I know that we can develop application in Vala for elemantry. But, do we have support for Genie? If I write application in Genie, should it has elemantry support?
Support for Genie is built into the Vala compiler. You can install the Vala compiler as follows:
$ sudo apt install valac
From this you may compile either Vala or Genie, as, for example:
$ valac example.gs -o example
Then run as:
You can also compile both Vala and Genie as, for example:
$ valac example.vala example.gs -o example
If you are feeling more intrepid, Gnome-Builder is an IDE that can be used to build projects for both Vala and Genie using the Meson build system.
Stackoverflow has a tag for Genie; it can be found here.
The best documentation on the internet may be found at the Elementary OS Developer Center.