Boris Johnson is a positive, good humoured individual with great generosity of spirit. That is a key aspect of his popularity. But his campaign team realise that it is also important to be negative. I don't think there is anything wrong with negative campaigning, provided it is honest and relevant. When your opponent happens to be Ken Livingstone negative campaigning is too good a chance to miss. Reminding London voters of Mr Livingstone's record and his extremist connections is a democratic duty.
Anyway this YouTube video and poster in York Road, Camden offer examples of where the Boris campaign going into attack mode.