MVVM ViewModel-Model

Question

Can I create an instance of the ViewModel in the View or it's inappropriate? I would need it, because the Command of a MenuFlyoutItem dynamically changes depending on the content of the Button (the Button content depends on a Json file).

Answer 1

Usually there are two approaches for ViewModel lifetime - singleton and tied to a specific instance of the view. For global ViewModels that are tied to a certain View for the whole lifetime of the app you will create a singleton instance of the ViewModel . Example for this could be the ViewModel of the root page of the app. This page will definitely appear only once in the navigation stack so you can safely use just one instance. A problem appears when there is a way for the user to repeat one View multiple times in the navigation stack. If you were using just one singleton instance of the ViewModel , by navigating to the second instance of the View you would essentially lose the the data tied to the previous instance and after navigating back you would see different data. Example would be a shopping app, where you have a product (A) and in the product view you have a list of related items. If the user selected one of them (say product B), then in cas of singleton ViewModel the instance would be refreshed to contain data of product B and when going back you would have to manually change the data back to product A. By having multiple instances of one ViewModel for each instance of the tied View in the navigation stack you can avoid reloading the data, which could be costly in case they are acquired from a remote service, for example. You can always manage all active ViewModels in a global array and remove the references to the ones no longer needed, so they can be freed up from memory.