When attempting to fetch only the cities based on the selected country, which step is not valid?

The selected step that is not valid involves calling the GetCities aggregate in the On After Fetch event of GetCountries. This step is not appropriate because the On After Fetch event occurs after the countries have been fetched but does not inherently provide a context or mechanism to initiate a new fetch for the cities based on a country selection. Instead, it is more logical to trigger the fetching of cities based on a user's selection immediately after the relevant country is selected or on an event that directly relates to the user interaction.

In contrast, it makes sense to call GetCities in the On Initialize event since this is a good time to set up initial data loads. Setting the Fetch property of GetCities to At Start ensures that the fetching process starts when the app loads, while using a filtering mechanism to retrieve cities based on the selected country is essential for ensuring that only the relevant cities are displayed according to user input. Each of these steps contributes to a coherent flow of data retrieval based on user interaction, which is why they are valid in the context of this operation.