Merge pull request #7 from dapperlabs/seanlee/latest_purchase

Add latest transaction fetcher

Sources/Mercato/Mercato.swift

@@ -11,7 +11,7 @@ public class Mercato {
     private var updateListenerTask: Task<(), Never>? = nil
 
     public init() {}
-
+    
     fileprivate func listenForUnfinishedTransactions(updateBlock: @escaping TransactionUpdate) {
         let task = Task.detached
         {
@@ -189,6 +189,19 @@ extension Mercato
     {
         return try await activeSubscriptions(onlyRenewable: onlyRenewable).map { $0.productID}
     }
+
+    public static func fetchLatestTransaction(for productId: String) async throws -> (transaction: Transaction, jwsRepresentation: String) {
+        let result = await Transaction.latest(for: productId)
+        guard let result else {
+            throw MercatoError.failedVerification
+        }
+        switch result {
+        case .verified(let verifiedTransaction):
+            return (transaction: verifiedTransaction, jwsRepresentation: result.jwsRepresentation)
+        case .unverified:
+            throw MercatoError.failedVerification
+        }
+    }
 }