shiny-mediator

Shiny Mediator Skill

You are an expert in Shiny Mediator, a mediator pattern library for .NET applications.

When to Use This Skill

Invoke this skill when the user wants to:

• Create request handlers, command handlers, event handlers, or stream handlers
• Generate contracts (IRequest, ICommand, IEvent, IStreamRequest)
• Add middleware (caching, resilience, validation, offline)
• Scaffold ASP.NET, MAUI, or Blazor projects with Shiny Mediator
• Configure Shiny Mediator in their application
• Set up ASP.NET Server-Sent Events (SSE) endpoints with stream handlers
• Use event subscriptions (WaitForSingleEvent, EventStream, Subscribe)
• Generate strongly-typed HTTP clients from OpenAPI/Swagger specs
• Create contract-first HTTP request handlers with [Get], [Post], etc.
• Migrate from MediatR to Shiny Mediator

Library Overview

Documentation: https://shinylib.net/mediator

Shiny Mediator is AOT & trimming friendly, using source generators for automatic DI registration.

Core Patterns

Pattern	Contract	Handler	Usage
Request	IRequest<TResult>	IRequestHandler<TRequest, TResult>	Queries returning data
Command	ICommand	ICommandHandler<TCommand>	Void state changes
Event	IEvent	IEventHandler<TEvent>	Pub/sub notifications
Stream	IStreamRequest<TResult>	IStreamRequestHandler<TRequest, TResult>	IAsyncEnumerable

Handler Registration

Always use registration attributes:

[MediatorSingleton]  // Stateless handlers
[MediatorScoped]     // Handlers needing per-request services (DbContext)

Critical: Partial Class Requirement

When using any middleware attribute ([Cache], [OfflineAvailable], [Resilient], [MainThread], [TimerRefresh], [Sample], [Throttle]), the handler class must be declared as partial:

[MediatorSingleton]
public partial class MyHandler : IRequestHandler<MyRequest, MyResult>  // partial required!
{
    [Cache(AbsoluteExpirationSeconds = 60)]
    public Task<MyResult> Handle(...) { }
}

This enables the source generator to create the IHandlerAttributeMarker implementation. Without partial, you'll get error SHINY001.

Basic Setup

ASP.NET:

builder.Services.AddShinyMediator(x => x
    .AddMediatorRegistry()
);
app.MapGeneratedMediatorEndpoints();

MAUI:

builder.AddShinyMediator(x => x
    .AddMediatorRegistry()
    .UseMaui()
    .AddMauiPersistentCache()
    .PreventEventExceptions()
);

Blazor:

builder.Services.AddShinyMediator(x => x
    .AddMediatorRegistry()
    .UseBlazor()
    .PreventEventExceptions()
);

Code Generation Instructions

When generating Shiny Mediator code:

1. Contracts

Always use records for immutability:

public record GetUserRequest(int UserId) : IRequest<UserDto>;
public record CreateUserCommand(string Name, string Email) : ICommand;
public record UserCreatedEvent(int UserId, string Name) : IEvent;

2. Handlers

Include all three parameters in Handle method:

[MediatorScoped]
public class GetUserRequestHandler : IRequestHandler<GetUserRequest, UserDto>
{
    public Task<UserDto> Handle(
        GetUserRequest request,
        IMediatorContext context,
        CancellationToken cancellationToken)
    {
        // Implementation
    }
}

3. Middleware Attributes

Apply to handler methods as needed:

• [Cache(AbsoluteExpirationSeconds = N)] - Cacheable queries
• [OfflineAvailable] - Offline storage for mobile
• [Resilient("policyName")] - Retry/timeout policies
• [MainThread] - MAUI main thread execution
• [TimerRefresh(milliseconds)] - Auto-refresh streams
• [Sample(milliseconds)] - Fixed-window sampling (last event in window executes)
• [Throttle(milliseconds)] - True throttle (first event executes, cooldown discards rest)
• [Validate] - Data annotation validation

When using ANY of these attributes, the handler class MUST be partial:

[MediatorSingleton]
public partial class CachedHandler : IRequestHandler<MyRequest, MyData>
{
    [Cache(AbsoluteExpirationSeconds = 60)]
    [OfflineAvailable]
    public Task<MyData> Handle(...) { }
}

4. Middleware Ordering

Use [MiddlewareOrder(int)] on custom middleware classes to control execution order. Lower values run first (outermost). Default is 0.

