What are the steps to implement a custom IMvxViewModelLocator

Step 1: Create a Custom ViewModelLocator Class

First, you need to create a class that inherits from `MvxDefaultViewModelLocator`. This class will override the methods necessary to implement your custom logic.

csharp
public class CustomViewModelLocator : MvxDefaultViewModelLocator
{
    // Implement custom logic here
}

Step 2: Override TryLoad Method

The `TryLoad` method is crucial for determining how view models are loaded. You can override this method to implement logic for loading specific view models.

csharp
public override bool TryLoad(Type viewModelType, IDictionary parameterValueLookup, out IMvxViewModel model)
{
    if (viewModelType == typeof(SpecialViewModel))
    {
        // Implement logic for SpecialViewModel
        model = new SpecialViewModel();
        return true;
    }
    else if (viewModelType == typeof(FooViewModel))
    {
        // Implement logic for FooViewModel
        model = new FooViewModel();
        return true;
    }
    return base.TryLoad(viewModelType, parameterValueLookup, out model);
}

Step 3: Implement Single Instance Logic (Optional)

If you want to ensure that certain view models are single instances, you can use a cache to store instances of view models.

csharp
private Dictionary _viewModelCache = new Dictionary();

public override bool TryLoad(Type viewModelType, IDictionary parameterValueLookup, out IMvxViewModel model)
{
    if (_viewModelCache.TryGetValue(viewModelType, out model))
    {
        return true;
    }

    if (viewModelType == typeof(SpecialViewModel))
    {
        model = new SpecialViewModel();
        _viewModelCache[viewModelType] = model;
        return true;
    }
    return base.TryLoad(viewModelType, parameterValueLookup, out model);
}

Step 4: Register the Custom ViewModelLocator

In your application's setup, specifically in the `App.cs` file, you need to override the `CreateDefaultViewModelLocator` method to return an instance of your custom locator.

csharp
protected override IMvxViewModelLocator CreateDefaultViewModelLocator()
{
    return new CustomViewModelLocator();
}

Step 5: Ensure Proper Initialization

Ensure that your custom locator is properly initialized and used by the MvvmCross framework. This might involve registering dependencies or ensuring that the locator is correctly set up in the application's startup process.

Additional Considerations

- Dependency Injection: If your view models require dependencies, ensure that these are properly injected using MvvmCross's IoC container.
- ViewModel Lifecycle: Understand the lifecycle methods (`Init`, `ReloadState`, `Start`) and how they are used in MvvmCross to manage view models.
- Testing: Test your custom locator thoroughly to ensure it behaves as expected under different scenarios.

By following these steps, you can effectively implement a custom `IMvxViewModelLocator` that meets your application's specific needs.