Ioc

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

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!

Posted by msicc in Dev Stories, Xamarin, 1 comment
Using Microsoft’s Extensions.DependencyInjection package in (Xamarin.Forms) MVVM applications (Part 1) [Updated]

Using Microsoft’s Extensions.DependencyInjection package in (Xamarin.Forms) MVVM applications (Part 1) [Updated]

As some of you might remember, I was always a big fan of the MVVMLight toolkit. As the later one is now deprecated and MAUI around, I took a look at the CommunityToolkit.Mvvm, which is officially the successor to MVVMLight.

As stated in the documentation of the new Toolkit, one could now use the Microsoft’s Extensions.DependencyInjection package for anything related to Inversion of Control (which used to be handled by the SimpleIoc implementation of MVVMLight before). Because this is also the built-in way for .NET 6 and web applications, I decided to adapt it already now for my Xamarin.Forms apps (especially my new one I am currently working on).

[Update] Nuget packages

Please note that while the toolkit’s source is now separated from the Windows CommunityToolkit, the documentation isn’t. This can be confusing (as it was for me). On top of that, there are now two Toolkit MVVM packages:

I thought I got it right when writing this blog post initially. After Brandon Minnick from Microsoft pointed me to the right package, I realized I was not. Up on further research, I found also this discussion in the GitHub repo, stating the one and only will be the CommunityToolkit package. Please use only this one if you are following my tutorials here. I updated all mentions of the Toolkit in this post accordingly.

Default IServiceProvider implementation

The toolkit has a default implementation for the IServiceProvider provided by the Extension.DependencyInjection package. You can read about it here in the documentation and see the source here on GitHub. It focuses heavily on thread safety, its usage is pretty strict, and it does not allow adding ViewModels dynamically. If you do not need stuff like this in your app, you’re probably fine using the Ioc.Default implementation of the toolkit.

Custom IServiceprovider implementation

In TwistReader, the application I am currently working on, I had my requirements easily resolved by the SimpleIoc implementation of the MVVMLight toolkit. With the Extensions.DependencyInjection package, I had to move on with a custom implementation, on which we will have a deeper look in this post. Before you move on reading, make sure you have read the documentation.

IIocManagerBase interface

Of course, I wanted my custom implementation to be reusable. So I extended my existing base interface that my applications need to implement:

public interface IIocManagerBase 
{
    IServiceProvider? Services { get; }
    ServiceCollection? ServiceDescriptors { get; }

    IServiceProvider? BuildServiceProvider(bool resetExisiting = false);

    void Initialize(bool useDefaultNavigationService = true);

    void RegisterServices(bool useDefaultNavigationService);

    void RegisterViewModels();
}

I added the ServiceDescriptors property as well as an IServiceProvider property including a method to (re)build the ServiceProvider if needed. Let’s continue by having a look at the Xamarin.Forms base implementation.

FormsIocManagerBase base class

Building up on the interface before, I created a base implementation for my Xamarin.Forms apps. Let’s go a bit into the details.

In the Initialize method, I am just calling the RegisterServices and the RegisterViewModels methods. One important thing to notice is that I am instantiating the ServiceDescriptors property in the RegisterViewModels method. I also add my default services already to collection there. The RegisterViewModels method remains empty in the base implementation.

public virtual void Initialize(bool useDefaultNavigationService = true)
{
    RegisterServices(useDefaultNavigationService);
    RegisterViewModels();
}

public virtual void RegisterServices(bool useDefaultNavigationService)
{
    this.ServiceDescriptors = new ServiceCollection();

    this.ServiceDescriptors.TryAddSingleton<IDialogService>(DependencyService.Get<IDialogService>());
    this.ServiceDescriptors.TryAddSingleton<IActionSheetService>(new ActionSheetService());

    if (useDefaultNavigationService)
        this.ServiceDescriptors.TryAddSingleton<INavigationService>(new NavigationService());
    else
        System.Diagnostics.Debug.WriteLine("***** DON'T FORGET TO REGISTER YOUR INavigationService INSTANCE(S)!  *****");
}

public virtual void RegisterViewModels()
{
}

Until I switched to CommunityToolkit.Mvvm, this was all I had in there (using SimpleIoc for service registrations). Now that I am using the Extensions.DependencyInjection package, there is some more work to do:

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 code is not that complex, but helps with the IServiceProvider instance handling. The BuildServiceProvider method has a reset flag that allows me to rebuild the provider at runtime. One scenario where we can use this one is for adding ViewModel registrations dynamically during the runtime of our app, but.

IocManager in-app implementation

The next code block shows a typical in-app implementation of my IocManager. You may have noticed I am using the TryAdd{Lifetime} methods already before when adding items to the ServiceCollection. This makes sure that I have always just one registration and does not throw an exception if I try to add it again. If you prefer the exception, just switch to the Add{Lifetime} version.

public class IocManager : FormsIocManagerBase
{
    private static IocManager _instance;

    public static IocManager Current => _instance ??= new IocManager();


    public override void Initialize(bool useDefaultNavigationService = true)
    {
        base.Initialize(useDefaultNavigationService);
    }

    public override void RegisterServices(bool useDefaultNavigationService)
    {
        base.RegisterServices(useDefaultNavigationService);

        this.ServiceDescriptors.TryAddSingleton<ITestService, TestService1>();
        this.ServiceDescriptors.TryAddScoped<ITestService, TestedTestService>();
    }

    public override void RegisterViewModels()
    {
        this.ServiceDescriptors.TryAddSingleton<MainViewModel>();
        this.ServiceDescriptors.TryAddSingleton<SecondaryViewModel>();
    }

    public MainViewModel MainVm => this.Services.GetRequiredService<MainViewModel>();
    public SecondaryViewModel SecondaryVm => this.Services.GetRequiredService<SecondaryViewModel>();
}

For my Xamarin.Forms applications, I always use the IocManager implementation as a singleton. This makes it pretty easy with the different lifetimes on all platforms. As you can see, there is nothing complicated in the registration process, I just add both my services and my ViewModels to the ServiceCollection.

I also have some convenience properties for the most important ViewModels that make Binding easier (as I tend to keep code behind files as clean as possible). If you need a service in another place in your app, and you are not using constructor injection (which gets automatically resolved by the Microsoft.Extensions.DepedencyInjection package), you can get the instance in the same way as I do with the ViewModel instances above.

Conclusion

Creating a custom IServiceProvider implementation is not that hard. The custom implementation allows one to recreate the IServiceProvider (handle with care!) if needed. In the next post, I will show you how to deal with keyed ViewModel instances when using the Extensions.DependencyInjection package.

Have you already used the Microsoft.Extensions.DependencyInjection package with Xamarin.Forms or other platforms (not web)? What are your experiences? If so, leave a comment or chat with me on Twitter!

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

Until the next post – happy coding, everyone!

Posted by msicc in Dev Stories, Xamarin, 4 comments