Add `@Default` property wrapper for SwiftUI

Fixes #28

Defaults.xcodeproj/project.pbxproj

@@ -29,6 +29,10 @@
 		E339B3B92449F10D00E7A40A /* UserDefaults.swift in Sources */ = {isa = PBXBuildFile; fileRef = E339B3B72449F10D00E7A40A /* UserDefaults.swift */; };
 		E339B3BA2449F10D00E7A40A /* UserDefaults.swift in Sources */ = {isa = PBXBuildFile; fileRef = E339B3B72449F10D00E7A40A /* UserDefaults.swift */; };
 		E339B3BB2449F10D00E7A40A /* UserDefaults.swift in Sources */ = {isa = PBXBuildFile; fileRef = E339B3B72449F10D00E7A40A /* UserDefaults.swift */; };
+		E38C9F27244ADA2F00A6737A /* SwiftUI.swift in Sources */ = {isa = PBXBuildFile; fileRef = E38C9F26244ADA2F00A6737A /* SwiftUI.swift */; };
+		E38C9F28244ADA2F00A6737A /* SwiftUI.swift in Sources */ = {isa = PBXBuildFile; fileRef = E38C9F26244ADA2F00A6737A /* SwiftUI.swift */; };
+		E38C9F29244ADA2F00A6737A /* SwiftUI.swift in Sources */ = {isa = PBXBuildFile; fileRef = E38C9F26244ADA2F00A6737A /* SwiftUI.swift */; };
+		E38C9F2A244ADA2F00A6737A /* SwiftUI.swift in Sources */ = {isa = PBXBuildFile; fileRef = E38C9F26244ADA2F00A6737A /* SwiftUI.swift */; };
 		E3EB3E33216505920033B089 /* util.swift in Sources */ = {isa = PBXBuildFile; fileRef = E3EB3E32216505920033B089 /* util.swift */; };
 		E3EB3E35216507AE0033B089 /* Observation.swift in Sources */ = {isa = PBXBuildFile; fileRef = E3EB3E34216507AE0033B089 /* Observation.swift */; };
 		E3EB3E36216507B50033B089 /* Observation.swift in Sources */ = {isa = PBXBuildFile; fileRef = E3EB3E34216507AE0033B089 /* Observation.swift */; };
@@ -79,6 +83,7 @@
 		E286D0C623B8D51100570D1E /* Observation+Combine.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; lineEnding = 0; path = "Observation+Combine.swift"; sourceTree = "<group>"; usesTabs = 1; };
 		E339B3B22449ED2000E7A40A /* Reset.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; lineEnding = 0; path = Reset.swift; sourceTree = "<group>"; usesTabs = 1; };
 		E339B3B72449F10D00E7A40A /* UserDefaults.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; lineEnding = 0; path = UserDefaults.swift; sourceTree = "<group>"; usesTabs = 1; };
+		E38C9F26244ADA2F00A6737A /* SwiftUI.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; lineEnding = 0; path = SwiftUI.swift; sourceTree = "<group>"; usesTabs = 1; };
 		E3EB3E32216505920033B089 /* util.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; lineEnding = 0; path = util.swift; sourceTree = "<group>"; usesTabs = 1; };
 		E3EB3E34216507AE0033B089 /* Observation.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; lineEnding = 0; path = Observation.swift; sourceTree = "<group>"; usesTabs = 1; };
 /* End PBXFileReference section */
@@ -215,6 +220,7 @@
 				E339B3B22449ED2000E7A40A /* Reset.swift */,
 				E3EB3E34216507AE0033B089 /* Observation.swift */,
 				E286D0C623B8D51100570D1E /* Observation+Combine.swift */,
+				E38C9F26244ADA2F00A6737A /* SwiftUI.swift */,
 				E3EB3E32216505920033B089 /* util.swift */,
 			);
 			path = Defaults;
@@ -502,6 +508,7 @@
 			buildActionMask = 2147483647;
 			files = (
 				E286D0C823B8D54C00570D1E /* Observation+Combine.swift in Sources */,
+				E38C9F28244ADA2F00A6737A /* SwiftUI.swift in Sources */,
 				8933C7851EB5B820000D00A4 /* Defaults.swift in Sources */,
 				E339B3B92449F10D00E7A40A /* UserDefaults.swift in Sources */,
 				E3EB3E35216507AE0033B089 /* Observation.swift in Sources */,
@@ -523,6 +530,7 @@
 			buildActionMask = 2147483647;
 			files = (
 				E286D0CA23B8D54E00570D1E /* Observation+Combine.swift in Sources */,
+				E38C9F2A244ADA2F00A6737A /* SwiftUI.swift in Sources */,
 				E3EB3E3A216507C40033B089 /* util.swift in Sources */,
 				E339B3BB2449F10D00E7A40A /* UserDefaults.swift in Sources */,
 				E3EB3E37216507B50033B089 /* Observation.swift in Sources */,
@@ -536,6 +544,7 @@
 			buildActionMask = 2147483647;
 			files = (
 				E286D0C923B8D54D00570D1E /* Observation+Combine.swift in Sources */,
+				E38C9F29244ADA2F00A6737A /* SwiftUI.swift in Sources */,
 				E3EB3E3B216507C40033B089 /* util.swift in Sources */,
 				E339B3BA2449F10D00E7A40A /* UserDefaults.swift in Sources */,
 				E3EB3E38216507B60033B089 /* Observation.swift in Sources */,
@@ -549,6 +558,7 @@
 			buildActionMask = 2147483647;
 			files = (
 				E286D0C723B8D51100570D1E /* Observation+Combine.swift in Sources */,
+				E38C9F27244ADA2F00A6737A /* SwiftUI.swift in Sources */,
 				E3EB3E39216507C30033B089 /* util.swift in Sources */,
 				E339B3B82449F10D00E7A40A /* UserDefaults.swift in Sources */,
 				E3EB3E36216507B50033B089 /* Observation.swift in Sources */,

