This is a simple example of a completion that works both in bash and in zsh. The zsh code actually makes use of the bash code so it can be reused and not have to be maintained twice.
The bash version works by simply loading the script:
source completion.bash
The zsh version needs to be placed inside any directory in your $fpath as file
called _git (or your command):
cp completion.zsh ~/.zsh/_git
This is a companion for my video: Creating a shell completion from scratch (bash and zsh).