skill-events

Skill: AL Event-Driven Architecture

Purpose

Discover, design, and implement event-driven patterns in AL Business Central extensions: event discovery via Event Recorder, subscriber/publisher creation, IsHandled patterns, and extensibility design.

When to Load

This skill should be loaded when:

• Extending standard BC functionality (posting, validation, master data changes)
• Designing custom integration events for a new codeunit or extension
• A subscriber is not firing and needs investigation
• An architecture requires extensibility points for future consumers
• Implementing document posting interventions, workflow triggers, or UI customizations

Core Patterns

Pattern 1: Event Discovery with Event Recorder

Before writing any subscriber, discover which events are raised during the business process:

1. Open the Event Recorder in the BC client / VS Code (a human step — no agent tool here)
2. Start recording
3. Execute the business process in BC (e.g., post a sales order)
4. Stop recording
5. Analyze the event list — filter by object or keyword
6. Identify the OnBefore/OnAfter event closest to your extension point

Use al_search_objects and al_get_object_definition to inspect publisher signatures. Use al_find_references to see who else subscribes to the same event.

Pattern 2: Event Subscriber

// Subscriber codeunit — name with "Handler" suffix
codeunit 50100 "Sales Document Events Handler"
{
    // Attribute breakdown:
    //   ObjectType  — Table, Codeunit, Page, Report, etc.
    //   ObjectId    — Database::"Sales Header" or Codeunit::"Customer Management"
    //   EventName   — exact name of the publisher procedure (e.g., OnBeforeInsert)
    //   ElementName — '' for table/codeunit events; field name for page field triggers
    //   SkipOnMissingLicense  — false (recommended: always run)
    //   SkipOnMissingPermission — false (recommended: always run)

    [EventSubscriber(ObjectType::Table, Database::"Sales Header",
                     OnBeforeInsert, '', false, false)]
    local procedure OnBeforeInsertSalesHeader(
        var SalesHeader: Record "Sales Header";
        RunTrigger: Boolean)
    begin
        ValidateCustomFields(SalesHeader);
    end;
}