Document Interface Composition Working Example

[cmeyertons]

I'd like to re-visit the solutioning / documentation around interface composition on latest .NET 8 and Refit.HttpClientFactory 7.

Related to: #49

It appears interface composition now is possible, but is undocumented. I think this would help a population of folks that have larger API surfaces they are trying to create / maintain with Refit.

Interface Composition provides a better UX as you can dive into sub-concerns and not have a globally inherited interface where intellisense, etc is returning everything all the time.

Describe the solution you'd like

Document Refit interface composition.

Describe alternatives you've considered

n/a, it now works (at least with HttpClientFactory)

Describe suggestions on how to achieve the feature

Please see my below code example:

public interface IMainApi
{
    IUsersApi Users { get; }
    IApplicationsApi Applications { get; }
    // could have many more
}

public interface IUsersApi
{
    [Get("/users")]
    Task<object> GetUsers();
}

public interface IApplicationsApi
{
    [Get("/applications")]
    Task<object> GetApplications();
}

internal record MainApi(IUsersApi Users, IApplicationsApi Applications) : IMainApi;

public static class DISetup
{
    private static readonly RefitSettings _settings = new()
    {
        // don't set the primary handler or anything like that that refit would invoke multiple times
    };

    public static IHttpClientBuilder AddMainApiClient(this IServiceCollection services, Uri baseAddress)
    {
        services.AddSingleton<IMainApi, MainApi>();

        var mainBuilder = services.AddRefitClient<IUsersApi>((services) => _settings, nameof(IMainApi));

        //using the same name here causes the underlying http client to be shared
        services.AddRefitClient<IApplicationsApi>((services) => _settings, nameof(IMainApi));

        mainBuilder.ConfigureHttpClient(client => {
            client.BaseAddress = baseAddress;
        });

        return mainBuilder;
    }
}