how to hook and extract text from powerpoint

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COM

i want to extract text from microsoft powerpoint .i have found some code that can be used to extract text using com automation .the problem is that i want to extract text whenever i turn pages in slide show. i think that every time ppt goes to a different page ,a certain function will be called .i just want to hook the function ,but i don't know which function it is
. i know how to hook from ordnary dll files ,but i have no experience exploiting com api ,there are almost no functions exported if use some tools like depends ,but some interface for you to call functions the com contains, is there any way to hook virtual table ,please help .
as i don't know if this is a feasible approch, if anyone have done something similar please help me,thanks!