**Course Title:** Qt 6 Application Development with C++ **Section Title:** Model-View-Controller (MVC) Architecture in Qt **Topic:** Working with custom models and proxy models **Introduction** In the previous topics, we explored the basics of the Model-View-Controller (MVC) pattern in Qt and learned how to use built-in data models such as `QAbstractListModel` and `QAbstractTableModel`. However, sometimes you may need to create custom data models that fit your specific use case. In this topic, we will delve into the world of custom models and proxy models in Qt, and learn how to create and use them effectively. **What are custom models?** Custom models are data models that are tailored to specific use cases or applications. They can provide additional functionality or data structures that are not available in the standard Qt data models. Custom models are typically derived from the `QAbstractItemModel` class, which provides the base functionality for data models in Qt. **Why use custom models?** Custom models are useful when you need to: * Provide a custom data structure that doesn't fit into the standard table, list, or tree structure * Add additional functionality to the data model, such as data validation or business logic * Integrate with existing data sources or libraries **Creating a custom model** To create a custom model, you need to derive a class from `QAbstractItemModel` and implement the following abstract methods: * `index()`: returns a `QModelIndex` for a given row, column, and parent index * `parent()`: returns the parent `QModelIndex` for a given index * `rowCount()`: returns the number of rows in the model * `columnCount()`: returns the number of columns in the model * `data()`: returns the data for a given index * `setData()`: sets the data for a given index Here is an example of a simple custom model that stores a list of strings: ```cpp class CustomListModel : public QAbstractListModel { Q_OBJECT public: CustomListModel(QObject *parent = nullptr) : QAbstractListModel(parent) {} // Implement the abstract methods QModelIndex index(int row, int column, const QModelIndex &parent) const override { if (column == 0 && row < mList.size()) { return createIndex(row, column); } return QModelIndex(); } QModelIndex parent(const QModelIndex &index) const override { Q_UNUSED(index); return QModelIndex(); } int rowCount(const QModelIndex &parent = QModelIndex()) const override { Q_UNUSED(parent); return mList.size(); } int columnCount(const QModelIndex &parent = QModelIndex()) const override { Q_UNUSED(parent); return 1; } QVariant data(const QModelIndex &index, int role) const override { if (index.isValid() && role == Qt::DisplayRole) { return mList.at(index.row()); } return QVariant(); } bool setData(const QModelIndex &index, const QVariant &value, int role = Qt::EditRole) override { if (index.isValid() && role == Qt::EditRole) { mList.replace(index.row(), value.toString()); emit dataChanged(index, index); return true; } return false; } private: QStringList mList; }; ``` **What are proxy models?** Proxy models are models that sit on top of another model, transforming or filtering the data in some way. Proxy models are typically used to: * Filter out certain data * Sort or reorganize the data * Transform the data in some way **Using QSortFilterProxyModel** One common use case for proxy models is to filter or sort data. Qt provides a built-in proxy model called `QSortFilterProxyModel` that can be used to filter and sort data. Here is an example of how to use `QSortFilterProxyModel` to filter a list of strings: ```cpp // Create the original model CustomListModel *model = new CustomListModel(); // Create the proxy model QSortFilterProxyModel *proxy = new QSortFilterProxyModel(this); proxy->setSourceModel(model); // Set the filter string QString filter = "hello"; proxy->setFilterRegExp(filter); // Use the proxy model in a view QListView *view = new QListView(); view->setModel(proxy); ``` **Conclusion** In this topic, we learned how to create custom models and proxy models in Qt. Custom models are useful for creating tailored data structures or adding additional functionality to existing models. Proxy models are used to transform or filter data in some way. We also explored the built-in `QSortFilterProxyModel` class, which can be used to filter and sort data. **What's next?** In the next topic, we will explore Qt's styling system and Qt Stylesheets (QSS). **Do you have any questions or need help with this topic?** Please leave a comment below or ask for help. **Additional Resources:** * Qt documentation: [Model-View Programming](https://doc.qt.io/qt-6/model-view-programming.html) * Qt documentation: [QAbstractItemModel](https://doc.qt.io/qt-6/qabstractitemmodel.html) * Qt documentation: [QSortFilterProxyModel](https://doc.qt.io/qt-6/qsortfilterproxymodel.html)

class CustomListModel : public QAbstractListModel
{
    Q_OBJECT

public:
    CustomListModel(QObject *parent = nullptr) : QAbstractListModel(parent) {}

    // Implement the abstract methods
    QModelIndex index(int row, int column, const QModelIndex &parent) const override
    {
        if (column == 0 && row < mList.size())
        {
            return createIndex(row, column);
        }
        return QModelIndex();
    }

    QModelIndex parent(const QModelIndex &index) const override
    {
        Q_UNUSED(index);
        return QModelIndex();
    }

    int rowCount(const QModelIndex &parent = QModelIndex()) const override
    {
        Q_UNUSED(parent);
        return mList.size();
    }

    int columnCount(const QModelIndex &parent = QModelIndex()) const override
    {
        Q_UNUSED(parent);
        return 1;
    }

    QVariant data(const QModelIndex &index, int role) const override
    {
        if (index.isValid() && role == Qt::DisplayRole)
        {
            return mList.at(index.row());
        }
        return QVariant();
    }

    bool setData(const QModelIndex &index, const QVariant &value, int role = Qt::EditRole) override
    {
        if (index.isValid() && role == Qt::EditRole)
        {
            mList.replace(index.row(), value.toString());
            emit dataChanged(index, index);
            return true;
        }
        return false;
    }

private:
    QStringList mList;
};

// Create the original model
CustomListModel *model = new CustomListModel();

// Create the proxy model
QSortFilterProxyModel *proxy = new QSortFilterProxyModel(this);
proxy->setSourceModel(model);

// Set the filter string
QString filter = "hello";
proxy->setFilterRegExp(filter);

// Use the proxy model in a view
QListView *view = new QListView();
view->setModel(proxy);