Spice 🌶, a spicy cross-platform UI framework!

A prototype (and design) of API minimalism for mobile.

If you like this idea, star for approval! Read on for details!

Getting Started

Simply install the template:

dotnet new install Spice.Templates

Create either a plain Spice project, or a hybrid "Spice+Blazor" project:

dotnet new spice
# Or if you want hybrid/web support
dotnet new spice-blazor

Or use the project template in Visual Studio:

Build it as you would for other .NET MAUI projects:

dotnet build
# To run on Android
dotnet build -f net8.0-android -t:Run
# To run on iOS
dotnet build -f net8.0-ios -t:Run

Of course, you can also just open the project in Visual Studio and hit F5.

Startup Time & App Size

In comparison to a dotnet new maui project, I created a Spice project with the same layouts and optimized settings for both project types. (AndroidLinkMode=r8, etc.)

App size of a single-architecture .apk, built for android-arm64:

The average startup time of 10 runs on a Pixel 5:

This gives you an idea of how much "stuff" is in .NET MAUI.

In some respects the above comparison isn't completely fair, as Spice 🌶 has very few features. However, Spice 🌶 is fully trimmable, and so a Release build of an app without Spice.Button will have the code for Spice.Button trimmed away. It will be quite difficult for .NET MAUI to become fully trimmable -- due to the nature of XAML, data-binding, and other System.Reflection usage in the framework.

Background & Motivation

In reviewing, many of the cool UI frameworks for mobile:

• Flutter
• SwiftUI
• Jetpack Compose
• Fabulous
• Comet
• An, of course, .NET MAUI!

Looking at what apps look like today -- it seems like bunch of rigamarole to me. Can we build mobile applications without design patterns?

The idea is we could build apps in a simple way, in a similar vein as minimal APIs in ASP.NET Core but for mobile & maybe one day desktop:

public class App : Application
{
    public App()
    {
        int count = 0;
    
        var label = new Label
        {
            Text = "Hello, Spice 🌶",
        };
    
        var button = new Button
        {
            Text = "Click Me",
            Clicked = _ => label.Text = $"Times: {++count}"
        };
    
        Main = new StackView { label, button };
    }
}

These "view" types are mostly just POCOs.

Thus you can easily write unit tests in a vanilla net8.0 Xunit project, such as:

[Fact]
public void Application()
{
    var app = new App();
    var label = (Label)app.Main.Children[0];
    var button = (Button)app.Main.Children[1];

    button.Clicked(button);
    Assert.Equal("Times: 1", label.Text);

    button.Clicked(button);
    Assert.Equal("Times: 2", label.Text);
}

The above views in a net8.0 project are not real UI, while net8.0-android and net8.0-ios projects get the full implementations that actually do something on screen.

So for example, adding App to the screen on Android:

protected override void OnCreate(Bundle? savedInstanceState)
{
    base.OnCreate(savedInstanceState);

    SetContentView(new App());
}

And on iOS:

var vc = new UIViewController();
vc.View.AddSubview(new App());
Window.RootViewController = vc;

App is a native view on both platforms. You just add it to an the screen as you would any other control or view. This can be mix & matched with regular iOS & Android UI because Spice 🌶 views are just native views.

NEW Blazor Support

Currently, Blazor/Hybrid apps are strongly tied to .NET MAUI. The implementation is basically working with the plumbing of the native "web view" on each platform. So we could have implemented BlazorWebView to be used in "plain" dotnet new android or dotnet new ios apps. For now, I've migrated some of the source code from BlazorWebView from .NET MAUI to Spice 🌶, making it available as a new control:

public class App : Application
{
    public App()
    {
        Main = new BlazorWebView
        {
            HostPage = "wwwroot/index.html",
            RootComponents =
            {
                new RootComponent { Selector = "#app", ComponentType = typeof(Main) }
            },
        };
    }
}

From here, you can write Index.razor as the Blazor you know and love:

@page "/"

<h1>Hello, world!</h1>

Welcome to your new app.

To arrive at Blazor web content inside iOS/Android apps:

This setup might be particularly useful if you want web content to take full control of the screen with minimal native controls. No need for the app size / startup overhead of .NET MAUI if you don't actually have native content?

Scope

• No XAML. No DI. No MVVM. No MVC. No data-binding. No System.Reflection.
  • Do we need these things?
• Target iOS & Android only to start.
• Implement only the simplest controls.
• The native platforms do their own layout.
• Document how to author custom controls.
• Leverage C# Hot Reload for fast development.
• Measure startup time & app size.
• Profit?

Benefits of this approach are full support for trimming and eventually NativeAOT if it comes to mobile one day. 😉

Thoughts on .NET MAUI

.NET MAUI is great. XAML is great. Think of this idea as a "mini" MAUI.

