Do some action after stock quantity changed?

Hi, I want to set up a custom inventory management system for my store. It’s that my final products are made by combining many components together, and managing stock for components as important as for final products.

My basic idea to do this is using custom fields to store stock quantity for components. So if stock quantity of final products decrease by 1, all stock quantities for component are also decrease by 1. But after doing some research, I still dont know the hook for the changing of stock quantity. Could you briefly explain for me how I can achieve that? Thanks.

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