Peeking Inside Qt: Exploring QGuiApplication::topLevelWindows()

Purpose

• These windows are typically the main application window, any dialogs, or other windows that are not children (sub-windows) of another window.
• This function retrieves a list of all top-level windows currently managed by the Qt application.

Functionality

1. • It internally accesses a private list of windows maintained by the QGuiApplication class.

2. Filtering Top-Level Windows
   

   • It iterates through the complete list, checking each window for the following criteria:
     • No Parent
       The window must not have a parent window, indicating it's at the top level in the window hierarchy.
     • Not Desktop Type
       The window's type should not be Qt::Desktop. This excludes the desktop window itself, which isn't typically considered a top-level application window.
     • Embedded QAxServer Handling (Optional)
       
       • In some Qt versions, additional logic might filter out top windows of embedded QAxServers that don't have QWindow parents but aren't true top-level windows.

3. Returning the List
   

   • After filtering, it creates a new QWindowList object containing only the qualified top-level windows.
   • This list is returned to your code for further processing.

Usage

• The returned QWindowList object provides various methods to interact with the top-level windows:
  • size(): Get the number of windows in the list.
  • at(int index): Access a specific window by its index in the list.
  • Iterate through the list using a loop to access each window individually.
• You can call QGuiApplication::topLevelWindows() from anywhere in your Qt application code.

Example

#include <QApplication>
#include <QMainWindow>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    // Create a main window
    QMainWindow window;
    window.show();

    // Get the list of top-level windows (including the main window)
    QWindowList topLevelWindows = QGuiApplication::topLevelWindows();

    // Loop through and print the window titles (if they have titles)
    for (int i = 0; i < topLevelWindows.size(); ++i) {
        QWindow* window = topLevelWindows.at(i);
        if (window->windowTitle().isEmpty()) {
            qDebug() << "Window " << i+1 << " has no title.";
        } else {
            qDebug() << "Window " << i+1 << " title:" << window->windowTitle();
        }
    }

    return app.exec();
}

Key Points

• Consider alternative approaches if you need to interact with specific windows based on their type or other characteristics.
• Be cautious when modifying top-level windows directly using this list, as it might bypass your application's intended window management logic.
• This function is useful for programmatic interactions with top-level windows that might not be directly accessible through your code's structure.

---

Activating a Specific Top-Level Window

#include <QApplication>
#include <QMainWindow>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    // Create two main windows
    QMainWindow window1;
    window1.setWindowTitle("Window 1");
    window1.show();

    QMainWindow window2;
    window2.setWindowTitle("Window 2");
    window2.hide(); // Initially hidden

    // Get the list of top-level windows
    QWindowList topLevelWindows = QGuiApplication::topLevelWindows();

    // Find the window with the title "Window 2" and activate it
    for (int i = 0; i < topLevelWindows.size(); ++i) {
        QWindow* window = topLevelWindows.at(i);
        if (window->windowTitle() == "Window 2") {
            window->activateWindow();
            break;
        }
    }

    return app.exec();
}

Checking if a Specific Window Exists

#include <QApplication>
#include <QDialog>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    // Create a main window
    QMainWindow window;
    window.show();

    // Open a dialog
    QDialog dialog;
    dialog.setWindowTitle("My Dialog");
    dialog.show();

    // Get the list of top-level windows
    QWindowList topLevelWindows = QGuiApplication::topLevelWindows();

    // Check if the dialog with title "My Dialog" exists
    bool dialogFound = false;
    for (int i = 0; i < topLevelWindows.size(); ++i) {
        QWindow* window = topLevelWindows.at(i);
        if (window->windowTitle() == "My Dialog") {
            dialogFound = true;
            break;
        }
    }

    if (dialogFound) {
        qDebug() << "Dialog with title 'My Dialog' found.";
    } else {
        qDebug() << "Dialog with title 'My Dialog' not found.";
    }

    return app.exec();
}

#include <QApplication>
#include <QMainWindow>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    // Create a main window (don't close this one)
    QMainWindow mainWindow;
    mainWindow.setWindowTitle("Main Window");
    mainWindow.show();

    // Create other top-level windows
    QMainWindow window1;
    window1.setWindowTitle("Window 1");
    window1.show();

    QDialog dialog;
    dialog.setWindowTitle("My Dialog");
    dialog.show();

    // Get the list of top-level windows
    QWindowList topLevelWindows = QGuiApplication::topLevelWindows();

    // Close all windows except the main window
    for (int i = 0; i < topLevelWindows.size(); ++i) {
        QWindow* window = topLevelWindows.at(i);
        if (window->windowTitle() != "Main Window") {
            window->close();
        }
    }

    return app.exec();
}

---

Pointers or References to Specific Windows

• This is ideal when you know the structure of your application and the windows you want to work with.
• If you have direct pointers or references to the specific windows you need to interact with, it's the most efficient and controlled approach.

Example

#include <QApplication>
#include <QMainWindow>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    // Create a main window
    QMainWindow* mainWindow = new QMainWindow;
    mainWindow->setWindowTitle("Main Window");
    mainWindow->show();

    // Interact with the main window directly using the pointer
    mainWindow->raise(); // Bring the window to the foreground

    return app.exec();
}

Signals and Slots

• This promotes loose coupling and separation of concerns in your application.
• Connect signals emitted by the target windows to slots in your code that perform the desired actions.
• If you need to react to events or interactions happening within specific top-level windows, Qt's signals and slots mechanism provides a clean and decoupled way to achieve this.

Example

#include <QApplication>
#include <QMainWindow>
#include <QPushButton>

class MyWindow : public QMainWindow {
    Q_OBJECT

public:
    MyWindow(QWidget* parent = nullptr) : QMainWindow(parent) {
        // ... (window setup)
        connect(closeButton, &QPushButton::clicked, this, &MyWindow::onCloseButtonClicked);
    }

signals:
    void closeButtonClicked();

private slots:
    void onCloseButtonClicked() {
        close(); // Close the window in response to button click
    }

    QPushButton* closeButton = new QPushButton("Close");
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    // Create a main window
    MyWindow window;
    window.show();

    return app.exec();
}

Child-Parent Relationship

• This approach is useful when you know the nesting structure of your windows.
• If you have a hierarchical window structure where top-level windows create child windows, you can iterate through the child window hierarchy to find specific windows or perform operations on all child windows.

#include <QApplication>
#include <QMainWindow>
#include <QDialog>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    // Create a main window
    QMainWindow mainWindow;
    mainWindow.show();

    // Create a child dialog
    QDialog* dialog = new QDialog(&mainWindow);
    dialog->setWindowTitle("My Dialog");
    dialog->show();

    // Access child windows of the main window
    QList<QWidget*> childWindows = mainWindow.findChildren<QWidget*>();
    for (QWidget* child : childWindows) {
        if (child == dialog) {
            // Do something specific with the dialog window
            break;
        }
    }

    return app.exec();
}