[in_app_purchase_storekit] Add support to win back offers / promotional offers (#8474)

**Adds StoreKit2 support for**:  
 - **Win Back Offers** (requires Xcode 16 or later)
    - New `isWinBackOfferEligible` method for eligibility checks
 - **Promotional Offers**
- Fixes introductory pricing handling in promotional offers list
- Fixes  `appAccountToken` handling for StoreKit2 purchases

Fixes:
flutter/flutter#161393
flutter/flutter#160826

Author: kaiquegazola

packages/in_app_purchase/in_app_purchase_storekit/CHANGELOG.md

@@ -1,6 +1,11 @@
-## NEXT
+## 0.4.1
 
 * Updates minimum supported SDK version to Flutter 3.27/Dart 3.6.
+* Adds **Win Back Offers** support for StoreKit2:
+    - Includes new `isWinBackOfferEligible` function for eligibility verification
+* Adds **Promotional Offers** support in StoreKit2 purchases
+* Fixes introductory pricing handling in promotional offers list in StoreKit2
+* Ensures proper `appAccountToken` handling for StoreKit2 purchases
 
 ## 0.4.0
 

packages/in_app_purchase/in_app_purchase_storekit/darwin/in_app_purchase_storekit/Sources/in_app_purchase_storekit/StoreKit2/InAppPurchasePlugin+StoreKit2.swift

@@ -53,7 +53,36 @@ extension InAppPurchasePlugin: InAppPurchase2API {
           return completion(.failure(error))
         }
 
-        let result = try await product.purchase(options: [])
+        var purchaseOptions: Set<Product.PurchaseOption> = []
+
+        if let appAccountToken = options?.appAccountToken,
+          let accountTokenUUID = UUID(uuidString: appAccountToken)
+        {
+          purchaseOptions.insert(.appAccountToken(accountTokenUUID))
+        }
+
+        if #available(iOS 17.4, macOS 14.4, *) {
+          if let promotionalOffer = options?.promotionalOffer {
+            purchaseOptions.insert(
+              .promotionalOffer(
+                offerID: promotionalOffer.promotionalOfferId,
+                signature: promotionalOffer.promotionalOfferSignature.convertToSignature
+              )
+            )
+          }
+        }
+
+        if #available(iOS 18.0, macOS 15.0, *) {
+          if let winBackOfferId = options?.winBackOfferId,
+            let winBackOffer = product.subscription?.winBackOffers.first(where: {
+              $0.id == winBackOfferId
+            })
+          {
+            purchaseOptions.insert(.winBackOffer(winBackOffer))
+          }
+        }
+
+        let result = try await product.purchase(options: purchaseOptions)
 
         switch result {
         case .success(let verification):
@@ -88,6 +117,70 @@ extension InAppPurchasePlugin: InAppPurchase2API {
     }
   }
 
+  /// Checks if the user is eligible for a specific win back offer.
+  ///
+  /// - Parameters:
+  ///   - productId: The product ID associated with the offer.
+  ///   - offerId: The ID of the win back offer.
+  ///   - completion: Returns `Bool` for eligibility or `Error` on failure.
+  ///
+  /// - Availability: iOS 18.0+, macOS 15.0+, Swift 6.0+ (Xcode 16+).
+  func isWinBackOfferEligible(
+    productId: String,
+    offerId: String,
+    completion: @escaping (Result<Bool, Error>) -> Void
+  ) {
+    if #available(iOS 18.0, macOS 15.0, *) {
+      Task {
+        do {
+          guard let product = try await Product.products(for: [productId]).first else {
+            completion(
+              .failure(
+                PigeonError(
+                  code: "storekit2_failed_to_fetch_product",
+                  message: "Storekit has failed to fetch this product.",
+                  details: "Product ID: \(productId)")))
+            return
+          }
+
+          guard let subscription = product.subscription else {
+            completion(
+              .failure(
+                PigeonError(
+                  code: "storekit2_not_subscription",
+                  message: "Product is not a subscription",
+                  details: "Product ID: \(productId)")))
+            return
+          }
+
+          let isEligible = try await subscription.status.contains { status in
+            if case .verified(let renewalInfo) = status.renewalInfo {
+              return renewalInfo.eligibleWinBackOfferIDs.contains(offerId)
+            }
+            return false
+          }
+
+          completion(.success(isEligible))
+
+        } catch {
+          completion(
+            .failure(
+              PigeonError(
+                code: "storekit2_eligibility_check_failed",
+                message: "Failed to check offer eligibility: \(error.localizedDescription)",
+                details: "Product ID: \(productId), Error: \(error)")))
+        }
+      }
+    } else {
+      completion(
+        .failure(
+          PigeonError(
+            code: "storekit2_unsupported_platform_version",
+            message: "Win back offers require iOS 18+ or macOS 15.0+",
+            details: nil)))
+    }
+  }
+
   /// Wrapper method around StoreKit2's transactions() method
   /// https://developer.apple.com/documentation/storekit/product/3851116-products
   func transactions(

