SwiftUI Re-rendering

SwiftUI.swift

import Foundation
import PlaygroundSupport
import SwiftUI

// This View will be re-rendered because it's property value changed
struct Headline: View {
    var headline: String
    
    var body: some View {
        Text(headline)
            .foregroundColor(Color(red: 0.12, green: 0.73, blue: 0.82))
            .font(.headline)
            .padding(EdgeInsets(top: 0, leading: 0, bottom: 20, trailing: 0))
    }
}

// This View won't be re-rendered since it didn't change
struct SubHeadline: View {
    var subHeadline: String
    
    var body: some View {
        Text(subHeadline + " " + Date().description(with: .current))
            .lineLimit(nil)
            .font(.subheadline)
    }
}

struct Content: View {
    @ObservedObject var viewModel: ViewModel
    
    var text: String
    var headline: String
    var subHeadline: String
    
    var body: some View {
        HStack {
            VStack(alignment: .leading) {
                Headline(headline: "\(headline) \(viewModel.counter)")
                SubHeadline(subHeadline: subHeadline)
                Text(text)
                    .foregroundColor(viewModel.counter % 2 == 0 ? .black : .gray)
                Spacer()
            }.padding(20)
            Spacer()
        }
    }
}

final class ViewModel: ObservableObject {
    @Published var counter = 0
    
    init() {
        startTimer()
    }
    
    func startTimer() {
        Timer.scheduledTimer(withTimeInterval: 3, repeats: true, block: { _ in
            self.counter += 1
        })
    }
}

PlaygroundPage.current.liveView = UIHostingController(rootView: Content(viewModel: ViewModel(), text: "The future is reactive", headline: "High performance SwiftUI", subHeadline: "by Paul Weichhart"))