favoriting-component.swift

import SwiftUI

struct ContentView: View {
  @State var isFavorite = false

  var body: some View {
    FavoriteComponent(id: 42, isFavorite: self.$isFavorite)
  }
}

// A shareable "favoriting" component. Just instantiate it with the id
// of the thing you want to favorite, and a binding to the boolean
// representing its favorited state, and it will take care of the rest:
//
//  * It will optimistically toggle the favoriting state
//  * It will make an API request to try to favorite
//  * If the favoriting fails it will revert the favorited state
struct FavoriteComponent: View {
  let id: Int
  @Binding var isFavorite: Bool

  var body: some View {
    Button(action: {
      // Optimistically toggle the favorited state
      self.isFavorite.toggle()

      // Make an API request to your server to tell it to favorite the item
      // with id `self.id`.
      URLSession.shared
        .dataTask(with: URL(string: "https://www.site.com/item/\(self.id)/favorite")!) { _, _, error in
          // If we got an error then revert the state of the component
          if error != nil {
            self.isFavorite.toggle()
          }
      }.resume()
    }) {
      Image(systemName: self.isFavorite ? "heart" : "heart.fill")
        .foregroundColor(.blue)
    }
  }
}

struct ContentView_Previews: PreviewProvider {
  static var previews: some View {
    ContentView()
  }
}