[WIP] Fluent API + Micosoft.Extensions.Hosting for initialization of Xamarin.Forms

[KSemenenko]

Description of Change

What if we were to add a "Fluent API" for initialization of Xamarin.Forms? Instead of calling several Init methods that are not very intuitive or discoverable, we can use a fluent mechanism.
Based on @jamesmontemagno post
https://montemagno.com/add-asp-net-cores-dependency-injection-into-xamarin-apps-with-hostbuilder/

@nickrandolph thanks for the idea with the article.

Issues Resolved

• fixes #5037
• partially fixes #8560

var app = Forms.Create(this, bundle)
	            .WithFlags("UseLegacyRenderers")
	            .WithMaps()
	            .WithVisualMaterial()
	            .WithAppLinks()
	            .UseStartup<Startup>()
	            .NativeConfigureHostConfiguration(c =>
	            {
	                // do some native configuration
	            })
	            .NativeConfigureServices((h, s) =>
	            {
	            	// do some native configuration
                        s.AddSingleton<INativeDataService, MyNativeDataService>();
	            })
                    .NativeConfigureAppConfiguration((h, s) =>
	            {
	            	// do some native configuration
	            })
	            .Build<App>();

LoadApplication(app);

API Changes

new property in Application

IServiceProvider Application.ServiceProvider { get; set; }

new interfaces:

public interface IFormsBuilder
{
    IFormsBuilder PostInit(Action action);
    IFormsBuilder PreInit(Action action);
    
    void Init();

    Application Build(Type app);
    TApp Build<TApp>() where TApp : Application;
    TApp Build<TApp>(Func<TApp> createApp) where TApp : Application;

    IFormsBuilder UseStartup(Type startupType);
    IFormsBuilder UseStartup<TStartup>() where TStartup : IStartup, new();
    IFormsBuilder UseStartup<TStartup>(Func<TStartup> createStartup) where TStartup : IStartup;

    IFormsBuilder NativeConfigureServices(Action<HostBuilderContext, IServiceCollection> configureDelegate);
    IFormsBuilder NativeConfigureHostConfiguration(Action<IConfigurationBuilder> configureDelegate);
    IFormsBuilder NativeConfigureAppConfiguration(Action<HostBuilderContext, IConfigurationBuilder> configureDelegate);
}

public interface IStartup
{
    void ConfigureServices(HostBuilderContext hostBuilderContext, IServiceCollection services);
    void ConfigureHostConfiguration(IConfigurationBuilder configurationBuilder);
    void ConfigureAppConfiguration(HostBuilderContext hostBuilderContext, IConfigurationBuilder configurationBuilder);
}

Xamarin Forms Init part:

now we can write extensions for pre init:

public static class FormsBuilderExtensions
{
	public static IFormsBuilder WithFlags(this IFormsBuilder init, params string[] flags)
	{
		return init.PreInit(() => Forms.SetFlags(flags));
	}
}

and we can write extensions for post init:

public static class FormsBuilderExtensions
{
	public static IFormsBuilder WithAppLinks(this IFormsBuilder init, Activity activity)
	{
		return init.PostInit(() => AndroidAppLinks.Init(activity));
	}
}

DI + HostBuilder part:

We can create Startup.cs, and use it .UseStartup<Startup>()

public class Startup : IStartup
{
    public void ConfigureServices(HostBuilderContext hostBuilderContext, IServiceCollection services)
    {
	if (hostBuilderContext.HostingEnvironment.IsDevelopment())
        {
            var world = hostBuilderContext.Configuration["Hello"];
        }

	services.AddSingleton<IDataService, MyDataService>();
	services.AddTransient<MyViewModel>();
	services.AddTransient<MainPage>();
	services.AddSingleton<App>();
    }

    public void ConfigureHostConfiguration(IConfigurationBuilder configurationBuilder)
    {

    }

    public void ConfigureAppConfiguration(HostBuilderContext hostBuilderContext, IConfigurationBuilder configurationBuilder)
    {
	
    }
}

minimal call

var app = Forms.Create()
	            .Build<App>();

LoadApplication(app);

or just Init for backward compatibility

Forms.Create().Init();
var app = new App();

We can use the service provider to get instances of the class.

public App()
{
    InitializeComponent();
    MainPage = Application.ServiceProvider.GetService<MainPage>();
}

