objc.io’s geometryPreference modifier and Preference view for propagating up proxied geometries and reading them.

geometryPreference.swift

import SwiftUI

struct Preference<PreferenceKey: SwiftUI.PreferenceKey, Content: View>: View
	where PreferenceKey.Value: Equatable {

	private let content: (PreferenceKey.Value) -> Content
	@State private var value: PreferenceKey.Value = PreferenceKey.defaultValue

	init(
		_ key: PreferenceKey.Type,
		@ViewBuilder content: @escaping (PreferenceKey.Value) -> Content
	) {
		self.content = content
	}

	var body: some View {
		content(value)
			.onPreferenceChange(PreferenceKey.self) { self.value = $0 }
	}
}

extension View {
	func geometryPreference<PreferenceKey: SwiftUI.PreferenceKey>(
		_ key: PreferenceKey.Type,
		value: @escaping (GeometryProxy) -> PreferenceKey.Value
	) {
		overlay(
			GeometryReader { proxy in
				Color
					.clear
					.preference(key: key, value: value(proxy))
			}
		)
	}
}