非消耗性产品一般需要一个Restore按钮，用户重新安装的时候用来恢复购买。

流程：
1.获取 products
2.添加到 payment 队列
3.根据 updatedTransactions 判断是否购买成功
实现代理:SKPaymentTransactionObserver,SKProductsRequestDelegate

swift
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var payment : SKPayment?
var product : SKProduct?
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//load request
SKPaymentQueue.defaultQueue().addTransactionObserver(self)
productRequest = SKProductsRequest(productIdentifiers: ["me.zhangxi.xxxx"])
productRequest?.delegate = self
productRequest?.start()
//restore
SKPaymentQueue.defaultQueue().restoreCompletedTransactions()
SKProductsRequestDelegate
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func productsRequest(request: SKProductsRequest, didReceiveResponse response: SKProductsResponse) {
    product = response.products.first
    if product != nil
    {
        payment = SKPayment(product: product!)
    }
}
SKPaymentTransactionObserver
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func paymentQueue(queue: SKPaymentQueue, updatedTransactions transactions: [SKPaymentTransaction]) {
    for transaction in transactions
    {
        switch transaction.transactionState {
        case .Purchasing: // Transaction is being added to the server queue.
            break;
        case .Purchased: // Transaction is in queue, user has been charged.  Client should complete the transaction.
            self.buyOK()
            SKPaymentQueue.defaultQueue().finishTransaction(transaction)
        case .Failed: // Transaction was cancelled or failed before being added to the server queue.
            break;
        case .Restored: // Transaction was restored from user's purchase history.  Client should complete the transaction.
            self.buyOK()
            self.alert("Restore Success.")
            SKPaymentQueue.defaultQueue().finishTransaction(transaction)
        case .Deferred: // The transaction is in the queue, but its final status is pending external action.
            break;
        }
    }
}