What are the best practices for using NavigationFacades in MvvmCross

Introduction to NavigationFacades in MvvmCross

NavigationFacades in MvvmCross are a powerful tool for managing complex navigation scenarios, especially when the choice of ViewModel depends on dynamic conditions or parameters. They allow you to decouple the navigation logic from the ViewModels themselves, making your application more flexible and easier to maintain.

Purpose of NavigationFacades

NavigationFacades are used to determine which ViewModel should be displayed based on a given URI. This is particularly useful when you need to show different views depending on the type of data or the state of the application. For example, in a task management app, you might want to display a read-only view for completed tasks and an editable view for pending tasks.

Implementing NavigationFacades

To implement a NavigationFacade, you need to create a class that inherits from `IMvxNavigationFacade`. This interface requires you to implement the `BuildViewModelRequest` method, which takes a URI and returns a `MvxViewModelRequest` object specifying the ViewModel to navigate to.

Here's an example of how you might implement a simple NavigationFacade:

csharp
[assembly: MvxRouting(typeof(SimpleNavigationFacade), @"mvx://task/\?id=(?[A-Z0-9]{32})$")]
namespace YourApp.NavigationFacades
{
    public class SimpleNavigationFacade : IMvxNavigationFacade
    {
        public Task BuildViewModelRequest(string url, 
            IDictionary currentParameters, MvxRequestedBy requestedBy)
        {
            var viewModelType = currentParameters["id"] == Guid.Empty.ToString("N") 
                ? typeof(ViewModelA) : typeof(ViewModelB);

            return Task.FromResult(new MvxViewModelRequest(viewModelType, new MvxBundle(), null, requestedBy));
        }
    }
}

In this example, the `SimpleNavigationFacade` decides whether to navigate to `ViewModelA` or `ViewModelB` based on the `id` parameter in the URI.

Best Practices for Using NavigationFacades

1. Keep Navigation Logic Centralized: Use NavigationFacades to keep navigation logic separate from your ViewModels. This makes it easier to manage complex navigation flows without cluttering your ViewModels with navigation code.

2. Use Dependency Injection: NavigationFacades can benefit from dependency injection, allowing you to access services or data that might be needed to determine which ViewModel to display.

3. Minimize Work in BuildViewModelRequest: Since the `BuildViewModelRequest` method is called as part of the navigation process, it should be lightweight. Avoid performing heavy operations or database queries here, as they can delay the UI response.

4. Use URI Parameters Effectively: NavigationFacades rely on URI parameters to make decisions. Ensure that your URIs are well-structured and that you use regular expressions effectively to capture and process these parameters.

5. Test NavigationFacades Thoroughly: Given their central role in navigation, it's crucial to test NavigationFacades thoroughly to ensure they behave as expected under different conditions.

Conclusion

NavigationFacades in MvvmCross provide a flexible way to manage complex navigation scenarios by decoupling navigation logic from ViewModels. By following best practices such as keeping logic centralized, using dependency injection, minimizing work in `BuildViewModelRequest`, and testing thoroughly, you can leverage NavigationFacades to create robust and maintainable navigation flows in your cross-platform applications.