Custom Delegate Programming for Qt Item Views: QStyledItemDelegate::setModelData()

Purpose

• It's invoked when the editing process in a view item (like a table cell) is complete, allowing you to transfer the modified data from the editor widget back to the model.
• This function serves as a bridge between the edited data in a custom widget (the editor) and the underlying model that manages the item data in your Qt application.

Context

• Once editing is finished (e.g., by pressing Enter or leaving the cell), setModelData() is called to commit the changes.
• When a user edits an item in a view, Qt creates an editor widget based on the delegate's createEditor() function.
• QStyledItemDelegate is a base class for creating custom delegates that control how items in views (e.g., QTableView, QListView) are displayed and edited.

Arguments

• index (const QModelIndex&): The index of the item that was edited. This provides information about the specific item within the model that was modified.
• model (QAbstractItemModel*): The model that holds the item data.
• editor (QWidget*): The editor widget used for editing the item. This could be a QLineEdit, QComboBox, or any custom widget you've created.

Functionality

1. • You (the developer) need to implement this function in your custom delegate class derived from QStyledItemDelegate.
   • Within setModelData(), your implementation should extract the edited data from the editor widget. This might involve methods specific to the type of editor widget being used.
   • For instance, if the editor is a QLineEdit, you might use editor->text() to get the entered text.

2. Updating the Model
   

   • Once you have the edited data, utilize the model argument to update the corresponding item in the underlying data model.
   • Call appropriate methods on the model to set the new data. Qt's item models typically provide functions like setData(), Qt::EditRole (specifying editing role), and the index to indicate the item to modify.

Example (Custom Delegate with QLineEdit)

#include <QStyledItemDelegate>
#include <QLineEdit>

class MyDelegate : public QStyledItemDelegate {
    Q_OBJECT

public:
    QWidget* createEditor(QWidget* parent, const QStyleOptionViewItem& option, const QModelIndex& index) const override {
        // Create a QLineEdit for editing
        QLineEdit* editor = new QLineEdit(parent);
        return editor;
    }

    void setModelData(QWidget* editor, QAbstractItemModel* model, const QModelIndex& index) const override {
        // Get the edited text from the QLineEdit
        QLineEdit* lineEdit = qobject_cast<QLineEdit*>(editor);
        if (lineEdit) {
            QString newText = lineEdit->text();

            // Update the model with the new data (assuming setData supports QString)
            model->setData(index, newText, Qt::EditRole);
        }
    }
};

Key Points

• Ensure your custom delegate class reimplements this function to handle data updates correctly.
• The implementation involves retrieving data from the editor and setting it in the model using appropriate methods.
• setModelData() is a crucial function for synchronizing changes made in custom editors with the underlying data model.

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Using QComboBox for Selection

class EnumDelegate : public QStyledItemDelegate {
    Q_OBJECT

public:
    QWidget* createEditor(QWidget* parent, const QStyleOptionViewItem& option, const QModelIndex& index) const override {
        // Create a QComboBox for selecting from a predefined list
        QComboBox* comboBox = new QComboBox(parent);
        QStringList options = {"Option 1", "Option 2", "Option 3"};
        comboBox->addItems(options);

        // Set the current selection based on the model data (assuming data is an integer)
        int currentValue = model->data(index, Qt::DisplayRole).toInt();
        comboBox->setCurrentIndex(currentValue);

        return comboBox;
    }

    void setModelData(QWidget* editor, QAbstractItemModel* model, const QModelIndex& index) const override {
        QComboBox* comboBox = qobject_cast<QComboBox*>(editor);
        if (comboBox) {
            int newSelection = comboBox->currentIndex();

            // Update the model with the selected index (assuming data is stored as an integer)
            model->setData(index, newSelection, Qt::EditRole);
        }
    }
};

Handling Data Type Conversions

class DoubleSpinBoxDelegate : public QStyledItemDelegate {
    Q_OBJECT

public:
    QWidget* createEditor(QWidget* parent, const QStyleOptionViewItem& option, const QModelIndex& index) const override {
        // Create a QDoubleSpinBox for editing double values
        QDoubleSpinBox* spinBox = new QDoubleSpinBox(parent);
        spinBox->setDecimals(2); // Set desired number of decimal places

        // Set the initial value based on the model data (assuming data is a double)
        double currentValue = model->data(index, Qt::DisplayRole).toDouble();
        spinBox->setValue(currentValue);

        return spinBox;
    }

    void setModelData(QWidget* editor, QAbstractItemModel* model, const QModelIndex& index) const override {
        QDoubleSpinBox* spinBox = qobject_cast<QDoubleSpinBox*>(editor);
        if (spinBox) {
            double newValue = spinBox->value();

            // Update the model, potentially converting the double to a string for storage
            QString newData = QString::number(newValue);
            model->setData(index, newData, Qt::EditRole);
        }
    }
};

class ColorPickerDelegate : public QStyledItemDelegate {
    Q_OBJECT

public:
    QWidget* createEditor(QWidget* parent, const QStyleOptionViewItem& option, const QModelIndex& index) const override {
        // Create a custom color picker widget (implementation omitted for brevity)
        ColorPickerWidget* picker = new ColorPickerWidget(parent);
        return picker;
    }

    void setModelData(QWidget* editor, QAbstractItemModel* model, const QModelIndex& index) const override {
        ColorPickerWidget* picker = qobject_cast<ColorPickerWidget*>(editor);
        if (picker) {
            QColor newColor = picker->selectedColor();

            // Validate the color (e.g., ensure it's within a valid range)
            if (isValidColor(newColor)) {
                // Update the model with the validated color (assuming data is stored as a QColor)
                model->setData(index, newColor, Qt::EditRole);
            } else {
                // Handle invalid color case (e.g., display an error message)
                // ...
            }
        }
    }

private:
    bool isValidColor(const QColor& color) {
        // Implement your color validation logic here
        return true; // Placeholder for actual validation
    }
};

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1. • If your delegate only handles simple editing scenarios and doesn't require complex editor logic, you could potentially modify the model directly within the createEditor() function.
   • However, this approach is generally discouraged as it tightly couples the delegate with the model and reduces code reusability.
   • Caution
     Exercise caution with this method as it bypasses the delegate's editing workflow and might lead to unexpected behavior if not implemented carefully.

2. Custom Signals and Slots
   

   • You can create custom signals and slots between your delegate and a separate class responsible for data validation and model updates.
   • The delegate emits a signal when editing is complete, passing the edited data.
   • The separate class receives the signal, performs validation (if needed), and updates the model using appropriate methods.
   • This approach decouples the delegate from model specifics and allows for more centralized data handling and validation logic.

3. Data Validation in createEditor() (Limited)
   

   • In some cases, you might perform basic data validation within createEditor().
   • For example, if the editor is a QSpinBox, you could set its range to restrict user input.
   • However, this approach only handles validation at the editor level and doesn't guarantee consistency with your model's data types or validation rules.

Choosing the Right Approach

• Direct model modification in createEditor() should be used cautiously and only for very simple editing scenarios.
• If you need complex validation or centralized data handling, consider using custom signals and slots.
• For most custom delegates, QStyledItemDelegate::setModelData() is the recommended approach as it provides a clear separation between editor logic and model updates.