Alternatives to QCompleter::completionRole for Fine-Tuning Autocompletion in Qt

QCompleter and Autocompletion

• When the user types in a widget associated with a QCompleter, the completer suggests matching options from a data source (like a string list or model) as they type.
• QCompleter is a class in Qt Widgets that provides autocompletion functionality for various widgets like QLineEdit and QComboBox.

completionRole Property

• By setting the completionRole, you specify which role's data will be used for the completions.
• Qt data models often have multiple roles associated with each item. This allows storing different types of data for each item (e.g., display text, internal ID, etc.).
• The completionRole property of QCompleter determines which data role within the data source (model or string list) is used to populate the completion suggestions.

Example with a QAbstractItemModel

#include <QApplication>
#include <QLineEdit>
#include <QCompleter>
#include <QStandardItemModel>

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

    // Create a model with two roles: display role (Qt::DisplayRole) and a custom ID role
    QStandardItemModel model(3, 2);
    model.setDataModel(Qt::DisplayRole, QStringList() << "Item 1" << "Item 2" << "Item 3");
    model.setData(model.index(0, 1), QVariant(10), Qt::UserRole + 1); // Custom role for ID
    model.setData(model.index(1, 1), QVariant(20), Qt::UserRole + 1);
    model.setData(model.index(2, 1), QVariant(30), Qt::UserRole + 1);

    // Create a QLineEdit and a QCompleter
    QLineEdit lineEdit;
    QCompleter completer(&model, &lineEdit);

    // Set the completion role to the custom ID role (Qt::UserRole + 1)
    completer.setCompletionRole(Qt::UserRole + 1);

    // Connect the completer's activated signal to a slot (optional)
    QObject::connect(&completer, &QCompleter::activated, [](const QString &text) {
        // Handle selection of a completion suggestion (e.g., display a message)
        qDebug() << "Selected completion:" << text;
    });

    lineEdit.show();

    return app.exec();
}

In this example, the completionRole is set to Qt::UserRole + 1, indicating that the completer should use the data stored in the custom role (containing item IDs) for suggestions.

• This property is especially useful when working with QAbstractItemModels.
• You can use custom roles to provide more context in the completions.
• The default value is Qt::DisplayRole, which is commonly used for displaying text in views.
• Use completionRole to specify which data within the data source is used for autocompletions.

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Using a String List with a Custom Role

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

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

    // Create a string list with a custom role for storing additional data
    QStringList list;
    list << "Apple (Fruit)" << "Banana (Fruit)" << "Orange (Fruit)";

    // Create a QCompleter and set the completion role to a custom index (1)
    QCompleter completer(list);
    completer.setCompletionRole(1); // Assuming additional data is stored after the display text

    QLineEdit lineEdit;
    lineEdit.setCompleter(&completer);

    lineEdit.show();

    return app.exec();
}

In this example, the string list stores the display text followed by additional information in parentheses. The completionRole is set to 1, indicating that the completer should use the second part of the string (after the space) for suggestions.

Using Qt::UserRole for Internal Data

#include <QApplication>
#include <QLineEdit>
#include <QCompleter>
#include <QStandardItemModel>

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

    // Create a model with display role and a custom role for storing file paths
    QStandardItemModel model(3, 2);
    model.setDataModel(Qt::DisplayRole, QStringList() << "Image1.jpg" << "Image2.png" << "Image3.bmp");
    model.setData(model.index(0, 1), QVariant("path/to/Image1.jpg"), Qt::UserRole + 1);
    model.setData(model.index(1, 1), QVariant("path/to/Image2.png"), Qt::UserRole + 1);
    model.setData(model.index(2, 1), QVariant("path/to/Image3.bmp"), Qt::UserRole + 1);

    QLineEdit lineEdit;
    QCompleter completer(&model, &lineEdit);

    // Set the completion role to the custom role containing file paths
    completer.setCompletionRole(Qt::UserRole + 1);

    lineEdit.show();

    return app.exec();
}

This example builds upon the previous one, using a QStandardItemModel to store image names and their corresponding file paths. The completionRole is set to Qt::UserRole + 1 to use the file paths for suggestions.

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Customizing the Completion Model

• This approach offers more flexibility but requires writing more code compared to using completionRole.
• Override methods like data() and rowCount() to provide the desired data and filtering behavior for the completer.
• If you have more complex logic for filtering or displaying suggestions, you can create a custom model that inherits from QAbstractItemModel.

Lambda Function with QCompleter::setCompletionFilter

• Offers more control over filtering logic compared to just using a role, but might be less readable for simpler scenarios.
• The lambda function can return true to include the item in the suggestions or false to exclude it.
• This method takes a lambda function that receives the current completion prefix and the item data as arguments.
• If you need to filter suggestions based on more than just the data in a specific role, you can use QCompleter::setCompletionFilter().

Third-Party Autocompletion Libraries

• These libraries often provide more customization options but might come with additional dependencies and learning curves.
• For advanced autocompletion features like fuzzy matching, highlighting, or integration with specific data sources, you can explore third-party Qt-compatible libraries.

Choosing the Right Approach

The best alternative depends on your specific requirements:

• For advanced features, consider third-party libraries.
• If you need more control over filtering or displaying suggestions, a custom model or lambda filter might be appropriate.
• For basic customization based on a single data field, completionRole is often sufficient.

Approach	Description	Advantages	Disadvantages
QCompleter::completionRole	Use a specific role in the data source for suggestions.	Simple to use, works with various data sources.	Limited customization for filtering or displaying suggestions.
Custom Model	Create a custom QAbstractItemModel subclass.	Full control over data and filtering logic.	More complex to implement compared to completionRole.
Lambda Filter with setCompletionFilter	Filter suggestions based on a custom function.	Flexible filtering logic beyond data roles.	Less readable than completionRole for simpler cases.
Third-Party Libraries	Use external libraries for advanced autocompletion features.	Powerful features, easier for complex scenarios.	Adds dependencies, might require additional learning.