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ReactiveUI On The Web with Blazor

Sunday, July 12, 2020
Rich Bryant

ReactiveUI On The Web With Blazor!

It really works, honest.

I hope by now people are starting to learn about the existence of the ReactiveUI.Blazor package. Blazor means "writing Single Page Apps like Angular or Vue in C#" and it has all the potential in the world. And ReactiveUI supports it. The official samples are here but I don't feel that they really sell the reason why you'd want to do web work with ReactiveUI, and I should know, I wrote them.

For me, the big seller is that you've already got a WPF or Xamarin or WinForms app using ReactiveUI and you want a web version. In such a case, you can rewrite everything from scratch or you can use Blazor and keep the logic in your ViewModels absolutely unchanged. Sounds nice in theory but the web is a tricky thing, so does it actually work? I decided to try it.

The ReactiveUI Compelling Example

The ReactiveUI website Getting Started section links a Compelling Example, a simple Nuget browser that you can build very quickly with copypasta.


As you can see, it has text input, it does pipelines, it has data, it has views and viewmodels.... Why not, I thought? What are the steps?

  1. Follow the instructions Rich, you idiot

Yeah, that makes sense. Step 1 - build the Compelling Example just like the instructions say except that I used WPF in .NET Core 3.1 which worked flawlessly and I didn't have to change anything. I mostly did this because Blazor is a .NET Core app and I wanted to avoid any hassles later. The screenshot above is my working WPF project.

  1. Add a Blazor project.

I used Blazor AspNetCore Hosted for reasons which I hope will become clear later and I moved the actual "get the data" stuff to a service inside the Server project. I also created a ViewModels project (a .NET Standard 2.1 class library) because I don't want to copy my ViewModels, I want them shared.

Here's the project structure :

img Yes, I could have put the viewmodels in CompellingExample.Blazor.Shared but they contain no Blazor code and apply to the original WPF project too, so I didn't. You can throw eggs at me later. Let's move on. Because I needed an HttpClient calling an MVC WebApi, I put Refit on both the Blazor Client and the CompellingExample - they have to be the same, right? I won't go into the specifics because this isn't about Refit, I could have done it any number of ways, but I like Refit. The code's in a [GitHub repo]( if you want to check it for errors. This did require some wrangling of Splat, shown here in the App.xaml.cs of the WPF project:
public partial class App
    public App()
        Locator.CurrentMutable.Register(() => new MainWindow(), typeof(IViewFor<AppViewModel>));
        Locator.CurrentMutable.Register(() => new NugetDetailsView(), typeof(IViewFor<NugetDetailsViewModel>));
        Locator.CurrentMutable.RegisterLazySingleton(() =>
            RestService.For<INugetService>("https://localhost:44394/api"), typeof(INugetService));
  1. Wrangle Splat into Blazor

Here's my Program.cs. As you can see, I'm using Blazorise mostly because I suck at CSS and styling things, and I think <div> is ugly. You don't have to, obviously.

 public static async Task Main(string[] args)
        var builder = WebAssemblyHostBuilder.CreateDefault(args);
            .AddBlazorise( options =>
                options.ChangeTextOnKeyPress = false;
            } )

        builder.Services.UseMicrosoftDependencyResolver(); //Splat config
        var resolver = Locator.CurrentMutable;

        Locator.CurrentMutable.Register(() => new IndexView(), typeof(IViewFor<AppViewModel //Splat!
        Locator.CurrentMutable.Register(() => new NugetDetailsView(), typeof(IViewFor<NugetDetailsViewModel>)); //Splat!

        Locator.CurrentMutable.RegisterLazySingleton(() =>
            RestService.For<INugetService>("https://localhost:44394/api", new RefitSettings{ ContentSerializer = new JsonContentSerializer()}), typeof(INugetService)); //Annoying bit of Splat that I kicked myself for forgetting!

        var host = builder.Build();


        await host.RunAsync();

Other than that, you don't need much.

  1. Create your views!

Here's the IndexView.razor

@page "/"
@inherits ReactiveComponentBase<AppViewModel>

<Container class="mainContent">
            <Addon AddonType="AddonType.Body">
                <TextEdit @bind-Text="ViewModel.SearchTerm" Placeholder="Search" />
            <Addon AddonType="AddonType.End">
                <Button Color="Color.Secondary" Clicked="@SearchTextChanged">Search</Button>
    @if (ShowResults)
            @foreach (var result in ViewModel.SearchResults)
                <NugetDetailsView NugetViewModel="@result"/>

and here's its codebehind.