[MiddlewareOrder(-100)]  // Runs before middleware with higher order values
[MediatorSingleton]
public class EarlyMiddleware<TRequest, TResult> : IRequestMiddleware<TRequest, TResult>
    where TRequest : IRequest<TResult>
{ ... }

5. File Organization

Place files in appropriate folders:

• Contracts: Contracts/{Name}Request.cs, Contracts/{Name}Command.cs
• Handlers: Handlers/{Name}Handler.cs
• Middleware: Middleware/{Name}Middleware.cs

Usage Examples

Request:

var response = await mediator.Request(new GetUserRequest(1));
var user = response.Result;

Command:

await mediator.Send(new CreateUserCommand("John", "john@example.com"));

Event:

await mediator.Publish(new UserCreatedEvent(1, "John"));

Chaining via Context:

public async Task<UserDto> Handle(GetUserRequest request, IMediatorContext context, CancellationToken ct)
{
    // Use context to chain operations (shares scope)
    await context.Publish(new UserAccessedEvent(request.UserId));
    return new UserDto(...);
}

Event Subscriptions & Streaming

WaitForSingleEvent - Await a single event occurrence (with optional filter):

// Wait for a specific event (blocks until event fires or cancellation)
var evt = await mediator.WaitForSingleEvent<OrderCompletedEvent>(
    filter: e => e.OrderId == orderId,
    cancellationToken: ct
);

EventStream - Continuous IAsyncEnumerable stream of events (uses Channels internally):

// Consume events as an async stream
await foreach (var evt in mediator.EventStream<PriceUpdatedEvent>(cancellationToken: ct))
{
    Console.WriteLine($"New price: {evt.Price}");
}

Subscribe - Manual subscription returning IDisposable:

var sub = mediator.Subscribe<MyEvent>((ev, ctx, ct) =>
{
    Console.WriteLine($"Event received: {ev}");
    return Task.CompletedTask;
});
// Later: sub.Dispose() to unsubscribe

ASP.NET Server-Sent Events (SSE)

Stream handlers decorated with [MediatorHttpGet] or [MediatorHttpPost] on an IStreamRequestHandler are automatically generated as SSE endpoints by the source generator via MapGeneratedMediatorEndpoints().

Manual SSE endpoint with EventStream:

app.MapGet("/events", ([FromServices] IMediator mediator) =>
    TypedResults.ServerSentEvents(mediator.EventStream<MyEvent>())
);

Stream handler as auto-generated SSE endpoint:

public record TickerStreamRequest : IStreamRequest<int>;

[MediatorScoped]
public class TickerStreamHandler : IStreamRequestHandler<TickerStreamRequest, int>
{
    [MediatorHttpGet("/ticker")]
    public async IAsyncEnumerable<int> Handle(
        TickerStreamRequest request,
        IMediatorContext context,
        [EnumeratorCancellation] CancellationToken cancellationToken)
    {
        var i = 0;
        while (!cancellationToken.IsCancellationRequested)
        {
            yield return i++;
            await Task.Delay(1000, cancellationToken);
        }
    }
}

HTTP client-side SSE consumption: Implement IServerSentEventsStream marker on the contract to indicate the server returns SSE format. The generated HTTP handler will use ReadServerSentEvents<T>() to parse the data: prefixed SSE lines.

public record TickerStreamRequest : IStreamRequest<int>, IServerSentEventsStream;

Contract-First HTTP Clients

Shiny Mediator generates strongly-typed HTTP client handlers from contract classes decorated with HTTP method attributes. No manual HttpClient code needed.

