Alternatives to QTextDocument::createObject() for Text Formatting in Qt

Purpose

• You wouldn't typically call it directly in your application code.
• Qt internally calls it when encountering specific formatting within rich text content.
• This protected member function in QTextDocument serves as a factory method for creating text objects within the document.

Functionality

• Common QTextObject subclasses include:
  • QTextList: Represents an ordered or unordered list within the document.
  • QTextTable: Represents a table structure within the document.
  • QTextFrame: Represents a container for text objects that allows for independent positioning and formatting.
• Based on the properties in QTextFormat, createObject() constructs a new QTextObject subclass instance.
• It takes a QTextFormat object as input, which encapsulates text formatting attributes like font, color, alignment, etc.

When to Reimplement

• Within your overridden method, you'd handle the creation of your custom text object based on the provided QTextFormat.
• In such cases, you'd need to subclass QTextDocument and override createObject().
• The default behavior of createObject() might not suffice if you're working with custom text objects not natively supported by Qt.

Key Points

• Subclass Responsibility
  Your overridden version creates appropriate instances based on QTextFormat.
• Custom Text Objects
  Reimplement it only when dealing with non-standard text objects.
• Internal Usage
  Primarily used by Qt internally during rich text processing.

• QTextObject subclasses provide advanced text layout and formatting capabilities, but their use cases might be less frequent compared to basic formatting.
• For standard text formatting (like font, color, alignment), directly applying styles through QTextCharFormat or QTextBlockFormat within your document is often more convenient.

---

#include <QApplication>
#include <QTextEdit>
#include <QTextDocument>
#include <QTextList>

class CustomTextObject : public QTextObject {
    Q_OBJECT

public:
    CustomTextObject(const QTextFormat& format)
        : QTextObject(format) {}

    int type() const override { return UserType; }

protected:
    void draw(QPainter* painter, const QRectF& rect, QTextDocument* doc, int positionInDocument = 0) const override {
        // Implement custom drawing logic here based on format or other properties
        painter->fillRect(rect, Qt::lightGray);
        painter->drawText(rect, Qt::AlignCenter, "Custom Object");
    }
};

class RichTextEditor : public QTextEdit {
    Q_OBJECT

public:
    RichTextEditor(QWidget* parent = nullptr) : QTextEdit(parent) {}

protected:
    QTextDocument* createDocument(const QString& text) const override {
        QTextDocument* doc = new QTextDocument();
        QString richText = "This is some text with a ";
        richText += "[custom:lightgray]"; // Marker for custom object
        richText += " custom object.";
        doc->setHtml(richText);
        return doc;
    }
};

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

    // Subclass QTextDocument to handle custom object creation
    class MyTextDocument : public QTextDocument {
        Q_OBJECT

    protected:
        QTextObject* createObject(const QTextFormat& format) const override {
            if (format.property().contains("custom")) {
                return new CustomTextObject(format);
            }
            return QTextDocument::createObject(format);
        }
    };

    RichTextEditor editor;
    editor.setDocument(new MyTextDocument());
    editor.show();

    return app.exec();
}

1. We define a CustomTextObject class that inherits from QTextObject. It implements the draw() method to display a custom visual representation within the document (here, a light gray rectangle with text).
2. The RichTextEditor class overrides the createDocument() method to set the initial content with a marker [custom:lightgray] for the custom object.
3. The key part is the MyTextDocument class. It subclasses QTextDocument and overrides createObject(). When it encounters the custom property in the format, it creates a CustomTextObject instance instead of the default behavior.

• This is a simplified example to illustrate the concept. In practice, custom object creation through createObject() is less common than using built-in text formatting features.

---

Built-in Text Formatting Classes

• You can create instances of these classes and apply them to text selections using methods like setCurrentCharFormat(), setTabStopDistance(), setBulletChar(), and insertTable(). This is the recommended approach for most common formatting needs.

• Qt provides a rich set of classes for applying formatting to text content:

  • QTextCharFormat: Controls character-level formatting like font, color, background, etc.
  • QTextBlockFormat: Controls block-level formatting like alignment, indentation, spacing, etc.
  • QTextList: Represents ordered or unordered lists within the document.
  • QTextTable: Represents a table structure within the document.

QTextCursor and Character Styles

• This allows fine-grained control over the formatting of specific text portions.
• Define character styles using QTextCharFormat and apply them to the current selection using QTextCursor::setCharFormat().
• Use QTextCursor to navigate and manipulate specific text regions within your document.

Rich Text Formats (HTML, RTF)

• Qt attempts to interpret the formatting embedded within these formats and apply it to the document.
• If you need to handle rich text content from external sources, use QTextDocument::setHtml() or QTextDocument::setRtf() to load the content directly.

• You can embed these custom widgets within a layout or a rich text editor using techniques like QWidget::setParent() or QTextEdit::insertWidget().
• For highly specialized visual representations or control over text layout, consider creating custom widgets that render the desired content.