Qt Widgets: Customizing QCompleter Suggestion Popup Behavior

Purpose

• The QCompleter::popup() function plays a crucial role in this process by returning a pointer to the underlying widget that displays the completion suggestions. This widget is typically a QListView instance.
• In Qt, the QCompleter class is designed to provide auto-completion functionality for widgets like QLineEdit and QComboBox.

Functionality

• You can use QCompleter::popup() to retrieve a reference to this popup widget, allowing you to perform certain customizations or manipulations.
• QCompleter then creates a popup widget (usually a QListView) to display these suggestions.
• When the user starts typing in the associated widget (e.g., QLineEdit), QCompleter analyzes the entered text and generates a list of potential completions based on its model (usually a QStringListModel).

Customization and Manipulations (Optional)

• While QCompleter generally manages the popup widget's behavior, you might have specific requirements:
  • Controlling Position
    The default behavior positions the popup below the associated widget. You can't directly influence this placement using popup(), but you might be able to achieve custom positioning by subclassing QCompleter and overriding relevant methods.
  • Focus Management
    By default, the popup can take focus, potentially interfering with user interaction. To prevent this, you can call setFocusPolicy(Qt::NoFocus) on the retrieved popup widget using popup()->setFocusPolicy(Qt::NoFocus).
  • Appearance Customization
    Although not directly related to popup(), you can customize the look of the suggestions list using stylesheets or subclassing QAbstractItemView (the base class for QListView).

Things to Keep in Mind

• The specific type of popup widget used by QCompleter might vary depending on the Qt version and configuration. However, it's usually a QListView.
• QCompleter::popup() returns a read-only pointer. You cannot modify the popup widget's structure through this function.

• The core functionality of creating, displaying, and managing the popup is handled by QCompleter itself.
• This allows for limited customization of the popup's behavior, such as managing focus or potentially customizing appearance (through other means).
• QCompleter::popup() provides access to the popup widget that displays completion suggestions in Qt Widgets' QCompleter class.

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#include <QApplication>
#include <QLineEdit>
#include <QStringListModel>
#include <QCompleter>

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

  // Sample data for completions
  QStringList list;
  list << "apple" << "banana" << "cherry" << "date";

  // Create a line edit and model
  QLineEdit lineEdit;
  QStringListModel model(list);

  // Create a completer and set the model
  QCompleter completer(&model);
  lineEdit.setCompleter(&completer);

  // Connect the completer's activated signal to a slot (optional)
  QObject::connect(&completer, &QCompleter::activated, &lineEdit, &QLineEdit::setText);

  // Access the popup widget (usually a QListView) and prevent focus
  QAbstractItemView* popup = completer.popup();
  if (popup) {
    popup->setFocusPolicy(Qt::NoFocus);
  }

  lineEdit.show();

  return app.exec();
}

1. Include necessary headers
   QApplication, QLineEdit, QStringListModel, and QCompleter.
2. Create sample data and model
   A QStringList holds potential completions, and a QStringListModel is created to provide the data to the QCompleter.
3. Create line edit and completer
   A QLineEdit is created, and a QCompleter is instantiated with the QStringListModel. The lineEdit is then associated with the completer.
4. (Optional) Connect activation signal
   Here, we connect the activated signal of the completer to the setText slot of the lineEdit. This automatically sets the selected suggestion in the line edit when an item is activated.
5. Access and manipulate popup
   We call completer.popup() to get a pointer to the popup widget. If it exists (usually a QListView), we set its focusPolicy to Qt::NoFocus to prevent it from taking focus when displayed.
6. Show line edit
   Finally, the lineEdit is displayed on the screen.

• For more advanced customizations, consider subclassing QCompleter and overriding relevant methods.
• The exact type of popup widget might vary between Qt versions.
• This code demonstrates a basic usage of QCompleter::popup().

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Subclassing QCompleter and Overriding Methods

• This approach offers greater flexibility but requires more effort and understanding of the internal workings of QCompleter.
• For example, you could override createModelPopup() to create a custom widget instead of the default one.
• If you need more control over the popup's behavior or appearance, you can subclass QCompleter and override specific methods.

Inline Completion Mode (Limited Customization)

• This eliminates the need for a separate popup, but it also limits customization options.
• QCompleter provides a setCompletionMode() function. Setting it to QCompleter::InlineCompletion displays suggestions directly in the associated widget (e.g., QLineEdit) as the user types.

Customizing Suggestion List Appearance (Indirect)

• Stylesheets offer a declarative way to style widgets, while subclassing provides more control but requires a deeper understanding of Qt's widget hierarchy.
• Use Qt Stylesheets or subclass QAbstractItemView (the base class for QListView commonly used by QCompleter) to achieve this.
• While you can't directly modify the popup widget using QCompleter::popup(), you can customize the appearance of the suggestion list indirectly.

Choosing the Right Approach

The best approach depends on your specific needs:

• If you want to avoid the popup altogether, use the InlineCompletion mode.
• If you need to completely replace the default popup widget, subclassing QCompleter is necessary.
• For minor appearance tweaks, consider stylesheets.

• Inline completion might not be suitable if you have a large number of suggestions or if you need advanced filtering or interaction with the suggestions.
• Subclassing QCompleter can be complex, so consider alternative approaches if they fulfill your requirements.