Alternatives to PaintContext::cursorPosition for Custom Cursor Rendering in Qt

Understanding PaintContext

• These parameters include:
  • cursorPosition: The position within the document where the cursor line should be drawn (if applicable).
  • palette: The default text color to use when no specific color is specified.
  • clip: A hint to the layout system about the area that needs painting (can improve performance for large documents).
  • selections: A collection of selections to be rendered during the painting process.
• It provides a set of parameters used when rendering the layout of a QTextDocument.
• In Qt, PaintContext is a class associated with the QAbstractTextDocumentLayout class.

PaintContext::cursorPosition Explained

• It's primarily used for custom layouts implemented using QAbstractTextDocumentLayout::draw().
• A value of -1 indicates that no cursor line should be drawn.
• This member variable is an integer that holds the character index within the QTextDocument where the cursor line should be drawn.

When to Use PaintContext::cursorPosition

• This could be useful for custom text editors or other applications that require a custom rendering of text documents.
• You might use PaintContext::cursorPosition in scenarios where you're creating a custom layout for a QTextDocument and want to visually represent the cursor position within that layout.

Example (Conceptual)

#include <QAbstractTextDocumentLayout>
#include <QTextDocument>

// ... (other code)

void MyCustomTextLayout::draw(QPainter *painter, const QTextDocument *document, const QRectF &rect) {
    PaintContext context;
    context.cursorPosition = document->cursor().position(); // Get current cursor position

    // ... (other painting logic)

    // Draw the cursor line at the specified position
    if (context.cursorPosition != -1) {
        QTextLine line = document->cursorForPosition(context.cursorPosition).currentLine();
        QPointF cursorPos = line.position();
        painter->drawLine(cursorPos.x(), rect.top(), cursorPos.x(), rect.bottom());
    }
}

• Consider using standard Qt widgets for most text editing scenarios unless you have a specific need for a custom layout.
• It doesn't directly affect the behavior of standard Qt GUI widgets like QTextEdit.
• PaintContext::cursorPosition is specifically for custom layouts.

---

#include <QtWidgets>
#include <QAbstractTextDocumentLayout>
#include <QTextDocument>

class CustomTextWidget : public QWidget {
    Q_OBJECT

public:
    CustomTextWidget(QWidget *parent = nullptr);

protected:
    void paintEvent(QPaintEvent *event) override;

private:
    QTextDocument *textDocument;
};

CustomTextWidget::CustomTextWidget(QWidget *parent) : QWidget(parent) {
    textDocument = new QTextDocument(this);
    textDocument->setPlainText("This is some text with a cursor.");
}

void CustomTextWidget::paintEvent(QPaintEvent *event) {
    QPainter painter(this);
    painter.fillRect(event->rect(), Qt::white);

    // Create a PaintContext object and set cursor position
    PaintContext context;
    context.cursorPosition = textDocument->cursor().position();

    // Use QTextLayout to layout the text
    QTextLayout textLayout(textDocument->toPlainText(), textDocument->defaultFont());
    textLayout.beginLayout();

    // Define layout area with margins
    qreal margin = 10;
    QRectF layoutRect(margin, margin, width() - 2 * margin, height() - 2 * margin);

    // Create lines of text and draw them
    QTextBlock block = textDocument->firstBlock();
    while (block.isValid()) {
        QTextLine line = textLayout.createLine(block);
        if (line.isValid()) {
            painter.drawText(layoutRect.topLeft() + line.position(), line.text());

            // Draw cursor line if applicable
            if (context.cursorPosition >= block.position() &&
                context.cursorPosition < block.position() + line.textLength()) {
                qreal cursorX = layoutRect.left() + line.cursorPosition(context.cursorPosition - block.position());
                painter->drawLine(cursorX, layoutRect.top(), cursorX, layoutRect.bottom());
            }
        }
        block = block.next();
    }

    textLayout.endLayout();
}

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    CustomTextWidget widget;
    widget.show();
    return app.exec();
}

This code:

1. Creates a CustomTextWidget class that inherits from QWidget.
2. Defines a textDocument member variable to hold the text content.
3. In the paintEvent(), it creates a QPainter object and sets the background color.
4. It creates a PaintContext object and sets the cursorPosition based on the current cursor position in the textDocument.
5. It uses QTextLayout to layout the text based on the document and font.
6. It defines a layout area with margins.
7. It iterates through text blocks and lines, drawing the text and the cursor line (if within the current line) using the cursorPosition information.

---

Using QAbstractTextDocumentLayout Signals

• If you primarily need to react to cursor position changes within a custom layout, you can utilize signals provided by QAbstractTextDocumentLayout:
  • cursorPositionChanged(int): This signal emits whenever the cursor position changes within the document. You can connect to this signal and update your custom rendering logic accordingly.

Custom Cursor Rendering with QTextCursor

This approach offers more control over the cursor's appearance and behavior within your custom layout.

Leveraging Standard Qt Widgets

• In most scenarios, standard Qt widgets like QTextEdit or QPlainTextEdit already handle cursor rendering and editing functionality. Consider using these widgets unless you have a specific requirement for a custom text layout that can't be achieved with these existing components.

The choice between these approaches depends on your specific needs:

• If your primary goal is text editing functionality, standard Qt widgets like QTextEdit are generally the recommended approach.
• If you require more control over the cursor's visual appearance or behavior, directly managing it with QTextCursor is an option.
• If you only need to react to cursor position changes and update your rendering logic, using signals might be sufficient.