๐๏ธ Test a ViewModel
ViewModels in codeonthe_rocks are state objects so in order to test them, you need to build the corresponding StatefulWidget (ex. for the CounterViewModel, you need to build the CounterViewModelBuilder).
๐๏ธ Test a ViewModelBuilder
ViewModelBuilders are StatefulWidgets which means they need to be tested using the WidgetTester. They can be tested like any other widget as shown here.