What's the correct way to update the UI from a QUndoCommand?

I have a QImage which can be drawn on to using a custom QWidget class, it actually draws a 40x40 pixel rectangle but that's not important:

void assetEditor::drawPoint(const QPoint &scaledPoint)
{
    QPainter painter(&foregroundImage);

    /* Set the fill colour and no border */
    painter.setBrush(currentEditorState.paint.penColour);
    painter.setPen(QPen(currentEditorState.paint.penColour, currentEditorState.paint.penSize, Qt::NoPen, Qt::RoundCap, Qt::RoundJoin));

    /* Draw a 40x40 rectangle as the pixel */
    painter.drawRect( QRect( QPoint( (scaledPoint.x() * 40), (scaledPoint.y() * 40) ), QSize(40, 40) ) );
    update();
}

Then on the mouse release event, the undo command is called:

/* Scale the foreground down to the correct size to reduce storage on the undo stack */
QImage scaledDownForeground = scaleDownImage(foregroundImage);

undoStack->push(new assetEditorImageChange(currentEditorConfig.imageSize, &foregroundImage, scaledDownForeground, undoStoreImage, &currentEditorState, preUndoStoreEditorState, postUndoStoreEditorState));

Note the max size of an image will be 16x16 pixels, so I figured it won't kill the memory to store a copy of the image in the undo stack.

Undo command is then defined here:

assetEditorImageChange::assetEditorImageChange(
            int imageHeight,
            QImage *passForegroundImage,
            const QImage &newImageState,
            const QImage &oldImageState,
            assetEditor::editorState *passCurrentEditorConfig,
            const assetEditor::editorState &newConfigState,
            const assetEditor::editorState &oldConfigState,
            QUndoCommand *parent)
    : QUndoCommand(parent),
      editorImageHeight(imageHeight),
      foregroundImage(passForegroundImage),
      oldImage(oldImageState),
      newImage(newImageState),
      editorConfig(passCurrentEditorConfig),
      oldConfig(newConfigState),
      newConfig(oldConfigState),
{
}

QImage assetEditorImageChange::scaleUpImage(QImage originalImage) {

    QImage scaledUpImage;

    scaledUpImage = originalImage.scaledToHeight(( editorImageHeight * 40 ), Qt::FastTransformation);
    QPainter p(&scaledUpImage);
    p.drawImage(QPoint(0, 0), scaledUpImage);

    return scaledUpImage;
}

void assetEditorImageChange::undo()
{
    /* Clear the editor foreground */
    foregroundImage->fill(qRgba(0, 0, 0, 0));

    /* Scale up the image to restore */
    QImage scaledUp = scaleUpImage(oldImage);

    /* Restore the image */
    QPainter painter(foregroundImage);
    painter.drawImage(QPoint(0, 0), scaledUp);

    /* Revert the editor config back */
    memcpy((void*)editorConfig, (void*)&oldConfig, sizeof(assetEditor::editorState));

    /* Update the editor */
    qDebug() << "Call update here";
}

void assetEditorImageChange::redo()
{
    /* Clear the editor foreground */
    foregroundImage->fill(qRgba(0, 0, 0, 0));

    /* Scale up the image to restore */
    QImage scaledUp = scaleUpImage(newImage);

    /* Restore the image */
    QPainter painter(foregroundImage);
    painter.drawImage(QPoint(0, 0), scaledUp);

    /* Revert the editor config back */
    memcpy((void*)editorConfig, (void*)&newConfig, sizeof(assetEditor::editorState));

    /* Update the editor */
    qDebug() << "Call update here";
}

This all works fine if I create an undo action in mainWindow.cpp and call the undo function and the update function:

void MainWindow::on_actionUndo_triggered()
{
    /* Perform the undo commands */
    undoStack->undo();

    /* Update the UI */
    ui->imageEditor->update();
}

But the undo stack holds lots of commands that aren't related to the image editor, so it doesn't need to be updated for every undo/redo action.

What's the correct way to update the UI once an undo/redo command is called?