Demonstrates that SwiftUI (in it's initial beta form) is unable to properly handle diffing of class objects. The counter will not increment as you might expect it to.

SwiftUI-Class-Update-Failure.swift

import SwiftUI
import Combine

struct ContentView : View {
    @ObjectBinding var viewModel: ViewModel
    
    var body: some View {
        CounterView(counter: viewModel.counter)
    }
}

struct CounterView : View {
    let counter: Counter
    
    var body: some View {
        Text("\(counter.index)")
    }
}

// Changing this to struct fixes the issue.
class Counter {
    var index: Int
    
    init(index: Int) {
        self.index = index
    }
}

class ViewModel: BindableObject {
    let didChange = PassthroughSubject<ViewModel, Never>()
    
    var counter = Counter(index: 0)
    
    var timer: Timer!
    
    init() {
        timer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true, block: { [weak self] timer in
            guard let self = self else { return }
            
            self.counter.index += 1
            self.didChange.send(self)
        })
    }
}

#if DEBUG
struct ContentView_Previews : PreviewProvider {
    static var previews: some View {
        ContentView(viewModel: ViewModel())
    }
}
#endif