Exploring Alternatives to QLineEdit::completer() for Auto-Completion in Qt

Purpose

• It provides suggestions to the user as they type, making data entry faster and more accurate.
• QLineEdit::completer() is a method used in Qt Widgets to enable auto-completion functionality in a QLineEdit widget.

How it Works

1. • You create a QCompleter object, which acts as the source of suggestions.
   • This object can be populated with data from various sources, such as:
     • String lists
     • String models (like QStringListModel)
     • Custom models that handle data retrieval dynamically

2. Setting the Completer
   

   • Use the setCompleter(QCompleter *completer) method on your QLineEdit instance.
   • This associates the completer with the line edit, enabling it to provide suggestions.

3. Triggering Completion
   

   • By default, suggestions typically appear as the user types and a minimum number of characters (usually one) has been entered.
   • You can customize the trigger behavior using the setCompletionMode(QCompleter::CompletionMode) method of the completer.

4. Selecting a Suggestion
   

   • The user can accept a suggestion by:
     • Pressing the Tab key (or a custom key you set)
     • Selecting the desired suggestion from the displayed list (usually a popup)

5. Interaction with Validators (Optional)
   

   • If you have a validator set on the QLineEdit, the completer will only suggest options that are valid according to the validator's rules.
   • This ensures that the user can only enter valid data.

Code Example (String List)

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

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

    // Create a line edit
    QLineEdit lineEdit;

    // Create a string list for suggestions
    QStringList suggestions = QStringList() << "apple" << "banana" << "cherry";

    // Create a completer and set the model (list)
    QCompleter *completer = new QCompleter(suggestions, &lineEdit);

    // Set the completer for the line edit
    lineEdit.setCompleter(completer);

    lineEdit.show();

    return app.exec();
}

Additional Customization

• The popup's appearance can be modified using stylesheets or custom widgets.
• You can control the case sensitivity of suggestions using setCaseSensitivity(Qt::CaseSensitivity).

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Using a String Model (QStringListModel)

This example shows how to dynamically add and remove suggestions from the completer using a QStringListModel.

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

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

    // Create a line edit
    QLineEdit lineEdit;

    // Create a string list model
    QStringListModel *model = new QStringListModel(&lineEdit);

    // Initially empty list
    model->setStringList(QStringList());

    // Create a completer and set the model
    QCompleter *completer = new QCompleter(model, &lineEdit);

    // Set the completer for the line edit
    lineEdit.setCompleter(completer);

    // Add some suggestions dynamically (e.g., from a database)
    QStringList newSuggestions = QStringList() << "grape" << "orange";
    model->setStringList(newSuggestions);

    lineEdit.show();

    return app.exec();
}

Custom Model for File Path Completion

This example implements a custom model that retrieves file paths starting with the entered text in the line edit.

#include <QApplication>
#include <QLineEdit>
#include <QCompleter>
#include <QDir>
#include <QFileSystemModel>

class FilePathModel : public QAbstractListModel {
    Q_OBJECT

public:
    explicit FilePathModel(QObject *parent = nullptr) : QAbstractListModel(parent) {}

    QStringList data(const QModelIndex &index, int role = Qt::DisplayRole) const override {
        if (role == Qt::DisplayRole) {
            QDir dir(filter);
            QStringList entries = dir.entryList(QStringList("*.txt"), QDir::Files);
            return entries;
        }
        return QStringList();
    }

    void setFilter(const QString &text) { filter = text; }

private:
    QString filter;
};

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

    // Create a line edit
    QLineEdit lineEdit;

    // Create a custom file path model
    FilePathModel *model = new FilePathModel(&lineEdit);

    // Create a completer and set the model
    QCompleter *completer = new QCompleter(model, &lineEdit);
    completer->setCompletionMode(QCompleter::PopupCompletion);

    // Set the completer for the line edit
    lineEdit.setCompleter(completer);

    // Connect textEdited signal to update suggestions
    QObject::connect(&lineEdit, &QLineEdit::textEdited, model, &FilePathModel::setFilter);

    lineEdit.show();

    return app.exec();
}

Case-Insensitive Completion

This example shows how to make the completer suggestions case-insensitive.

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

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

    // Create a line edit
    QLineEdit lineEdit;

    // Create a string list for suggestions
    QStringList suggestions = QStringList() << "Apple" << "Banana" << "Cherry";

    // Create a completer and set the model (list)
    QCompleter *completer = new QCompleter(suggestions, &lineEdit);

    // Set the completer for the line edit
    lineEdit.setCompleter(completer);

    // Make suggestions case-insensitive
    completer->setCaseSensitivity(Qt::CaseInsensitive);

    lineEdit.show();

    return app.exec();
}

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QComboBox

• This is a good choice for situations where the number of suggestions is limited and you want a more structured selection process.
• It provides a dropdown menu with suggested options that the user can select directly.
• Use a QComboBox with a custom model if you have a predefined set of options for the user to choose from.

Custom Widget with Popup

• This approach offers greater control over the appearance and behavior of the auto-completion feature, but requires more development effort.
• You can trigger the popup based on user input or other events.
• Create a custom widget that inherits from QWidget and implements its own popup mechanism for displaying suggestions.

Third-Party Libraries

• These libraries often provide additional features like highlighting matched text, filtering suggestions based on user input, and customizing suggestion formatting.

Choosing the Right Alternative

The best alternative depends on your specific requirements. Consider these factors:

• Development Effort
  Using QLineEdit::completer() is a relatively low-effort solution, while custom approaches require more work.
• Customization
  If you need fine-grained control over appearance and behavior, a custom widget might be necessary. Third-party libraries can also offer advanced customization options.
• Number of suggestions
  If you have a limited set of suggestions, a QComboBox might be sufficient. However, for a larger list, a QLineEdit with a completer might offer a more efficient user experience.

• Evaluate the performance implications of each approach, especially if you're dealing with a very large number of suggestions.
• You can combine different techniques. For example, you could use a QLineEdit with a completer for initial suggestions and then switch to a dropdown list (QComboBox) if the number of suggestions exceeds a certain threshold.