How do I get all option value id’s for a given attribute in a specific category in my custom block?

• My shop has an attribute manufacturer.
• This attribute has option values like adidas and nike etc.
• I have a custom module with a block
• I placed the block on a specific category through a widget
• The category has several products assigned to it
• The products each have an attribute value set to it (adidas and nike etc.)

How can retrieve all the option value id’s (for adidas and nike etc.) in my custom block for the category which is currently loaded?