translate a GtkTextBuffer

[Binary Chen <binary chen gmail com>]

Hi,

I want to translate a GtkTextBuffer content to another format, there are
mixed types in the text buffer, such as plain text, images, animations,
and some other widgets(the text buffer supports all of this). 

The question when I iterate a text buffer, how can I know which type of
the object is? text, image or other widget?

Thanks in advance.
Bin