EmbeddedResourceLoader for load embedded resources.
An important condition is set the calling assembly by calling SetExecutingAssembly
this is done in the Build method in the FormsBuild class

public static class EmbeddedResourceLoader
{
    public static void SetExecutingAssembly(Assembly assembly);
    public static byte[] GetEmbeddedResourceBytes(string resourceFileName);
    public static byte[] GetEmbeddedResourceBytes(string resourceFileName, Assembly assembly);
    public static string GetEmbeddedResourcePath(string resourceFileName);
    public static string GetEmbeddedResourcePath(string resourceFileName, Assembly assembly);
    public static Stream GetEmbeddedResourceStream(string resourceFileName);
    public static Stream GetEmbeddedResourceStream(string resourceFileName, Assembly assembly);
    public static string GetEmbeddedResourceString(string resourceFileName);
    public static string GetEmbeddedResourceString(string resourceFileName, Assembly assembly);
    public static ImageSource GetImageSource(string name);
    public static ImageSource GetImageSource(string name, Assembly assembly);
}

Platforms Affected

• Core (all platforms)
• iOS
• Android
• UWP
• Tizen
• WPF
• MacOS
• GTK

.NET Standards 2.0 required.

Links to the following NuGet are required.

Microsoft.Extensions.Hosting

Settings file should be added to the Forms project

appsettings.json 

Testing Procedure

Make sure that all test projects are launched as before.

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[KSemenenko]

@samhouts @jsuarezruiz @jfversluis Can I ask you to run the tests please?

[StephaneDelcroix]

I don't like t have 2 APIs to achieve the same goal. If you prefer having everything fluent, feel free to pack that in a separate nuget, as extensions methods, use it and share it

[KSemenenko]

I added System.Threading.Tasks.Extensions @jfversluis can you run build please?

[KSemenenko]

@jfversluis can you run build one more time? sorry for this :( seems I found real error

[KSemenenko]

Maybe I need to do something with nuspec? @jfversluis

[KSemenenko]

Can you run the build again @jfversluis? I already added the link wherever possible.

[KSemenenko]

I added some tests, @jfversluis can you please run the build when you have time.

[jfversluis]

Triggered another build. I don't think that adding another enhancement to this PR is really something we want though. This one is big enough as it is, no need to make it bigger. This will only make it harder for us to review and thus is will take longer.

Please keep your PRs as small as possible :)

[KSemenenko]

Thanks @jfversluis :) this PR uses EmbeddedResourceLoader to load appsettings.json so this is part of this PR.
and I thought that this should be highlighted somehow, so as not to forget.

[KSemenenko]

this is strange, all errors remained, although I already added links to all projects.

[KSemenenko]

I added links to Nuget which were in error, @jfversluis can I ask you to run the build again?

[KSemenenko]

this is so strange, I added all package links :(

[KSemenenko]

If someone knows what is wrong with NuGet I will be happy for any help

[anaximander23]

I'd love something like this - in fact, I wrote my own mini-framework on top of Xamarin.Forms that does exactly this. It's on GitHub, and while it's still in its infancy, there's enough there for you to see the rough pattern. For example, this is how you initialise the app:

var appCore = ApplicationCoreBuilder
    .Create()
    .UseStartup<Startup>()
    .OnAndroid(this)
    .Build();

LoadApplication(appCore);

Seeing a syntax like this become part of Xamarin.Forms itself would be great.

[KSemenenko]

Basically, all the code is ready but I found that UWP is using .net standard 1.0 and for work I need .net standard 2.0
So we need to make a decision about how to deal with the .net standard and NuGet versions of dependencies, I think that I can not add dependencies and change versions at my discretion.
Therefore, I need your opinion and advice.
@jfversluis @davidortinau @jamesmontemagno @jsuarezruiz

[jfversluis]

I think we're not going to drop netstandard1 anytime soon there #8570

[KSemenenko]

@jfversluis Microsoft.Extensions.Hosting is only for .netstandard 2.0 so this is a problem.

[KSemenenko]

@jfversluis Or maybe we can make our own analogue? With the same api, but with support .net standard 1?
https://github.com/aspnet/Extensions/tree/master/src/Hosting

And call it Xamarin.Forms.Hosting