Swift unit testing view model interface

As I understand, it is best to only test public methods of a class.

Let's have a look at this example. I have a view model for the view controller.

protocol MyViewModelProtocol {

   var items: [SomeItem] { get }
   var onInsertItemsAtIndexPaths: (([IndexPath]) -> Void)? { get set }

   func viewLoaded()
}

class MyViewModel: MyViewModelProtocol {

   func viewLoaded() {
        let items = createDetailsCellModels()
        updateCellModels(with: items)
        requestDetails()
   }
}

I want to test class viewLoaded(). This class calls two other methods - updateItems() and requestDetails() One of the methods sets up the items and the other one call API to retrieve data and update those items. Items array us updated two times and onInsertItemsAtIndexPaths are called two times - when setting up those items and when updating with new data.

I can test whether after calling viewLoaded() expected items are set up and that onInsertItemsAtIndexPaths is called.

However, the test method will become rather complex.

What is your view, should I test those two methods separately or just write this one huge test?

By testing only viewLoaded(), my idea is that the implementation can change and I only care that results are what I expect.