@lucamar
Understood.
What I'm thinking is, I ever wrote a simple pdf viewer for my friend so he can preview, print, browse and manage the pdf files into categories. I used ActiveXContainer to 'import' Acrobat Reader 7 into the program. That worked and still working good. I wondered how that can be done into Lazarus so I looked into the source code. It used a lot of interface 'things'. So, if I am good in using interface, I will able to, not simply 'import' but better control the objects.
@jamie
Thank you for the demo. It show the usage of interface delegation and the "like abstract method" you mentioned. It is a bit hard for me to 'digest' for now. I believe I can fully understand it when I really use it into my code in the future.