagation() {
 		let key1 = Defaults.Key<Bool?>("preventPropagation0", default: nil)
 		let expect = expectation(description: "No infinite recursion")

readme.md

@@ -123,6 +123,14 @@ if let name = Defaults[.name] {
 
 The default value is then `nil`.
 
+You can also specify a dynamic default value. This can be useful when the default value may change during the lifetime of the app:
+
+```swift
+extension Defaults.Keys {
+	static let camera = Key<AVCaptureDevice?>("camera") { .default(for: .video) }
+}
+```
+
 ---
 
 ### Enum example
@@ -383,6 +391,9 @@ print(UserDefaults.standard.bool(forKey: Defaults.Keys.isUnicornMode.name))
 //=> true
 ```
 
+> **Note** 
+> A `Defaults.Key` with a dynamic default value will not register the default value in `UserDefaults`.
+
 ## API
 
 ### `Defaults`