facebook自己的开源软件，，有很多人用此开发自己的程序，节省很多时间，而且，开发的程序上传appstore是没问题的。 Three20 is a collection of iPhone UI classes, like a photo viewer, and general utilities, like an HTTP disk cache. Three20 is derived from the [Facebook iPhone app][], which is one of the most downloaded iPhone apps ever.