--- a/Simoleon/Helpers/FavoriteButton.swift	Thu Dec 23 11:30:38 2021 +0100
+++ b/Simoleon/Helpers/FavoriteButton.swift	Thu Dec 23 16:12:22 2021 +0100
@@ -6,12 +6,21 @@
 //
 
 import SwiftUI
+import CoreData
 
 struct FavoriteButton: View {
+    @Binding var baseCurrency: SupportedCurrencyResult
+    @Binding var quoteCurrency: SupportedCurrencyResult
     @State private var scale: CGFloat = 1
+    @Environment(\.managedObjectContext) private var viewContext
+    @FetchRequest(sortDescriptors: []) private var favoritePairs: FetchedResults<FavoritePair>
     
     var body: some View {
-        Button(action: {}) {
+        Button(action: {
+            animate()
+            if isFavorite() { remove() }
+            else { add() }
+        }) {
             RoundedRectangle(cornerRadius: 15)
                 .foregroundColor(Color(.secondarySystemBackground))
                 .frame(width: 60, height: 60)
@@ -33,17 +42,49 @@
     
     // Add currency conversion to favorites
     private func add() {
+        let favoritePair = FavoritePair(context: viewContext)
+        favoritePair.baseCurrency = baseCurrency.code
+        favoritePair.quoteCurrency = quoteCurrency.code
         
+        do {
+            try viewContext.save()
+        } catch {
+            // Replace this implementation with code to handle the error appropriately.
+            // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
+            let nsError = error as NSError
+            fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
+        }
     }
     
     // Remove currency conversion from favorites
     private func remove() {
+        let favoritePair = favoritePairs.first(
+            where: {
+                $0.baseCurrency == baseCurrency.code && $0.quoteCurrency == quoteCurrency.code
+        })
         
+        viewContext.delete(favoritePair!)
+        
+        do {
+            try viewContext.save()
+        } catch {
+            // Replace this implementation with code to handle the error appropriately.
+            // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
+            let nsError = error as NSError
+            fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
+        }
     }
     
     // Check if currency conversion is in favorites
     private func isFavorite() -> Bool {
-        return false
+        let favoritePair = favoritePairs.first(
+            where: {
+                $0.baseCurrency == baseCurrency.code && $0.quoteCurrency == quoteCurrency.code
+        })
+        
+        guard let _ = favoritePair else { return false }
+        
+        return true
     }
     
     // Animate favorite button 
@@ -57,6 +98,10 @@
 
 struct FavoriteButton_Previews: PreviewProvider {
     static var previews: some View {
-        FavoriteButton()
+        FavoriteButton(
+            baseCurrency: .constant(SupportedCurrencyResult(code: "EUR", name: "Euro", isCrypto: 0)),
+            quoteCurrency: .constant(SupportedCurrencyResult(code: "USD", name: "U.S. Dollar", isCrypto: 0))
+        )
+            .environment(\.managedObjectContext, PersistenceController.preview.container.viewContext)
     }
 }