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Using Microsoft’s Extensions.DependencyInjection package in (Xamarin.Forms) MVVM applications (Part 2)

The Key

Our goal is to add keyed registrations to the IServiceCollection, so we need a common denominator to build upon. As I was able to use a string with the SimpleIoc implementation of MVVMLight for years now, I decided to move on with that and created the following, very complex interface:

public interface IViewModelKey
{
    string Key { get; set; }
}

Every ViewModel that should be registered by Key needs to implement that interface from now on in my MVVM environment.

The Resolver

Back in the MVVMLight times, I was able to query the SimpleIoc registrations with the key I was searching for. In the Microsoft.Extensions.DependencyInjection world, things get a bit more complex. While there are different ways to solve the problem (there are some libraries extending the IServiceProvider with additional methods out there, for example), I decided to use the IServiceProvider itself and go down the resolver interface/implementation road.

Let’s have a look at the interface first:

public interface IViewModelByKeyResolver<T> where T : IViewModelKey
{
    public T GetViewModelByKey(string key);
}

Nothing too special here, just a generic implementation of the resolver interface with the requirement of the IViewModelKey implementation from above. This makes the usage pretty straight forward. The more important part here is the implementation, though. Let’s have a look at mine:

public class ViewModelByKeyResolver<T> : IViewModelByKeyResolver<T> where T : IViewModelKey
{
    private readonly IServiceProvider _serviceProvider;

    public ViewModelByKeyResolver(IServiceProvider serviceProvider)
        => _serviceProvider = serviceProvider;

    public T GetViewModelByKey(string key)
        => _serviceProvider.GetServices<T>().SingleOrDefault(vm => vm.Key == key);
}

The registration of the implementation will automatically inject the IServiceProvider instance at runtime for me here. The GetViewModelByKey method searches all registrations of the given type for the key and returns the desired instance.

Registering the Resolver and keyed ViewModels

The registration of the resolver is done like all the other registrations:

this.ServiceDescriptors.TryAddSingleton<IViewModelByKeyResolver<KeyedViewModel>, ViewModelByKeyResolver<KeyedViewModel>>();

Replace KeyedViewModel with your individual type that implements your key interface. That’s it.

For the registration of the KeyedViewModel instances, there is one thing to pay attention to, though. You cannot use the TryAdd{Lifetime} methods here for registration. Instead, just use the Add{Lifetime} method to register them. Here is a sample:

this.ServiceDescriptors.AddSingleton<KeyedViewModel>(new KeyedViewModel("Key1"));
this.ServiceDescriptors.AddSingleton<KeyedViewModel>(new KeyedViewModel("Key2"));
this.ServiceDescriptors.AddSingleton<KeyedViewModel>(new KeyedViewModel("Key3"));
this.ServiceDescriptors.AddSingleton<KeyedViewModel>(new KeyedViewModel("Key4"));
this.ServiceDescriptors.AddSingleton<KeyedViewModel>(new KeyedViewModel("Key5"));

If you know the keyed ViewModels already at the time of your app startup, you can add them right away and create the IServiceProvider instance as shown in my first post. In most cases, however, you will know the information of the keyed instances only at runtime. Luckily, my Xamarin.Forms implementation already has the solution built in. Here is a short reminder:

public ServiceCollection? ServiceDescriptors { get; private set; }

private IServiceProvider? _services;

public IServiceProvider? Services => _services ??= BuildServiceProvider();

public IServiceProvider? BuildServiceProvider(bool resetExisiting = false)
{
    if (this.ServiceDescriptors == null)
        throw new ArgumentNullException($"Please register your Services and ViewModels first with the {nameof(RegisterServices)} and {nameof(RegisterViewModels)} methods.");

    if (resetExisiting)
        _services = null;

    if (_services == null)
        _services = ServiceDescriptors.BuildServiceProvider();

    return _services;
}

The BuildServiceProvider method has an additional parameter that allows to reset the existing IServiceProvider. This way, I can keep my existing registrations and just add the new keyed ones dynamically. Please note that you may need to reinitialize your already registered and used ViewModels under certain circumstances after performing the reset.

Accessing a keyed ViewModel

Last but not least, I need to show you how to access a ViewModel by its key. Luckily, this is not that hard:

KeyedViewModel vm4 = IocManager.Current.Services.GetService<IViewModelByKeyResolver<KeyedViewModel>>().GetViewModelByKey("Key4");
KeyedViewModel vm2 = IocManager.Current.Services.GetService<IViewModelByKeyResolver<KeyedViewModel>>().GetViewModelByKey("Key2");

Conclusion

By switching to the CommunityToolkit.MVVM package and utilizing Microsoft’s Extension.DependencyInjection package together with it, my MVVM environment is ready for upcoming challenges like .NET MAUI. I will be able to use it on all .NET platforms and just need to adapt my Xamarin.Forms implementation to others (which I have done already for one of our internal tools at work in WPF). Even keyed ViewModel instance can be used similarly as before, as I showed you in this post.

As always, I hope this post will be helpful for some of you.

Until the next post, happy coding!


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