Add Observer Pattern Source Generator

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Observer Pattern Source Generator - Third Review Round

Comment 2828802620 Addressed ✅

Removed Unused Unsubscribe Method

• Removed GenerateUnsubscribeMethod call and implementation
• The generated Unsubscribe method was never called - Subscription.Dispose handles removal directly
• This reduces generated code size and improves maintainability
• No behavior change - disposal logic remains in Subscription.Dispose exactly as before

Test Results: 11/13 Passing (85%)

Same as before - all changes backward compatible, no regressions.

Summary

Eliminated dead code from the generator. The Subscription.Dispose method already contains all the unsubscription logic inline based on the threading policy, making the separate Unsubscribe method unnecessary. This simplifies the generated code and reduces the size of generated types.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Generator: Create Observer Pattern</issue_title>
<issue_description>## Summary

Add a source generator that produces a complete implementation of the Observer pattern for event publication and subscription, with safe-by-default lifetimes and deterministic behavior.

The generator lives in PatternKit.Generators and emits code that is:

• reflection-free
• allocation-aware
• explicit about threading policy
• self-contained (no runtime PatternKit dependency)

---

Motivation / Problem

Observer is easy to misuse:

• leaking subscriptions
• nondeterministic invocation order
• unclear exception behavior (one subscriber breaks others?)
• ad-hoc concurrency policies

We want a generated implementation that makes the “rules of engagement” explicit and testable.

---

Supported Targets (must-have)

The generator must support:

• partial class
• partial struct
• partial record class
• partial record struct

Two consumption modes must be supported:

1. Event type (a type represents one observable event stream).
2. Event hub (a type groups multiple generated events).

---

Proposed User Experience

A) Single event, payload-based

[Observer]
public partial class TemperatureChanged { }

Generated (representative shape):

public partial class TemperatureChanged
{
    public IDisposable Subscribe(Action<Temperature> handler);
    public IDisposable Subscribe(Func<Temperature, ValueTask> handler);

    public void Publish(Temperature value);
    public ValueTask PublishAsync(Temperature value, CancellationToken ct = default);
}

B) Event hub (multi-event grouping)

[ObserverHub]
public static partial class SystemEvents
{
    [ObservedEvent]
    public static partial TemperatureChanged TemperatureChanged { get; }

    [ObservedEvent]
    public static partial ShutdownRequested ShutdownRequested { get; }
}

Generated semantics:

• Each [ObservedEvent] property returns a singleton instance of that event stream.
• Hub generation is optional, but if present must be deterministic and self-contained.

---

Attributes / Surface Area

Namespace: PatternKit.Generators.Observer

Core

• [Observer] on an event stream type
• [ObserverHub] on a hub type
• [ObservedEvent] on hub properties

Configuration

ObserverAttribute suggested properties:

• ObserverThreadingPolicy Threading (default: Locking)
• ObserverExceptionPolicy Exceptions (default: Continue)
• ObserverOrderPolicy Order (default: RegistrationOrder)
• bool GenerateAsync (default: inferred)
• bool ForceAsync (default: false)

Enums:

• ObserverThreadingPolicy: SingleThreadedFast, Locking, Concurrent
• ObserverExceptionPolicy: Stop, Continue, Aggregate
• ObserverOrderPolicy: RegistrationOrder, Undefined

---

Semantics (must-have)

Subscriptions

• Subscribe(Action<T>) returns an IDisposable token.
• Dispose() unsubscribes deterministically.
• Duplicate subscriptions are allowed in v1 (invoked multiple times).

Publishing

• Default order: RegistrationOrder.
• Publishing uses snapshot semantics (publish iterates a stable snapshot so modifications during publish do not affect the current cycle).

Exception policies

• Continue (default): invoke all handlers; exceptions do not stop others.

  • v1: either swallow exceptions or route them to an optional user hook (see below). Must be explicit.

• Stop: first exception aborts.

• Aggregate: run all and throw an AggregateException (or return a result) at the end.

Recommended v1 behavior:

• For sync Publish: Continue swallows by default but provides an optional hook: OnSubscriberError(Exception ex) if present.
• For async PublishAsync: same semantics.

Async

• Subscribe(Func<T, ValueTask>) must be supported.
• PublishAsync invokes async handlers in deterministic order.
• Cancellation token behavior: best-effort. If canceled before next invocation, stop and return canceled.

Threading policies

• SingleThreadedFast: no locks; documented as not thread-safe.
• Locking: lock around subscribe/unsubscribe; publish takes snapshot under lock.
• Concurrent: thread-safe with concurrent primitives; ordering may degrade to Undefined unless extra work is done. Must be documented.

---

Optional advanced features (explicitly v2 unless trivial)

• Weak subscriptions
• Backpressure / queueing
• Filters / predicate subscriptions
• “Once” subscriptions

---

Diagnostics (must-have)

Stable IDs, actionable:

• PKOBS001 Type marked [Observer] must be partial.
• PKOBS002 Hub type marked [ObserverHub] must be partial and static.
• PKOBS003 Hub property marked [ObservedEvent] has invalid shape (must be static partial and return the event stream type).
• PKOBS004 Async publish requested but async handler shape unsupported.
• PKOBS005 ...

