Based on the tunes of "monodrama", the Shanghai Rap has been evolved by drawing on the performing skills of Suzhou Tanci Storytelling. In the early days, when the monodrama actor was to change a scene, there tended to be a performer to sing a tune, which was accompanied by qin played by another performer. The libretto of the tune was mostly improvised, no script prepared beforehand. This is the early form of Shanghai Rap. It is mainly performed in Shanghai dialect, but will be blended with other dialects if the plot requires.
Source:
http://traditions.cultural-china.com/en/17Traditions3832.html