Optional Bindable

ContentView.swift

//
//  ContentView.swift
//  BindableOptional
//
//  Created by Craig Hockenberry on 4/12/24.
//

import SwiftUI

@Observable
class Whatever {
	var number: Int = 0
}

struct OtherView: View {
	@Binding var whatever: Whatever
	// This allows the view to work with @Bindable, but breaks the traditional $ binding
	//@Bindable var whatever: Whatever
	
	var body: some View {
		Button("Do Whatever") {
			whatever.number += 1
		}
		.buttonStyle(.borderedProminent)
	}
}

struct ContentView: View {
	@State private var optionalWhatever: Whatever?
	@State private var whatever = Whatever()
	
	var body: some View {
#if DEBUG
		let _ = Self._printChanges()
#endif
		VStack {
			if let whateverInstance = optionalWhatever {
				Text("Optional Whatever = \(whateverInstance.number)")
				
				@Bindable var whateverBinding = whateverInstance
				// Cannot convert value of type 'Bindable<Whatever>' to expected argument type 'Binding<Whatever>'
				//OtherView(whatever: $whateverBinding)
			}
			else {
				Text("No Optional Whatever")
				
				Button("Make Optional Whatever") {
					optionalWhatever = Whatever()
					optionalWhatever!.number = 999
				}
				.buttonStyle(.borderedProminent)
			}
		}
		.padding()
		
		VStack {
			Text("Whatever = \(whatever.number)")
			OtherView(whatever: $whatever)
		}
		.padding()
	}
}

#Preview {
	ContentView()
}

The problem here is that you need to use properties of the Observable, not the Observable itself:

struct OtherViewNumber: View {
	@Binding var whateverNumber: Double
	
	var body: some View {
		Button("Do Whatever Number") {
			whateverNumber += 1
		}
		.buttonStyle(.borderedProminent)
	}
}

The view can then be used in both contexts:

	@Bindable var whateverBindable = whateverInstance
	OtherViewNumber(whateverNumber: $whateverBindable.number)

and:

	OtherViewNumber(whateverNumber: $whatever.number)

It's a subtle distinction, but the dynamic member lookup for both Bindable and Binding makes it work.