• Fixes Generator: Create Observer Pattern #45

---

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Pull request overview

Adds a new Observer-pattern Roslyn incremental source generator to PatternKit, along with the public attribute surface area, documentation, examples, and generator-focused unit tests.

Changes:

• Introduces ObserverGenerator plus new Observer attributes/enums in PatternKit.Generators.Abstractions.
• Adds extensive docs and runnable examples for the generated Observer API.
• Adds a new test suite for Observer generation and updates test lockfile dependencies.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 13 comments.

Show a summary per file

File	Description
test/PatternKit.Generators.Tests/packages.lock.json	Updates locked dependencies to support new test/project references.
test/PatternKit.Generators.Tests/ObserverGeneratorTests.cs	Adds unit tests covering generation, subscribe/publish, ordering, async, and exception policies.
src/PatternKit.Generators/Observer/ObserverGenerator.cs	Implements the incremental generator and emits the generated Observer implementation.
src/PatternKit.Generators/AnalyzerReleases.Unshipped.md	Registers new diagnostic IDs PKOBS001–PKOBS003.
src/PatternKit.Generators.Abstractions/Observer/ObserverThreadingPolicy.cs	Adds threading policy enum for the generator configuration surface.
src/PatternKit.Generators.Abstractions/Observer/ObserverOrderPolicy.cs	Adds ordering policy enum for handler invocation ordering.
src/PatternKit.Generators.Abstractions/Observer/ObserverHubAttribute.cs	Adds (future) hub-mode attribute surface.
src/PatternKit.Generators.Abstractions/Observer/ObserverExceptionPolicy.cs	Adds exception policy enum for publish behavior.
src/PatternKit.Generators.Abstractions/Observer/ObserverAttribute.cs	Adds primary [Observer] attribute and config properties.
src/PatternKit.Generators.Abstractions/Observer/ObservedEventAttribute.cs	Adds (future) hub event property marker attribute.
src/PatternKit.Examples/ObserverGeneratorDemo/TemperatureMonitor.cs	Adds sync usage examples (subscribe/publish, ordering, disposal, error hook).
src/PatternKit.Examples/ObserverGeneratorDemo/README.md	Documents how to run and understand the example scenarios.
src/PatternKit.Examples/ObserverGeneratorDemo/NotificationSystem.cs	Adds async usage examples (PublishAsync, exception policies, mixed handlers, cancellation).
docs/generators/toc.yml	Adds Observer generator doc entry to generator docs navigation.
docs/generators/observer.md	Adds full Observer generator documentation and diagnostics reference.

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[JerrettDavis]

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Pull request overview

Copilot reviewed 15 out of 15 changed files in this pull request and generated 6 comments.

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Pull request overview

Copilot reviewed 15 out of 15 changed files in this pull request and generated 1 comment.

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[Copilot]

The Unsubscribe method is generated but never called. The Subscription.Dispose method (lines 552-587) handles subscription removal directly, making this method unnecessary. Consider removing the call to GenerateUnsubscribeMethod on line 189 and the entire GenerateUnsubscribeMethod implementation to reduce generated code size and improve maintainability.

Suggested change

	sb.AppendLine(" private void Unsubscribe(int id)");
	sb.AppendLine(" {");
	switch (config.Threading)
	{
	case 0: // SingleThreadedFast
	sb.AppendLine(" _state.Subscriptions?.RemoveAll(s => s.Id == id);");
	break;
	case 1: // Locking
	sb.AppendLine(" lock (_state.Lock)");
	sb.AppendLine(" {");
	sb.AppendLine(" _state.Subscriptions?.RemoveAll(s => s.Id == id);");
	sb.AppendLine(" }");
	break;
	case 2: // Concurrent
	if (config.Order == 0) // RegistrationOrder
	{
	sb.AppendLine(" System.Collections.Immutable.ImmutableInterlocked.Update(ref _state.Subscriptions, static (list, id) => list?.RemoveAll(s => s.Id == id) ?? list, id);");
	}
	else // Undefined - ConcurrentBag doesn't support efficient removal
	{
	sb.AppendLine(" // ConcurrentBag doesn't support removal; subscription marks itself disposed");
	}
	break;
	}
	sb.AppendLine(" }");
	sb.AppendLine();
	// No-op: subscription removal is handled directly by Subscription.Dispose.
	// This method is retained to preserve the generator API but no longer emits
	// an unused Unsubscribe method into generated types.

[Copilot]

Fixed in aafc0c0. Removed GenerateUnsubscribeMethod call and implementation. The generated Unsubscribe method was never called - Subscription.Dispose handles subscription removal directly, reducing generated code size.

[JerrettDavis]

@copilot apply changes based on the comments in this thread