Event Subscribers in AL

Build extension-safe customizations using event publishers and subscribers without modifying the base application.

35 minAdvanced

Learning Objectives

Subscribe to table, page, and codeunit events
Use OnBefore and OnAfter event patterns
Implement validation and business logic via event subscribers

Why Events?

Event subscribers let you extend Business Central without modifying Microsoft code. This is the primary customization pattern for AppSource apps and upgrade-safe extensions.

Publisher — Microsoft (or you) raises an event
Subscriber — Your code reacts to the event

Event Subscriber Syntax

CustomerEventSub.Codeunit.al

codeunit 50103 "Customer Event Subscriber"
{
    [EventSubscriber(ObjectType::Table, Database::Customer, 'OnBeforeInsertEvent', '', false, false)]
    local procedure OnBeforeCustomerInsert(var Rec: Record Customer; RunTrigger: Boolean)
    begin
        if Rec.Name = '' then
            Error('Customer name cannot be empty.');
    end;

    [EventSubscriber(ObjectType::Table, Database::Customer, 'OnAfterInsertEvent', '', false, false)]
    local procedure OnAfterCustomerInsert(var Rec: Record Customer; RunTrigger: Boolean)
    begin
        LogCustomerCreation(Rec."No.");
    end;
}

Event Types

Event Pattern	When It Fires
`OnBeforeInsertEvent`	Before a record is inserted
`OnAfterInsertEvent`	After a record is inserted
`OnBeforeModifyEvent`	Before a record is modified
`OnAfterModifyEvent`	After a record is modified
`OnBeforeDeleteEvent`	Before a record is deleted
`OnAfterValidateEvent`	After a field is validated

Subscribing to Codeunit Events

SalesPostingSub.Codeunit.al

[EventSubscriber(ObjectType::Codeunit, Codeunit::"Sales-Post", 'OnBeforePostSalesDoc', '', false, false)]
local procedure OnBeforePostSalesDoc(var SalesHeader: Record "Sales Header")
begin
    SalesHeader.TestField("Approval Status", SalesHeader."Approval Status"::Approved);
end;

EventSubscriber Parameters

The last two boolean parameters control skip and priority. Use false, false for standard subscribers. Set priority when execution order matters.

Integration Events (Custom Publishers)

You can publish your own events for other extensions to subscribe to:

Publisher.Codeunit.al

codeunit 50110 "Bonus Publisher"
{
    [IntegrationEvent(false, false)]
    procedure OnBeforeCalculateBonus(var CustomerNo: Code[20]; var BonusAmount: Decimal)
    begin
    end;

    procedure CalculateBonus(CustomerNo: Code[20]): Decimal
    var
        BonusAmount: Decimal;
    begin
        OnBeforeCalculateBonus(CustomerNo, BonusAmount);
        // Calculation logic...
        exit(BonusAmount);
    end;
}

SingleInstance Codeunits

For stateful event handling across a session:

SessionState.Codeunit.al

codeunit 50111 "Session State Manager"
{
    SingleInstance = true;

    var
        IsProcessing: Boolean;

    procedure SetProcessing(Value: Boolean)
    begin
        IsProcessing := Value;
    end;

    procedure IsCurrentlyProcessing(): Boolean
    begin
        exit(IsProcessing);
    end;
}

Best Practices

Keep subscribers small — Delegate complex logic to separate procedures
Don't modify base app code — Always use events for customization
Test upgrade compatibility — Events are stable; triggers on base tables are not
Use IntegrationEvent for your own extensibility points
Avoid infinite loops — Be careful with Modify/Insert inside OnAfter events

RunTrigger Parameter

Check RunTrigger before executing subscriber logic. When false, the trigger was called internally and your subscriber should typically skip processing.

Key Revision Points (MB-820)

Event subscribers are the upgrade-safe customization pattern
OnBefore events can prevent operations; OnAfter events react to completed operations
IntegrationEvent lets you build extensible apps
SingleInstance codeunits maintain state across a user session

Learning Objectives

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