Spice 🌶 will even leverage various parts of .NET MAUI:

• The iOS and Android workloads for .NET.
• The .NET MAUI "Single Project" system.
• The .NET MAUI "Asset" system, aka Resizetizer.
• Microsoft.Maui.Graphics for primitives like Color.

And, of course, you should be able to use Microsoft.Maui.Essentials by opting in with UseMauiEssentials=true.

It is an achievement in itself that I was able to invent my own UI framework and pick and choose the pieces of .NET MAUI that made sense for my framework.

Implemented Controls

• View: maps to Android.Views.View and UIKit.View.
• Label: maps to Android.Widget.TextView and UIKit.UILabel
• Button: maps to Android.Widget.Button and UIKit.UIButton
• StackView: maps to Android.Widget.LinearLayout and UIKit.UIStackView
• Image: maps to Android.Widget.ImageView and UIKit.UIImageView
• Entry: maps to Android.Widget.EditText and UIKit.UITextField
• WebView: maps to Android.Webkit.WebView and WebKit.WKWebView
• BlazorWebView extends WebView adding support for Blazor. Use the spice-blazor template to get started.

Custom Controls

Let's review an implementation for Image.

First, you can write the cross-platform part for a vanilla net8.0 class library:

public partial class Image : View
{
    [ObservableProperty]
    string _source = "";
}

[ObservableProperty] comes from the MVVM Community Toolkit -- I made use of it for simplicity. It will automatically generate various partial methods, INotifyPropertyChanged, and a public property named Source.

We can implement the control on Android, such as:

public partial class Image
{
    public static implicit operator ImageView(Image image) => image.NativeView;

    public Image() : base(c => new ImageView(c)) { }

    public new ImageView NativeView => (ImageView)_nativeView.Value;

    partial void OnSourceChanged(string value)
    {
        // NOTE: the real implementation is in Java for performance reasons
        var image = NativeView;
        var context = image.Context;
        int id = context!.Resources!.GetIdentifier(value, "drawable", context.PackageName);
        if (id != 0) 
        {
            image.SetImageResource(id);
        }
    }
}

This code takes the name of an image, and looks up a drawable with the same name. This also leverages the .NET MAUI asset system, so a spice.svg can simply be loaded via new Image { Source = "spice" }.

Lastly, the iOS implementation:

public partial class Image
{
    public static implicit operator UIImageView(Image image) => image.NativeView;

    public Image() : base(_ => new UIImageView { AutoresizingMask = UIViewAutoresizing.None }) { }

    public new UIImageView NativeView => (UIImageView)_nativeView.Value;

    partial void OnSourceChanged(string value) => NativeView.Image = UIImage.FromFile($"{value}.png");
}

This implementation is a bit simpler, all we have to do is call UIImage.FromFile() and make sure to append a .png file extension that the MAUI asset system generates.

Now, let's say you don't want to create a control from scratch. Imagine a "ghost button":

class GhostButton : Button
{
    public GhostButton() => NativeView.Alpha = 0.5f;
}

In this case, the NativeView property returns the underlying Android.Widget.Button or UIKit.Button that both conveniently have an Alpha property that ranges from 0.0f to 1.0f. The same code works on both platforms!

Imagine the APIs were different, you could instead do:

class GhostButton : Button
{
    public GhostButton
    {
#if ANDROID
        NativeView.SomeAndroidAPI(0.5f);
#elif IOS
        NativeView.SomeiOSAPI(0.5f);
#endif
    }
}

Accessing the native views don't require any weird design patterns. Just #if as you please.

Hot Reload

C# Hot Reload (in Visual Studio) works fine, as it does for vanilla .NET iOS/Android apps:

Note that this only works for Button.Clicked because the method is invoked when you click. If the method that was changed was already run, something has to force it to run again. MetadataUpdateHandler is the solution to this problem, giving frameworks a way to "reload themselves" for Hot Reload.

Unfortunately, MetadataUpdateHandler does not currently work for non-MAUI apps in Visual Studio 2022 17.5:

[assembly: System.Reflection.Metadata.MetadataUpdateHandler(typeof(HotReload))]

static class HotReload
{
    static void UpdateApplication(Type[]? updatedTypes)
    {
        if (updatedTypes == null)
            return;
        foreach (var type in updatedTypes)
        {
            // Do something with the type
            Console.WriteLine("UpdateApplication: " + type);
        }
    }
}

The above code works fine in a dotnet new maui app, but not a dotnet new spice or dotnet new android application.

And so we can't add proper functionality for reloading ctor's of Spice 🌶 views. The general idea is we could recreate the App class and replace the views on screen. We could also create Android activities or iOS view controllers if necessary.

Hopefully, we can implement this for a future release of Visual Studio.