要解决这个问题，您可以使用以下代码示例：

import StoreKit

class ViewController: UIViewController, SKProductsRequestDelegate, SKPaymentTransactionObserver {

    override func viewDidLoad() {
        super.viewDidLoad()
        // 添加观察者
        SKPaymentQueue.default().add(self)
    }

    func checkIfProductPurchased(productId: String) -> Bool {
        return UserDefaults.standard.bool(forKey: productId)
    }

    func purchaseProduct(productId: String) {
        if SKPaymentQueue.canMakePayments() {
            let productRequest = SKProductsRequest(productIdentifiers: [productId])
            productRequest.delegate = self
            productRequest.start()
        } else {
            // 用户禁用应用内购买
            print("应用内购买被禁用")
        }
    }

    // MARK: - SKProductsRequestDelegate

    func productsRequest(_ request: SKProductsRequest, didReceive response: SKProductsResponse) {
        if let product = response.products.first {
            let payment = SKPayment(product: product)
            SKPaymentQueue.default().add(payment)
        } else {
            print("找不到产品")
        }
    }

    // MARK: - SKPaymentTransactionObserver

    func paymentQueue(_ queue: SKPaymentQueue, updatedTransactions transactions: [SKPaymentTransaction]) {
        for transaction in transactions {
            switch transaction.transactionState {
            case .purchased:
                completeTransaction(transaction)
            case .failed:
                failedTransaction(transaction)
            case .restored:
                restoreTransaction(transaction)
            default:
                break
            }
        }
    }

    func completeTransaction(_ transaction: SKPaymentTransaction) {
        let productId = transaction.payment.productIdentifier
        UserDefaults.standard.set(true, forKey: productId)
        SKPaymentQueue.default().finishTransaction(transaction)
        print("购买完成")
    }

    func failedTransaction(_ transaction: SKPaymentTransaction) {
        if let error = transaction.error {
            print("购买失败：\(error.localizedDescription)")
        }
        SKPaymentQueue.default().finishTransaction(transaction)
    }

    func restoreTransaction(_ transaction: SKPaymentTransaction) {
        let productId = transaction.payment.productIdentifier
        UserDefaults.standard.set(true, forKey: productId)
        SKPaymentQueue.default().finishTransaction(transaction)
        print("已恢复购买")
    }
}

使用上述代码，您可以在应用中检查产品是否已购买，如果尚未购买，则可以进行购买操作。购买完成后，将在UserDefaults中设置产品的购买状态为true。如果用户尝试再次购买已购买的产品，将显示"此应用内购买已经购买过"的消息，并且不会再次进行购买。