How to do it...

1. Open the alExtensibleAppProject folder in Visual Studio Code.
2. In Explorer, select Transaction Entry.al. In the Editor, add a new event publisher to the table:

[IntegrationEvent(false, false)]
local procedure OnBeforeGetLastDirectUnitCost(var TransactionEntry: Record "Transaction Entry"; var IsHandled: Boolean)
begin
end;

3. In the GetLastDirectUnitCost() function, create a new local variable:

IsHandled: Boolean;

4. Now, raise the event at the start of the GetLastDirectUnitCost() function:

OnBeforeGetLastDirectUnitCost(Rec, IsHandled);

5. After the call to the event, add logic to exit the function if the subscriber has set IsHandled to true:

if IsHandled then
    exit;

Your function should now look like this, with the new code highlighted as follows:

procedure GetLastDirectUnitCost()
var
    PurchaseInvLine: Record "Purch. Inv. Line";
    IsHandled: Boolean;
begin
    OnBeforeGetLastDirectUnitCost(Rec, IsHandled);
    if IsHandled then
        exit;
    clear("Unit Cost");

    if "Item No." = '' then
        exit;

    PurchaseInvLine.SetRange(Type, PurchaseInvLine.Type::Item);
    PurchaseInvLine.SetRange("No.", "Item No.");  
    if PurchaseInvLine.FindLast() then
        validate("Unit Cost", PurchaseInvLine."Direct Unit Cost");
end;

It's that easy!

Now, a subscriber can take over the function, as well as perform its own calculation and bypass the rest of the logic within the function.