MvvmCross: Does ShowViewModel always construct new instances?

Question

Whenever I call ShowViewModel, somehow a ViewModel and a View of the requested types are retrieved and are bound together for display on the screen. When are new instances

Answer 1

In MvvmCross v3.5 you can use this Class:

public class CacheableViewModelLocator : MvxDefaultViewModelLocator{
public override IMvxViewModel Load(Type viewModelType, IMvxBundle parameterValues, IMvxBundle savedState)
{
    if (viewModelType.GetInterfaces().Any(x=>x == typeof(ICacheableViewModel)))
    {
        var cache = Mvx.Resolve<IMvxMultipleViewModelCache>();
        var cachedViewModel = cache.GetAndClear(viewModelType);

        if (cachedViewModel == null)
            cachedViewModel = base.Load(viewModelType, parameterValues, savedState);

        cache.Cache(cachedViewModel);

        return cachedViewModel;
    }

    return base.Load(viewModelType, parameterValues, savedState);
}}

in your App Code override this method:

protected override IMvxViewModelLocator CreateDefaultViewModelLocator(){
return new CacheableViewModelLocator();}

Create an interface "ICacheableViewModel" and implement it on your ViewModel.

Now you can share the same ViewModel instance with multiple Views.

Answer 2

When are new instances of the ViewModel and View created versus looked up and retrieved from a cache somewhere?

Never - for new navigations the default behaviour is always to create new instances.

if... how do I show my cached ViewModel instance?

If for whatever reason you want to override the ViewModel location/creation, then there's information available about overriding the DefaultViewModelLocator in your App.cs in:

• MVVMCross Passing values to ViewModel that has 2 constructors
• http://slodge.blogspot.co.uk/2013/01/navigating-between-viewmodels-by-more.html

Put simply, implement your code:

public class MyViewModelLocator
  : MvxDefaultViewModelLocator
{
    public override bool TryLoad(Type viewModelType, IDictionary<string, string> parameterValueLookup,
                             out IMvxViewModel model)
    {
        // your implementation
    }
}

then return it in App.cs:

protected override IMvxViewModelLocator CreateDefaultViewModelLocator()
{
    return new MyViewModelLocator();
}

---

Note that older posts like How to replace MvxDefaultViewModelLocator in MVVMCross application are still conceptually compatible - but the details in those older posts are now out of date.