Getting Started with Qt Purchasing in C++

This guide assumes that you have registered the in-app products for your application in the external store. For more information about registering products, see Registering Products in Google Play, Registering Products in App Store, and Registering Products in Windows Store.

Preparing the Application

Use the following include statement to access the C++ classes:

#include <QtPurchasing>

Before building your application, add the following statement to your .pro file to link against the Qt Purchasing library:

QT += purchasing

Registering Products

In order to allow in-app purchases in your application, register the products in your application. Start by creating an application-global instance of QInAppStore, and use the registerProduct() function to register each product.

The following example is a hypothetical role-playing game, which provides two in-app products to the user.

MyApplication::MyApplication(QObject *parent)
    : QObject(parent)
{
    m_myStore = new QInAppStore(this);
    setupConnections();

    m_myStore->registerProduct(QInAppProduct::Consumable,
                               QStringLiteral("healthPotion"));
    m_myStore->registerProduct(QInAppProduct::Unlockable,
                               QStringLiteral("dlcForestOfFooBar"));
}

As you can see, there are consumable products and unlockable products. The former can be purchased any number of times by the same user, whereas the latter can only be purchased once.

In our example, the "healthPotion" is a consumable product, because the user should be able to buy any number of health potions and add them to their in-game inventory.

The "dlcForestOfFooBar" is downloadable content, which unlocks a new part of the game, and once it is bought, the purchase should be persistent across the user's devices and across reinstallations.

Making Connections

Registering a product is an asynchronous operation, as are all operations supported by Qt Purchasing. Before you start registering a product, you must listen to the QInAppStore::productRegistered() and QInAppStore::productUnknown() signals in QInAppStore to know the status of your registration.

If the application intends to allow users to purchase products, it also needs to listen for the QInAppStore::transactionReady() signal to be notified when a transaction is pending.

void MyApplication::setupConnections()
{
    connect(m_myStore, SIGNAL(productRegistered(QInAppProduct*)),
            this, SLOT(markProductAvailable(QInAppProduct*)));
    connect(m_myStore, SIGNAL(productUnknown(QInAppProduct*)),
            this, SLOT(handleErrorGracefully(QInAppProduct*)));

    connect(m_myStore, SIGNAL(transactionReady(QInAppTransaction*)),
            this, SLOT(handleTransaction(QInAppTransaction*)));
}

Purchasing A Product

When the user wants to purchase a product, call QInAppProduct::purchase() on the product. This launches a platform-specific, asynchronous process to purchase the product, for example by requesting the user's password and confirmation of the purchase. In most cases, you must make sure that the application UI is not accepting input while the purchase request is being processed, as this is not handled automatically on all platforms.

void MyApplication::purchaseHealthPotion()
{
    QInAppProduct *product = m_myStore->registeredProduct(QStringLiteral("healthPotion"));

    // Should not get here if product is not registered
    Q_ASSERT(product != 0);

    product->purchase();
}

When this function is called, the purchase process is initiated. At some point during the process, the QInAppStore::transactionReady() signal is emitted, and the slot registered earlier is called. In this function, you can save data about a successful purchase so that it survives across application runs. After verifying that the data has been stored, finalize the transaction. If the transaction fails, display information about the failure to the user and finalize the transaction.

void MyApplication::handleTransaction(QInAppTransaction *transaction)
{
    if (transaction->status() == QInAppTransaction::PurchaseApproved
        && transaction->product()->identifier() == QStringLiteral("healthPotion")) {
        if (!hasAlreadyStoredTransaction(transaction->orderId())) {
            ++m_healthPotions;
            if (!addHealthPotionToPersistentStorage(transaction->orderId()))
                popupErrorDialog(tr("Unable to write to persistent storage. Please make sure there is sufficient space and restart."));
            else
                transaction->finalize();
        }
    } else if (transaction->status() == QInAppTransaction::PurchaseFailed) {
        popupErrorDialog(tr("Purchase not completed."));
        transaction->finalize();
    }
}

If a transaction is not finalized, the transactionReady() signal is emitted again for the same transaction the next time the product is registered, providing another chance to store the data. The transaction for a consumable product must be finalized before the product can be purchased again.

Restoring Previously Purchased Products

If the application is uninstalled and subsequently reinstalled (or installed by the same user on a different device), you must provide a way to restore the previously purchased unlockable products.

To start the process of restoring purchases, call the QInAppStore::restorePurchases() function. This emits the QInAppStore::transactionReady() signal for each of the application's unlockable products that were purchased previously by the current user. The status of these transactions will be QInAppTransaction::PurchaseRestored.

Continuing on the example from earlier, lets imagine that the game has downloadable content that you can buy to extend the game further. This must be an unlockable product, because the user need not purchase it more than once.

void MyApplication::handleTransaction(QInAppTransaction *transaction)
{
    if ((transaction->status() == QInAppTransaction::PurchaseApproved
         || transaction->status() == QInAppTransaction::PurchaseRestored)
        && transaction->product()->identifier() == QStringLiteral("dlcForestOfFooBar"))
    {

        if (!hasMap(QStringLiteral("forestOfFooBar.map")) {
            if (!downloadExtraMap(QStringLiteral("forestOfFooBar.map")))
                popupErrorDialog(tr("Unable to download The Forest of FooBar map. Please make sure there is sufficient space and restart."))
            else
                transaction->finalize()
        }

    } else if (transaction->status() == QInAppTransaction::PurchaseApproved
        && transaction->product()->identifier() == QStringLiteral("healthPotion")) {

        // ... handle healthPotion purchase

    } else {
        popupErrorDialog(tr("Purchase not completed."));
        transaction->finalize();
    }
}

If a user buys the downloadable content and later either installs the game on another device or uninstalls and reinstalls the game, you can provide a way to restore the purchase by calling QInAppStore::restorePurchases(). Purchases must be restored in response to a user input, as it may present a password dialog on some platforms.