struct StateUpdatedPreferenceKey: PreferenceKey {
	static var defaultValue: Int = Int.min
	static func reduce(value: inout Int, nextValue: () -> Int) {
		value = nextValue()
	}
}

struct StateUpdated: View {
	var value: Int
	var onChange: () -> Void

	var body: some View {
		Color.clear
			.preference(key: StateUpdatedPreferenceKey.self, value: self.value)
			.onPreferenceChange(StateUpdatedPreferenceKey.self) { _ in
				self.onChange()
		}
	}
}

struct ViewPlayground: View {
	@State var isEven: Bool = true
	@State var count: Int = 0

	var body: some View {
		VStack {
			Button("Update") {
				self.count += 1
			}
			Text("\(self.count)")
			Text("Is Even: \(self.isEven.description)")
				.background(StateUpdated(value: self.count, onChange: {
					DispatchQueue.main.async {
						self.isEven = self.count%2 == 0
					}
				}))
			Text("Is really even: \(self.reallyIsEven().description)")
			Text("Matches " + self.reallyIsEven().description)
		}
		.execute { // extension on View to execute a block
			print("Matches " + self.matches().description)
		}
	}

	func reallyIsEven() -> Bool {
		self.count%2 == 0
	}

	func matches() -> Bool {
		self.isEven == self.reallyIsEven()
	}
}