Beyond QWindow::transientParent: Alternative Window Management in Qt

Purpose

• Influences how the window manager displays the child window relative to the parent window.
• Establishes a relationship between two windows in Qt, specifically a child window (the "transient window") and its parent window.

Behavior

• When a child window has a transient parent set, the window manager typically:
  • Positions the child window near or on top of the parent window.
  • Disables or dims the parent window while the child window is active (depending on the window manager's behavior).
  • Prevents the user from interacting with other windows until the child window is closed or hidden.

Common Use Cases

• Splash Screens and Progress Dialogs
  While not strictly modal, these child windows with transient parents can help indicate the application's startup or progress without completely obscuring the main window.
• Tooltips and Popups
  These are typically child windows with transient parents, often appearing near or next to the element they provide information about (e.g., a tooltip next to a button).
• Modal Dialogs
  A classic example is a modal dialog box used for confirmation, input, or error messages. The modal dialog is a child window with a transient parent (often the main application window). When the dialog is open, the user cannot interact with the main window until they close the dialog.

Setting the Transient Parent

#include <QtWidgets>

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

    // Create the parent window
    QWidget *parentWindow = new QWidget();
    parentWindow->show();

    // Create the child window (e.g., a modal dialog)
    QDialog *childWindow = new QDialog(parentWindow, Qt::WindowCloseButtonHint);  // Set flags as needed

    // Set the transient parent for the child window
    childWindow->setWindowModality(Qt::ApplicationModal);  // Optionally set modality

    childWindow->show();

    return app.exec();
}

In this example, childWindow is set to have parentWindow as its transient parent. The setWindowModality call (optional) further controls how the user interacts with other windows while childWindow is open.

• Use transientParent appropriately to create a user-friendly experience that guides the user's focus and interaction within your Qt application.
• The specific behavior of transient windows can vary depending on the platform and window manager in use. Qt provides some control through setWindowModality, but the ultimate behavior is determined by the window manager.

---

Modal Dialog with Transient Parent (Basic)

#include <QtWidgets>

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

    // Create the parent window
    QWidget *parentWindow = new QWidget();
    parentWindow->resize(400, 300);
    parentWindow->setWindowTitle("Parent Window");
    parentWindow->show();

    // Create the child window (modal dialog)
    QDialog *childWindow = new QDialog(parentWindow, Qt::WindowCloseButtonHint);
    childWindow->setWindowTitle("Modal Dialog");
    childWindow->resize(200, 150);
    QVBoxLayout *layout = new QVBoxLayout(childWindow);
    QLabel *label = new QLabel("This is a modal dialog");
    QPushButton *closeButton = new QPushButton("Close");
    layout->addWidget(label);
    layout->addWidget(closeButton);
    QObject::connect(closeButton, &QPushButton::clicked, childWindow, &QDialog::close);
    childWindow->setLayout(layout);

    // Set the transient parent and show the dialog modally
    childWindow->setWindowModality(Qt::ApplicationModal);
    childWindow->show();

    return app.exec();
}

This example creates a parent window and a modal dialog that pops up on top of it, preventing interaction with the parent window until the dialog is closed.

Non-Modal Dialog with Transient Parent (Tooltip-like)

#include <QtWidgets>

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

    // Create the parent window
    QWidget *parentWindow = new QWidget();
    parentWindow->resize(400, 300);
    parentWindow->setWindowTitle("Parent Window");
    QLabel *label = new QLabel("Hover over the button below for a tooltip-like message", parentWindow);
    label->setGeometry(100, 100, 200, 50);
    parentWindow->show();

    // Create the child window with information
    QDialog *infoWindow = new QDialog(parentWindow, Qt::FramelessWindowHint); // Remove unnecessary window decorations
    infoWindow->setWindowTitle("Tooltip Information");
    infoWindow->resize(150, 100);
    QVBoxLayout *layout = new QVBoxLayout(infoWindow);
    QLabel *infoLabel = new QLabel("This is additional information about the button");
    layout->addWidget(infoLabel);
    infoWindow->setLayout(layout);
    infoWindow->hide(); // Initially hide the window

    // Connect button hover to show/hide the child window
    QPushButton *button = new QPushButton("Hover me!", parentWindow);
    button->setGeometry(150, 150, 100, 50);
    QObject::connect(button, &QPushButton::enterEvent, [=]() {
        infoWindow->move(button->mapToGlobal(button->rect().bottomLeft())); // Position near button
        infoWindow->show();
    });
    QObject::connect(button, &QPushButton::leaveEvent, infoWindow, &QDialog::hide);

    parentWindow->show();

    return app.exec();
}

This example demonstrates a non-modal dialog that appears near a button on hover, behaving like a tooltip that stays within the parent window.

---

Stacked Widget Approach (for Modal-like Windows)

• This approach offers more flexibility in terms of custom layout and interaction within the main window.
• Show/hide these child widgets based on user interaction to create a modal-like behavior.
• Each child widget can represent a different "state" or "mode" of your application.
• Use QStackedWidget to manage a stack of child widgets within your main window.

QMainWindow::centralWidget or Other Containers

• Show/hide these child widgets to create a temporary "modal-like" experience where the focus is visually directed to a specific area of the main window.
• Utilize containers within your main window (e.g., QWidget, QVBoxLayout, QHBoxLayout) to manage visibility and stacking order of child widgets.

setWindowFlags for Basic Control

• This can be useful for creating simple tooltip-like windows or basic popups.
• Employ setWindowFlags on your child window to control basic aspects like window decorations (borders, title bar) and stacking behavior.

Choosing the Right Approach

The best alternative depends on your desired functionality:

• For basic popups or tooltips that don't require full modality, setWindowFlags might suffice.
• For more complex scenarios where you want to manage a stack of views within the main window while showing/hiding specific areas, consider QStackedWidget or containers within your main window.
• If you need a strictly modal window that completely blocks user interaction with the rest of the application, QWindow::transientParent is generally the recommended approach.

• Flexibility
  Containers within the main window offer greater flexibility in positioning and customizing child widgets.
• Code Complexity
  QStackedWidget and layouts might require more code to manage stacking and visibility compared to transientParent.