packages/in_app_purchase/in_app_purchase_storekit/darwin/in_app_purchase_storekit/Sources/in_app_purchase_storekit/StoreKit2/StoreKit2Translators.swift

@@ -52,10 +52,23 @@ extension Product.ProductType {
 @available(iOS 15.0, macOS 12.0, *)
 extension Product.SubscriptionInfo {
   var convertToPigeon: SK2SubscriptionInfoMessage {
+    var allOffers: [SK2SubscriptionOfferMessage] = []
+
+    if #available(iOS 18.0, macOS 15.0, *) {
+      allOffers.append(contentsOf: winBackOffers.map { $0.convertToPigeon })
+    }
+
+    allOffers.append(contentsOf: promotionalOffers.map { $0.convertToPigeon })
+
+    if let introductory = introductoryOffer {
+      allOffers.append(introductory.convertToPigeon)
+    }
+
     return SK2SubscriptionInfoMessage(
-      promotionalOffers: promotionalOffers.map({ $0.convertToPigeon }),
+      promotionalOffers: allOffers,
       subscriptionGroupID: subscriptionGroupID,
-      subscriptionPeriod: subscriptionPeriod.convertToPigeon)
+      subscriptionPeriod: subscriptionPeriod.convertToPigeon
+    )
   }
 }
 
@@ -90,6 +103,33 @@ extension SK2SubscriptionOfferMessage: Equatable {
   }
 }
 
+extension SK2SubscriptionOfferSignatureMessage {
+  @available(iOS 17.4, macOS 14.4, *)
+  var convertToSignature: Product.SubscriptionOffer.Signature {
+    return Product.SubscriptionOffer.Signature(
+      keyID: keyID,
+      nonce: nonceAsUUID,
+      timestamp: Int(timestamp),
+      signature: signatureAsData
+    )
+  }
+
+  var nonceAsUUID: UUID {
+    guard let uuid = UUID(uuidString: nonce) else {
+      fatalError("Invalid UUID format for nonce: \(nonce)")
+    }
+    return uuid
+  }
+
+  var signatureAsData: Data {
+    guard let data = Data(base64Encoded: signature) else {
+      fatalError("Invalid Base64 format for signature: \(signature)")
+    }
+    return data
+  }
+
+}
+
 @available(iOS 15.0, macOS 12.0, *)
 extension Product.SubscriptionOffer.OfferType {
   var convertToPigeon: SK2SubscriptionOfferTypeMessage {
@@ -99,7 +139,12 @@ extension Product.SubscriptionOffer.OfferType {
     case .promotional:
       return SK2SubscriptionOfferTypeMessage.promotional
     default:
-      fatalError("An unknown OfferType was passed in")
+      if #available(iOS 18.0, macOS 15.0, *) {
+        if self == .winBack {
+          return SK2SubscriptionOfferTypeMessage.winBack
+        }
+      }
+      fatalError("An unknown or unsupported OfferType was passed in")
     }
   }
 }