Sources/Defaults/SwiftUI.swift

@@ -0,0 +1,113 @@
+#if canImport(Combine)
+
+import SwiftUI
+import Combine
+
+@available(macOS 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
+extension Defaults {
+	final class Observable<Value: Codable>: ObservableObject {
+		let objectWillChange = ObservableObjectPublisher()
+		private var observation: DefaultsObservation?
+		private let key: Defaults.Key<Value>
+
+		var value: Value {
+			get { Defaults[key] }
+			set {
+				objectWillChange.send()
+				Defaults[key] = newValue
+			}
+		}
+
+		init(_ key: Key<Value>) {
+			self.key = key
+
+			self.observation = Defaults.observe(key, options: [.prior]) { [weak self] change in
+				guard change.isPrior else {
+					return
+				}
+
+				DispatchQueue.mainSafeAsync {
+					self?.objectWillChange.send()
+				}
+			}
+		}
+
+		/// Reset the key back to its default value.
+		func reset() {
+			key.reset()
+		}
+	}
+}
+
+@available(macOS 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
+@propertyWrapper
+public struct Default<Value: Codable>: DynamicProperty {
+	public typealias Publisher = AnyPublisher<Defaults.KeyChange<Value>, Never>
+
+	private let key: Defaults.Key<Value>
+	@ObservedObject private var observable: Defaults.Observable<Value>
+
+	/**
+	Get/set a `Defaults` item and also have the view be updated when the value changes. This is similar to `@State`.
+
+	```
+	extension Defaults.Keys {
+		static let hasUnicorn = Key<Bool>("hasUnicorn", default: false)
+	}
+
+	struct ContentView: View {
+		@Default(.hasUnicorn) var hasUnicorn
+
+		var body: some View {
+			Text("Has Unicorn: \(hasUnicorn)")
+			Toggle("Toggle Unicorn", isOn: $hasUnicorn)
+		}
+	}
+	```
+	*/
+	public init(_ key: Defaults.Key<Value>) {
+		self.key = key
+		self.observable = Defaults.Observable(key)
+	}
+
+	public var wrappedValue: Value {
+		get { observable.value }
+		nonmutating set {
+			observable.value = newValue
+		}
+	}
+
+	public var projectedValue: Binding<Value> { $observable.value }
+
+	/// Combine publisher that publishes values when the `Defaults` item changes.
+	public var publisher: Publisher { Defaults.publisher(key) }
+
+	public mutating func update() {
+		_observable.update()
+	}
+
+	/**
+	Reset the key back to its default value.
+
+	```
+	extension Defaults.Keys {
+		static let opacity = Key<Double>("opacity", default: 1)
+	}
+
+	struct ContentView: View {
+		@Default(.opacity) var opacity
+
+		var body: some View {
+			Button("Reset") {
+				self._opacity.reset()
+			}
+		}
+	}
+	```
+	*/
+	public func reset() {
+		key.reset()
+	}
+}
+
+#endif

