Customizing Editor Widget Deletion in Qt Widgets with QAbstractItemDelegate::destroyEditor()

Purpose

• The destroyEditor() function is a virtual member function within QAbstractItemDelegate. It's designed to be reimplemented by subclasses if you need to customize the way editor widgets (created by createEditor()) are deleted.
• In Qt's model/view architecture, QAbstractItemDelegate is a fundamental class responsible for displaying and editing data from a model within views like QTableView or QListView.

Default Behavior

• By default, if you don't reimplement destroyEditor(), the QAbstractItemView (the view that uses the delegate) takes care of deleting the editor widget when it's no longer needed (e.g., after editing is complete or the view loses focus).

When to Reimplement

• In some scenarios, you might want to manage the editor widget's lifetime differently. Here are some reasons to reimplement:
  • Pooling Editors
    If you want to reuse editor widgets for efficiency, you could keep a pool of editors and manage their deletion yourself.
  • Custom Deletion Logic
    You might have specific requirements for how editors are deleted based on the editing state or other factors.

Reimplementation Example

#include <QtWidgets>

class MyDelegate : public QItemDelegate {
    Q_OBJECT

public:
    MyDelegate(QWidget *parent = nullptr) : QItemDelegate(parent) {}

    QWidget *createEditor(QWidget *parent, const QStyleOptionViewItem &option, const QModelIndex &index) const override {
        // ... (create editor widget)
    }

    void destroyEditor(QWidget *editor, const QModelIndex &index) const override {
        // Add the editor back to the pool for reuse
        myEditorPool.append(editor);
    }

private:
    QList<QWidget*> myEditorPool;
};

Important Considerations

• The QAbstractItemView might call destroyEditor() multiple times for the same editor widget during the editing process. Be prepared to handle this scenario in your reimplementation.
• When reimplementing destroyEditor(), it's crucial to ensure proper memory management. If you don't delete the editor widget or return it to a pool, you'll cause memory leaks.

• For more complex editor management, consider using Qt's QItemEditorFactory class, which provides a more structured approach to creating and managing editor widgets.

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Example 1: Pooled Editor with Usage Tracking

#include <QtWidgets>

class MyDelegate : public QItemDelegate {
    Q_OBJECT

public:
    MyDelegate(QWidget *parent = nullptr) : QItemDelegate(parent) {}

    QWidget *createEditor(QWidget *parent, const QStyleOptionViewItem &option, const QModelIndex &index) const override {
        // Check if there's an available editor in the pool
        if (!myEditorPool.isEmpty()) {
            QWidget* editor = myEditorPool.takeFirst();
            // Clear any previous data or state if needed
            return editor;
        }

        // Create a new editor widget if the pool is empty
        // ... (create editor widget)
    }

    void destroyEditor(QWidget *editor, const QModelIndex &index) const override {
        // Track usage count for the editor (optional)
        int usageCount = myEditorUsageCount.value(editor, 0);
        if (usageCount > 0) {
            myEditorUsageCount[editor] = usageCount - 1;
        } else {
            // Add the editor back to the pool only if not in use
            myEditorPool.append(editor);
        }
    }

private:
    QList<QWidget*> myEditorPool;
    QHash<QWidget*, int> myEditorUsageCount; // Optional for tracking usage
};

• It also introduces an optional myEditorUsageCount hash to track the usage count of each editor in the pool. This allows you to avoid adding editors back to the pool if they're still being used for editing in other parts of the view.
• This example enhances the basic pooling concept by checking if an available editor exists in the pool before creating a new one.

Example 2: Custom Deletion Based on Editing State

#include <QtWidgets>

class MyDelegate : public QItemDelegate {
    Q_OBJECT

public:
    MyDelegate(QWidget *parent = nullptr) : QItemDelegate(parent) {}

    QWidget *createEditor(QWidget *parent, const QStyleOptionViewItem &option, const QModelIndex &index) const override {
        // ... (create editor widget)
    }

    void destroyEditor(QWidget *editor, const QModelIndex &index) const override {
        // Get the current editing state (assuming you have a way to access it)
        bool editingComplete = isEditingComplete(index);

        if (editingComplete) {
            // Delete the editor if editing is complete
            delete editor;
        } else {
            // Keep the editor around if editing is still in progress
            // ... (optional handling, e.g., store for later use)
        }
    }

private:
    // Function to determine if editing is complete (implementation not shown)
    bool isEditingComplete(const QModelIndex &index) const;
};

• If editing is still in progress, you might choose to keep the editor around for later use (implementation not shown in this example).
• If editing is complete (e.g., the user has committed changes or editing has been canceled), the editor is deleted.
• This example demonstrates how to customize editor deletion based on the editing state of the item. You'll need to implement the isEditingComplete() function to suit your specific use case and data model.

• Consider using QItemEditorFactory for more complex editor management scenarios.
• Ensure proper memory management to avoid leaks.
• These are illustrative examples. Adapt them to your specific requirements and data model.

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Use Default Behavior

• The simplest approach is to rely on the default behavior of QAbstractItemView. It automatically deletes editor widgets when they're no longer needed, reducing the need for custom deletion logic in most cases. This is suitable for scenarios where basic editor lifecycle management suffices.

Manage Editors in Your View Class

Leverage QItemEditorFactory (Recommended for Complex Scenarios)

• QItemEditorFactory offers a more structured way to create and manage editor widgets. It allows you to:
  • Define custom editor creation logic based on model data or other criteria.
  • Reuse editors across different items in the view.
  • Manage the lifetime of editors centrally.
  • Handle complex scenarios like pooled editors or custom editor deletion strategies.

Choosing the Right Approach

The best approach depends on your specific use case and the level of control you need over editor lifecycle management:

• For complex scenarios with custom creation, reuse, or deletion logic, using QItemEditorFactory is generally recommended.
• If you need more control but don't want to modify the delegate, consider managing editors in the view class.
• For simple cases, the default behavior might suffice.

• Consider using signals and slots provided by QAbstractItemView (e.g., editorCreated(), editorDestroyed()) to react to editor lifecycle events if needed.
• When using custom deletion logic, ensure proper memory management to avoid leaks.