ios – Using protocol extension instead of class with static methods for reusable functions for specific app state

I have made ContactManager protocol that have viewModel class and then extend it with some fuctions that are making changes to some properties inside viewModel but alsoe provide logical calculations/manipulations that do not require storing data.

 protocol ContactManager{ 
   var viewModel : StreamViewModel { get } }

 extension ContactManager { 

func deselectContacts(contacts:[ContactInfo]){ 
    for contact in contacts { 
           if let index = viewModel.registeredContacts.firstIndex(where: {$0.id ==           contact.id}){ 

        var cnt = viewModel.registeredContacts[index] 
        cnt.selected = false 
        viewModel.registeredContacts[index] = cnt
}

I have couple views thath are using this protocol and its functions. Is this way correct or does it effect any other development stage, like testing etc…?