Detailed Description

The QDialog class is the base class of dialog windows.

A dialog window is a top-level window mostly used for short-term tasks and brief communications with the user. QDialogs may be modal or modeless. QDialogs can provide a return value, and they can have default buttons. QDialogs can also have a QSizeGrip in their lower-right corner, using setSizeGripEnabled().

Note that QDialog (and any other widget that has type Qt::Dialog) uses the parent widget slightly differently from other classes in Qt. A dialog is always a top-level widget, but if it has a parent, its default location is centered on top of the parent's top-level widget (if it is not top-level itself). It will also share the parent's taskbar entry.

Use the overload of the QWidget::setParent() function to change the ownership of a QDialog widget. This function allows you to explicitly set the window flags of the reparented widget; using the overloaded function will clear the window flags specifying the window-system properties for the widget (in particular it will reset the Qt::Dialog flag).

Modal Dialogs

A modal dialog is a dialog that blocks input to other visible windows in the same application. Dialogs that are used to request a file name from the user or that are used to set application preferences are usually modal. Dialogs can be application modal (the default) or window modal.

When an application modal dialog is opened, the user must finish interacting with the dialog and close it before they can access any other window in the application. Window modal dialogs only block access to the window associated with the dialog, allowing the user to continue to use other windows in an application.

The most common way to display a modal dialog is to call its exec() function. When the user closes the dialog, exec() will provide a useful return value. Typically, to get the dialog to close and return the appropriate value, we connect a default button, e.g. OK, to the accept() slot and a Cancel button to the reject() slot. Alternatively you can call the done() slot with Accepted or Rejected.

An alternative is to call setModal(true) or setWindowModality(), then show(). Unlike exec(), show() returns control to the caller immediately. Calling setModal(true) is especially useful for progress dialogs, where the user must have the ability to interact with the dialog, e.g. to cancel a long running operation. If you use show() and setModal(true) together to perform a long operation, you must call QApplication::processEvents() periodically during processing to enable the user to interact with the dialog. (See QProgressDialog.)

Modeless Dialogs

A modeless dialog is a dialog that operates independently of other windows in the same application. Find and replace dialogs in word-processors are often modeless to allow the user to interact with both the application's main window and with the dialog.

Modeless dialogs are displayed using show(), which returns control to the caller immediately.

If you invoke the show() function after hiding a dialog, the dialog will be displayed in its original position. This is because the window manager decides the position for windows that have not been explicitly placed by the programmer. To preserve the position of a dialog that has been moved by the user, save its position in your closeEvent() handler and then move the dialog to that position, before showing it again.

Default Button

A dialog's default button is the button that's pressed when the user presses Enter (Return). This button is used to signify that the user accepts the dialog's settings and wants to close the dialog. Use QPushButton::setDefault(), QPushButton::isDefault() and QPushButton::autoDefault() to set and control the dialog's default button.

Escape Key

If the user presses the Esc key in a dialog, QDialog::reject() will be called. This will cause the window to close: The close event cannot be ignored.

Extensibility

Extensibility is the ability to show the dialog in two ways: a partial dialog that shows the most commonly used options, and a full dialog that shows all the options. Typically an extensible dialog will initially appear as a partial dialog, but with a More toggle button. If the user presses the More button down, the dialog is expanded. The Extension Example shows how to achieve extensible dialogs using Qt.

Return Value (Modal Dialogs)

Modal dialogs are often used in situations where a return value is required, e.g. to indicate whether the user pressed OK or Cancel. A dialog can be closed by calling the accept() or the reject() slots, and exec() will return Accepted or Rejected as appropriate. The exec() call returns the result of the dialog. The result is also available from result() if the dialog has not been destroyed.

In order to modify your dialog's close behavior, you can reimplement the functions accept(), reject() or done(). The closeEvent() function should only be reimplemented to preserve the dialog's position or to override the standard close or reject behavior.

Code Examples

A modal dialog:

void EditorWindow::countWords()
{
    WordCountDialog dialog(this);
    dialog.setWordCount(document().wordCount());
    dialog.exec();
}

A modeless dialog:

void EditorWindow::find()
{
    if (!findDialog) {
        findDialog = new FindDialog(this);
        connect(findDialog, SIGNAL(findNext()), this, SLOT(findNext()));
    }

    findDialog->show();
    findDialog->raise();
    findDialog->activateWindow();
}