Manual HTTP Contracts

Decorate request classes with [Get], [Post], [Put], [Delete], [Patch] and use [Query], [Header], [Body] on properties:

[Get("/api/orders/{OrderId}")]
public class GetOrderRequest : IRequest<OrderDto>
{
    public int OrderId { get; set; }          // Route parameter (matches {OrderId})

    [Query("status")]
    public string? Status { get; set; }        // ?status=value

    [Header("Authorization")]
    public string? AuthToken { get; set; }     // HTTP header
}

[Post("/api/orders")]
public class CreateOrderRequest : IRequest<OrderDto>
{
    [Body]
    public CreateOrderBody? Body { get; set; } // JSON request body
}

The source generator creates handler classes inheriting BaseHttpRequestHandler that build routes, add query/header parameters, serialize bodies, and call IHttpClientFactory.

Registration:

builder.Services.AddShinyMediator(x => x
    .AddMediatorRegistry()
    .AddStrongTypedHttpClient()   // Registers generated HTTP handlers
);

OpenAPI Client Generation

Generate contracts, models, and handlers directly from OpenAPI/Swagger specs. Add a <MediatorHttp> item in your .csproj:

<ItemGroup>
    <!-- Remote URL: Include is a logical name, Uri points to the spec -->
    <MediatorHttp Include="MyApi"
                  Uri="https://api.example.com/swagger/v1/swagger.json"
                  Namespace="MyApp.ExternalApi"
                  ContractPostfix="HttpRequest"
                  GenerateJsonConverters="true"
                  Visible="false" />

    <!-- Local file: Include is the file path, no Uri needed -->
    <MediatorHttp Include="./specs/openapi.yaml"
                  Namespace="MyApp.LocalApi"
                  Visible="false" />
</ItemGroup>

Include can be a file path (for local specs) or a logical name when Uri is set separately.

MediatorHttp metadata options:

Metadata	Description
Uri	URL or local path to OpenAPI JSON/YAML spec
Namespace	C# namespace for generated types
ContractPrefix	Prefix for generated contract class names
ContractPostfix	Postfix for generated contract class names
UseInternalClasses	Generate internal classes instead of public
GenerateModelsOnly	Only generate models, no handlers
GenerateJsonConverters	Generate JsonConverter implementations for enums

Registration of generated OpenAPI client:

builder.Services.AddShinyMediator(x => x
    .AddMediatorRegistry()
    .AddGeneratedOpenApiClient()   // Registers all OpenAPI-generated handlers
);

Then use like any other mediator request:

var result = await mediator.Request(new GetPetsHttpRequest { Status = "available" });

Best Practices

1. Use records for contracts - Immutable, value equality
2. One handler per file - Focused and testable
3. Appropriate lifetime - Singleton for stateless, Scoped for DbContext
4. Use partial with middleware attributes - Required for [Cache], [OfflineAvailable], [Resilient], etc.
5. Chain via context - Use context.Request() not injecting IMediator
6. Implement IContractKey - For custom cache/offline keys
7. Always pass CancellationToken - Respect cancellation
8. Use IServerSentEventsStream marker - On stream contracts consumed via HTTP SSE
9. Stream handlers only support GET/POST - Other HTTP methods are not valid for SSE endpoints
10. Use EventStream for SSE push endpoints - Combine mediator.EventStream<T>() with TypedResults.ServerSentEvents() for event-driven SSE

Reference Files

For detailed templates and examples, see:

• reference/templates.md - Code generation templates (includes SSE endpoint templates)
• reference/scaffolding.md - Project structure templates
• reference/middleware.md - Middleware configuration
• reference/api-reference.md - Full API, event subscriptions, SSE, and NuGet packages

Common Packages

dotnet add package Shiny.Mediator                              # Core
dotnet add package Shiny.Mediator.Maui                         # MAUI
dotnet add package Shiny.Mediator.Blazor                       # Blazor
dotnet add package Shiny.Mediator.AspNet                       # ASP.NET
dotnet add package Shiny.Mediator.Resilience                   # Polly
dotnet add package Shiny.Mediator.Caching.MicrosoftMemoryCache # Caching
dotnet add package Shiny.Mediator.AppSupport                   # Offline