public partial class IndexView 
    private bool _showResults;
    public bool ShowResults
        get => _showResults;
            _showResults = value;

    public IndexView()
        ViewModel = new AppViewModel();

        this.WhenActivated(disposableRegistration =>
                    viewModel => viewModel.IsAvailable, 
                    view => view.ShowResults)

    private void SearchTextChanged()
        //This is really just here for sanity chacking and to make the textbox lose focus.
        Console.WriteLine($"SearchResults is {ViewModel.SearchResults.Count()}");

and now the NugetDetailsView

@inherits ReactiveComponentBase<NugetDetailsViewModel>
        <Figure Size="FigureSize.Is64x64">
            <FigureImage Source="@ViewModel.IconUrl.ToString()" AlternateText="@ViewModel.Title"/>
    <TableRowCell Class="boxContent">
            <Blazorise.Link To="@ViewModel.ProjectUrl.ToString()">

and its codebehind:

public partial class NugetDetailsView
    public NugetDetailsViewModel NugetViewModel { get; set; }

    protected override Task OnParametersSetAsync()
        ViewModel = NugetViewModel;
        return base.OnParametersSetAsync();

Basic stuff.

You could run this now and provided you've got your controllers wired up properly, it'll work. But I didn't much like it. I'll show you why.

Fugly Code

Have a look at this controller. Yuck .

public class NugetController : ControllerBase
    private readonly INugetService _nugetService;

    public NugetController(INugetService nugetService)
        _nugetService = nugetService;

    public async Task<IActionResult> GetNugetResults(string term)
            var result = await _nugetService.GetNugetPackages(term);
            return Ok(result);
        catch (Exception ex)
            return BadRequest(ex);

It might just be me, but I find Try/Catch personally offensive, especially in an async method. That had to go. And it wasn't just that, either. Take a look at the code to actually get package details, straight from the original CompellingExample.

public class NugetService : INugetService
    public async Task<IEnumerable<NugetPackageDto>> GetNugetPackages(string term)
        var providers = new List<Lazy<INuGetResourceProvider>>();
        var packageSource = new PackageSource("");
        var source = new SourceRepository(packageSource, providers);

        var filter = new SearchFilter(false);
        var resource = await source.GetResourceAsync<PackageSearchResource>().ConfigureAwait(false);
        var metadata = await resource.SearchAsync(term, filter, 0, 10, null, new CancellationToken())
        return metadata.Select(x => new NugetPackageDto(x));

Beautiful Functional C#

Wow. Imagine having to unit test that. Horrible. Imagine debugging it. No, it wouldn't do. Luckily, I have another tool in my toolbox especially for horrible things like these. It's called Functional Programming and ReactiveUI is pretty religious about it, except here for some reason. I don't know why. So I added a few static methods.

    public static SourceRepository NuGetLocalRepository()
        => new SourceRepository(new PackageSource(""),
            new List<Lazy<INuGetResourceProvider>>(Repository.Provider.GetCoreV3()));

    public static IEnumerable<NugetPackageDto> AsDtos(this IEnumerable<IPackageSearchMetadata> list)
        => list.Select(x => x.AsDto());

    public static NugetPackageDto AsDto(this IPackageSearchMetadata nuget)
        => new NugetPackageDto(nuget);

    public static async Task<PackageSearchResource> GetResource(this SourceRepository repo)
        => await repo.GetResourceAsync<PackageSearchResource>().ConfigureAwait(false);

    public static async Task<IEnumerable<IPackageSearchMetadata>> GetMetadata(PackageSearchResource source, string term)
        => await source.SearchAsync(term, new SearchFilter(false), 0, 10, null, new CancellationToken());

each one atomic, each one easy to test. And I added also my second favourite library ever, Language-Ext to get some nice helper methods and structs.

And here's that GetNugetPackages method now.

public async Task<IEnumerable<NugetPackageDto>> GetNugetPackages(string term) =>
        await NugetFunctions.NuGetLocalRepository()
            .Bind(x => NugetFunctions.GetMetadata(x, term))
            .Map(x => x.AsDtos());

Oh look, a Functional pipeline! Tell me you don't prefer that.

But I still had that nasty TryCatch in the controller. Luckily for me, along with fun things like Option<T> which protects you from null and Either<L,R> which takes away 99% of all if...then statements, Language-Ext also has a Try<T> which performs a Try and returns either your value or a safely wrapped exception that you can deal with you choose. And there's a TryAsync variant especially for asynchronous methods.

So I did this.

public TryAsync<IEnumerable<NugetPackageDto>> TryGetNugetPackagesAsync(string term)
        => TryAsync(GetNugetPackages(term));

And I still didn't want an exception in my controller method, so I put in some MVC helper statics, which you can find here if you care. And that left my controller as this

public class NugetController : ControllerBase
    private readonly INugetService _nugetService;

    public NugetController(INugetService nugetService) => _nugetService = nugetService;

    public async Task<IActionResult> GetNugetResults(string term) =>
        await _nugetService.TryGetNugetPackagesAsync(term)

Not only easy but also easy to throw in a few Functional C# improvements at the same time.

I hope you find this both useful, and a compelling example.

All code is here