Alternatives to QCompleter::widget() for Custom Auto-Completion in Qt

Purpose

The widget() method of QCompleter retrieves the widget that the QCompleter object is currently associated with for providing completions.
In Qt, the QCompleter class provides auto-completion functionality for widgets like QLineEdit and QComboBox.

How it Works

Creation
You create a QCompleter object, typically passing an item model (like QStringList or a custom model) to its constructor.
Association
- For QLineEdit and QComboBox, you use their respective setCompleter() methods to associate them with the QCompleter object. This association is automatically handled internally.
- For custom widgets that require auto-completion, you need to call setWidget() on the QCompleter object, explicitly setting the widget it should provide completions for.
Completion Trigger
When the user starts typing in the associated widget, the QCompleter object is notified.
Filtering and Display
Based on the entered text and the completer's configuration, it filters the items from its model and displays the filtered completions in a popup window (usually a QListView).

widget() Method

This might be useful in scenarios where you need to interact with both the QCompleter and its associated widget for advanced customization or behavior.
Once the association is established, you can use widget() on the QCompleter object to retrieve the associated widget.

Example

#include <QApplication>
#include <QLineEdit>
#include <QStringList>
#include <QCompleter>

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

    // Create a list of completion options
    QStringList options = QStringList() << "apple" << "banana" << "cherry";

    // Create the QCompleter object and set the completion model
    QCompleter *completer = new QCompleter(options);

    // Create a QLineEdit and associate it with the completer
    QLineEdit *lineEdit = new QLineEdit;
    lineEdit->setCompleter(completer);

    // ... (rest of your application code)

    // Later, if needed:
    QWidget *associatedWidget = completer->widget();
    // Now you have access to both the completer and its widget
    // (e.g., for custom behavior based on the widget state)

    return app.exec();
}

For more advanced scenarios, you might combine widget() with other QCompleter methods like setPopup() to customize the completion popup's behavior.
It's not strictly necessary for standard QLineEdit and QComboBox usage.
widget() is primarily used for custom widgets that require auto-completion.

Custom Completion Widget (Extending QLineEdit)

This example shows how to create a custom widget that inherits from QLineEdit and uses a QCompleter for auto-completion:

#include <QApplication>
#include <QLineEdit>
#include <QStringList>
#include <QCompleter>
#include <QHBoxLayout>

class MyLineEdit : public QLineEdit {
    Q_OBJECT

public:
    MyLineEdit(QWidget *parent = nullptr);

private slots:
    void textChanged(const QString &text);

private:
    QCompleter *completer;
};

MyLineEdit::MyLineEdit(QWidget *parent) : QLineEdit(parent) {
    // Create a list of completion options
    QStringList options = QStringList() << "apple" << "banana" << "cherry";

    // Create the QCompleter object and set the completion model
    completer = new QCompleter(options);

    // Set this custom widget as the completer's widget
    completer->setWidget(this);

    // Connect textChanged signal to update completions
    connect(this, &MyLineEdit::textChanged, this, &MyLineEdit::textChanged);
}

void MyLineEdit::textChanged(const QString &text) {
    // Update the completer's filtering based on the entered text
    completer->setCompletionPrefix(text);
    completer->complete(); // Trigger completion popup update
}

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

    MyLineEdit *lineEdit = new MyLineEdit;
    lineEdit->show();

    return app.exec();
}

Dynamic Completion Source (Using a Custom Model)

This example demonstrates associating a QCompleter with a custom model that provides completions on-the-fly:

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

class MyModel : public QAbstractListModel {
    Q_OBJECT

public:
    MyModel(QObject *parent = nullptr);
    void addEntry(const QString &entry);

    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override;

private:
    QStringList entries;
};

MyModel::MyModel(QObject *parent) : QAbstractListModel(parent) {}

void MyModel::addEntry(const QString &entry) {
    beginInsertRows(QModelIndex(), entries.size(), entries.size());
    entries.append(entry);
    endInsertRows();
}

QVariant MyModel::data(const QModelIndex &index, int role) const override {
    if (role == Qt::DisplayRole && index.isValid()) {
        return entries.at(index.row());
    }
    return QVariant();
}

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

    // Create a custom model
    MyModel *model = new MyModel;

    // Add some initial entries
    model->addEntry("apple");
    model->addEntry("banana");

    // Create the QCompleter object and set the custom model
    QCompleter *completer = new QCompleter(model);

    // Create a QLineEdit and associate it with the completer
    QLineEdit *lineEdit = new QLineEdit;
    lineEdit->setCompleter(completer);

    // ... (rest of your application code)

    // Later, you can access the widget using completer->widget()

    return app.exec();
}

Signals and Slots

#include <QApplication>
#include <QLineEdit>
#include <QStringList>
#include <QCompleter>

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

    // Create a list of completion options
    QStringList options = QStringList() << "apple" << "banana" << "cherry";

    // Create the QCompleter object and set the completion model
    QCompleter *completer = new QCompleter(options);

    // Create a QLineEdit
    QLineEdit *lineEdit = new QLineEdit;

    // Connect the completer's activated signal to a slot that modifies the line edit's text
    connect(completer, &QCompleter::activated, lineEdit, &QLineEdit::setText);

    // Associate the completer with the line edit
    lineEdit->setCompleter(completer);

    // ... (rest of your application code)

    return app.exec();
}

In this example, the activated signal of completer is connected to a slot that sets the text of the lineEdit when a completion is chosen. This approach avoids needing to directly access the widget through widget().

Subclassing QCompleter (Advanced)

For more advanced control over the auto-completion behavior, you can subclass QCompleter. This allows you to override methods or introduce new signals/slots to interact with the associated widget directly within the completer itself.

Third-Party Libraries

While less common, there might be third-party Qt libraries that provide alternative auto-completion solutions with their own mechanisms for interacting with the associated widget.

Third-party libraries should be evaluated based on their features and community support.
If you need more granular control or custom behavior, subclassing QCompleter might be considered.
For most standard use cases with QLineEdit or QComboBox, using signals and slots is often sufficient.