Refactor: Calculate total cart items in ViewModel

This commit refactors the calculation of the total number of items in the cart.
Previously, the total item count was calculated independently in both the Android `CartScreen.kt` and iOS `CartView.swift`.

This change moves the calculation into the shared `CartViewModel.kt`. A new `totalItemCount` property is added to `CartUiState`, which is now populated within the `map` function that transforms the `repository.cartDetails` flow.

This centralization simplifies the UI components and ensures consistency in how the total item count is determined across platforms.

Author: cartland

Fruitties/androidApp/src/main/java/com/example/fruitties/android/ui/CartScreen.kt

@@ -108,7 +108,7 @@ fun CartScreen(onNavBarBack: () -> Unit) {
                 .padding(paddingValues)
                 .padding(16.dp),
         ) {
-            val cartItemCount = cartState.cartDetails.sumOf { it.count }
+            val cartItemCount = cartState.totalItemCount
             Text(
                 text = "Cart has $cartItemCount items",
                 modifier = Modifier.padding(8.dp),

Fruitties/iosApp/iosApp/CartView.swift

@@ -29,7 +29,7 @@ struct CartView : View {
         Observing(cartViewModel.cartUiState) { cartUIState in
             VStack {
                 HStack {
-                    let total = cartUIState.cartDetails.reduce(0) { $0 + $1.count }
+                    let total = cartUIState.totalItemCount
                     Text("Cart has \(total) items").padding()
                     Spacer()
                 }

Fruitties/shared/src/commonMain/kotlin/com/example/fruitties/viewmodel/CartViewModel.kt

@@ -36,8 +36,12 @@ class CartViewModel(
 ) : ViewModel() {
     val cartUiState: StateFlow<CartUiState> =
         repository.cartDetails
-            .map { CartUiState(it) }
-            .stateIn(
+            .map { details ->
+                CartUiState(
+                    cartDetails = details,
+                    totalItemCount = details.sumOf { it.count },
+                )
+            }.stateIn(
                 scope = viewModelScope,
                 started = SharingStarted.WhileSubscribed(TIMEOUT_MILLIS),
                 initialValue = CartUiState(),
@@ -66,6 +70,7 @@ class CartViewModel(
  */
 data class CartUiState(
     val cartDetails: List<CartItemDetails> = listOf(),
+    val totalItemCount: Int = 0,
 )
 
 private const val TIMEOUT_MILLIS = 5_000L