Sources/Defaults/util.swift

@@ -132,3 +132,17 @@ extension Optional: _DefaultsOptionalType {
 func isOptionalType<T>(_ type: T.Type) -> Bool {
 	type is _DefaultsOptionalType.Type
 }
+
+
+extension DispatchQueue {
+	/**
+	Performs the `execute` closure immediately if we're on the main thread or asynchronously puts it on the main thread otherwise.
+	*/
+	static func mainSafeAsync(execute work: @escaping () -> Void) {
+		if Thread.isMainThread {
+			work()
+		} else {
+			main.async(execute: work)
+		}
+	}
+}

readme.md

@@ -13,9 +13,10 @@ It's used in production by apps like [Gifski](https://github.com/sindresorhus/Gi
 - **Strongly typed:** You declare the type and default value upfront.
 - **Codable support:** You can store any [Codable](https://developer.apple.com/documentation/swift/codable) value, like an enum.
 - **NSSecureCoding support:** You can store any [NSSecureCoding](https://developer.apple.com/documentation/foundation/nssecurecoding) value.
-- **Debuggable:** The data is stored as JSON-serialized values.
+- **SwiftUI:** Property wrapper that updates the view when the `UserDefaults` value changes.
-- **Observation:** Observe changes to keys.
 - **Publishers:** Combine publishers built-in.
+- **Observation:** Observe changes to keys.
+- **Debuggable:** The data is stored as JSON-serialized values.
 
 ## Compatibility
 
@@ -141,6 +142,29 @@ Defaults[isUnicorn]
 //=> true
 ```
 
+### SwiftUI support
+
+You can use the `@Default` property wrapper to get/set a `Defaults` item and also have the view be updated when the value changes. This is similar to `@State`.
+
+```swift
+extension Defaults.Keys {
+	static let hasUnicorn = Key<Bool>("hasUnicorn", default: false)
+}
+
+struct ContentView: View {
+	@Default(.hasUnicorn) var hasUnicorn
+
+	var body: some View {
+		Text("Has Unicorn: \(hasUnicorn)")
+		Toggle("Toggle Unicorn", isOn: $hasUnicorn)
+	}
+}
+```
+
+Note that it's `@Default`, not `@Defaults`.
+
+This is only implemented for `Defaults.Key`. PR welcome for `Defaults.NSSecureCoding` if you need it.
+
 ### Observe changes to a key
 
 ```swift
@@ -449,6 +473,12 @@ Break the lifetime tie created by `tieToLifetime(of:)`, if one exists.
 
 The effects of any call to `tieToLifetime(of:)` are reversed. Note however that if the tied-to object has already died, then the observation is already invalid and this method has no logical effect.
 
+### `@Default(_ key:)`
+
+Get/set a `Defaults` item and also have the view be updated when the value changes.
+
+This is only implemented for `Defaults.Key`. PR welcome for `Defaults.NSSecureCoding` if you need it.
+
 ## FAQ
 
 ### How can I store a dictionary of arbitrary values?