ViewModel's constructor is being called again on navigation, and so messenger subscriptions are being subscribed again

Question

I am building a cross-platform mobile application using MvvmCross framework.

Since I would like to share information between ViewModels, I am registerin

Answer 1

Explanation on why the constructor is being called again, and how to avoid that.

The ctor is not called twice on the same object - instead what might happen is that a new View and a new ViewModel are created each time.

By default I would expect a new ViewModel to be created on every forwards navigation on every platform.

By default I would **not expect this to happen during a back button on WindowsPhone - it doesn't happen here for my test cases - but it could happen if:

• WindowsPhone removes your first Page (and it's ViewModel) from memory - I guess this might happen if your app is tombstoned or if you are using a custom RootFrame - but I don't expect this to happen by default.
• you somehow null the ViewModel (DataContext) in your first Page

Without seeing more of your code I can't guess any more about why this might happen.

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I'd personally recommend you look deeper at why you are seeing new ViewModels created during Back, but if you just want a quick fix, then you could look at overriding the ViewModelLocator within MvvmCross - see MvvmCross: Does ShowViewModel always construct new instances?

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Note that on WindowsStore, I would expect this to happen - WindowsStore doesn't hold Pages from the backstack in memory by default - but you can overriding this by setting NavigationCacheMode = NavigationCacheMode.Enabled; if you need to.