feature: Provide a way to detect when navigation to a view-model is completed, even if no navigation-parameter is being passed

[AlexanderMelchers]

Is your feature request related to a problem? Please describe.
The problem I'm currently facing is as follows:

• I'm implementing a detail view for recipes. This detail view can be accessed either from a list that shows all available recipes by directly passing a reference to the recipe to show, or by reference to the identifier of the recipe to show - in which case the recipe will be resolved when the view gets activated.
• I've implemented WhenNavigatedTo(INavigationParameter) to pass said data - i.e. the recipe-object or its identifier - to the view-model.
• If neither recipe-object or identifier has been specified, I want to inform the user of this fact by showing them an error message.

Now I can show the user an error when INavigationParameter doesn't contain the expected parameters, but when the view is pushed without INavigationParameter, WhenNavigatedTo doesn't get called at all. As far as I'm aware there's no way to detect either 1) whether WhenNavigatedTo will be called at some point following activation of the view-model; or 2) that WhenNavigatedTo will not be called at all. That is, other than simply processing WhenNavigatedTo, I can't predict in the view-model constructor or WhenActivated whether to wait for WhenNavigatedTo to occur and have any error messages be handled there, or know that, since WhenNavigatedTo is not going to be called anyway, I should trigger my error from the constructor or WhenActivated.

Describe the solution you'd like
Either:

• call WhenNavigatedTo(INavigationParameter) with null to indicate the lack of parameters having been passed, but the navigation event having taken place; or
• implement a separate handler, such as WhenNavigatedTo(), WhenNavigatedToWithoutParameter() or WhenNavigationCompleted(ToOrFrom), to indicate that navigation is complete, but without parameters.

Describe alternatives you've considered
Alternatives I've considered are:

1. Using the view-model's WhenActivated to check if my data has come in. This doesn't work, though, since WhenNavigatedTo consistently occurs after WhenActivated.
2. Consistently passing an empty NavigationParameter when I want to trigger the intended error message. This, however, defeats the purpose, as the error is supposed to catch exceptions in the program.
3. Setting a timer to verify whether data has come in. This, however, may provide false-positives, as the data might need to be retrieved through the network and thus end up taking longer than the time-out that was set.

Describe suggestions on how to achieve the feature
As proposed above, the existing interface and method can be used to achieve this effect by simply always calling the WhenNavigatedTo(INavigationParameter)-method when a new view-model is navigated to. This, however, may break existing code that either does a null-check on the incoming parameters, or otherwise simply assumes the parameter to be set. In this case, extending the INavigated-interface with a new method would solve the issue.

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[winterdouglas]

Hey @AlexanderMelchers , I was looking the code here and actually what you are requesting can be done already in some way. The WhenNavigatingTo and WhenNavigatedTo are called every time a push happens, the only pre-requisite to have that working is to be using the ParameterViewStackService and call the Push passing in yourself an empty dictionary as a parameter. If you instead call the Push methods without the parameter signature (from the ViewStackService) then indeed those won't be triggered, so I guess that's your case.

Ideally I'd say we should have some kind of special "hook" to have that in a more consise way, there's a feature request I opened for that here #197

[winterdouglas]

I'd say this could be potentially closed and you could keep an eye on the #197 :)
@RLittlesII

[RLittlesII]

@AlexanderMelchers Apologies for not commenting on this sooner. What @winterdouglas says is accurate, the methods get called every navigation and there is nothing preventing you from hooking into them with an empty dictionary. Some base ViewModel class that handles it will get you that information.

We are looking into ways to provide better life cycle methods for the ViewModel. I feel like this is basically the same thing you are requesting. I am going to close this issue as it is similar to the other feature request and solving one, should solve both.