Qt/C++ Interview Notes [Quick Reference Four] — MVC Pattern in Qt

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In software development, design patterns are tools that help make code structure clearer, more maintainable, and extensible. The MVC (Model-View-Controller) pattern is one of the classic design patterns widely used in graphical user interface (GUI) applications. Qt, as a powerful cross-platform development framework, also adopts the MVC pattern to help developers efficiently manage data, interfaces, and logic in applications.

Today, we will explain the implementation principles of the MVC pattern in Qt in an easy-to-understand way, and provide simple examples to help everyone better understand.

Qt/C++ Interview Notes [Quick Reference Four] — MVC Pattern in Qt

1. What is the MVC Pattern?

The core idea of the MVC pattern is to divide the application into three parts:

Model: Responsible for managing the application’s data and business logic.
View: Responsible for displaying data and presenting it to the user.
Controller: Handles user interactions and updates the model and view.

By separating data, interface, and control logic, the application becomes easier to manage, maintain, and extend.

2. MVC Pattern in Qt

The Qt framework implements the MVC pattern very flexibly. Qt usesQAbstractItemModel and QAbstractItemView as two core classes to help us achieve the separation of the model and view, while the responsibilities of the controller are usually handled by the developer. Next, let’s take a look at the specific roles of these core components.

2.1 Model

The model is one of the most important parts of the application, responsible for managing data and business logic. In Qt, all model classes inherit fromQAbstractItemModel. The model is not just a container for data; it also provides an interface for data operations and signals to notify the view of data changes.

For example: Suppose we want to manage a set of contact information, where each contact has attributes like name, phone number, and email. We can create a class that inherits fromQAbstractItemModel specifically to manage this contact data and provide functionalities to add, delete, and modify contacts.

2.2 View

The view’s task is to display the data from the model and present it to the user. Qt has many view classes, such asQListView, QTableView, and QTreeView, all of which inherit fromQAbstractItemView. The view retrieves data from the model and updates the UI display based on the data.

For example: Suppose our data model is a contact list (as mentioned above), the view classQTableView can be used to display the names, phones, emails, etc., of these contacts. TheQTableView will automatically update based on the model’s data.

2.3 Controller

The controller is the part that handles user interactions, typically updating the model data based on user actions and notifying the view to update the interface. In Qt, although there is no explicitQController class, this role is usually taken by custom classes, such asQWidget, QDialog, etc.

For example: Suppose the user clicks the “Add Contact” button, the controller will receive this event, update the model (add a new contact), and then notify the view to update the interface through signals.

3. How the Qt MVC Pattern Works

When we combine these components, the working principle of the MVC pattern in Qt is very simple:

User Interaction: The user performs an operation in the view (e.g., clicking a table item).
Controller Processing: The controller receives the user’s operation and updates the model’s data.
Model Notifies View: When the model’s data changes, it notifies the view through signals.
View Updates: The view automatically re-renders the interface upon receiving the signal, displaying the latest data.

In this way, the interaction between the model, view, and controller becomes very clear, greatly enhancing the maintainability and extensibility of the code.

4. Using Signals and Slots Mechanism

One of the most powerful features in Qt is the “signals and slots” mechanism, which allows different components to communicate in a loosely coupled manner. In the MVC pattern, the role of the signals and slots mechanism is particularly important, as it simplifies and enhances the interaction between the model and view.

Model Emits Signal: When the model’s data changes, it emits a signal (e.g.,dataChanged()).
View Receives Signal: The view receives the data change signal by connecting signals and slots, and automatically updates the interface.

For example: Suppose we modify a contact’s information in the contact model, the model will emit thedataChanged() signal, and the view will automatically refresh to display the new information of that contact, without the developer needing to manually update the view.

5. Simple Example: Contact List

To better understand the MVC pattern in Qt, suppose we are developing a simple “Contact Management” application.

Model:

class ContactModel : public QAbstractTableModel {
    Q_OBJECT
public:
    ContactModel(QObject *parent = nullptr) : QAbstractTableModel(parent) {}

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

    int columnCount(const QModelIndex &parent = QModelIndex()) const override {
        return 3;  // Name, Phone, Email
    }

    QVariant data(const QModelIndex &index, int role) const override {
        if (role == Qt::DisplayRole) {
            switch (index.column()) {
                case 0: return contacts[index.row()].name;
                case 1: return contacts[index.row()].phone;
                case 2: return contacts[index.row()].email;
            }
        }
        return QVariant();
    }

private:
    struct Contact {
        QString name;
        QString phone;
        QString email;
    };
    QList<Contact> contacts;
};

View:

QTableView *tableView = new QTableView;
ContactModel *model = new ContactModel;
tableView->setModel(model);
tableView->show();

Controller:

QPushButton *addButton = new QPushButton("Add Contact");
connect(addButton, &QPushButton::clicked, [=]() {
    // When the button is clicked, update the model and notify the view to update
    model->addContact("New Contact", "1234567890", "[email protected]");
});

In this example, ContactModel is the model, responsible for managing contact data; QTableView is the view, responsible for displaying data; the button click event is handled by the controller, which updates the data in the model and automatically notifies the view to refresh.

6. Summary

The MVC pattern in Qt separates data and interface throughQAbstractItemModel and QAbstractItemView classes. The role of the controller is usually played by other classes in the application (such asQWidget). Through the signals and slots mechanism, Qt makes the interaction between the model and view more flexible and efficient. With this design pattern, we can write applications that are clearly structured, easy to extend, and maintain.

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