refactoring

Delphi Code Refactoring — Rules

Use these rules when detecting or fixing code smells in Object Pascal. Refactoring doesn't change behavior — it just improves the internal structure.

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🔴 Code Smells that Require Immediate Refactoring

Code Smell	Symptom	Technique
Long method	> 20 lines	Extract Method
Bloated class	> 300 lines / multiple responsibilities	Extract Class
Magic numbers	if Age > 18	Replace with Constant
Deep Nesting	if...if...if above 2 levels	Replace with Guard Clauses
Duplicate code	Same block in ≥ 2 places	Extract Method / Pull Up
Excessive parameters	Method with > 3 parameters	Introduce Parameter Object
Mixed UI logic	Business in OnClick	Move Method to Service
with statement	with DataSet do...	Remove With
Unnecessary temporary variable	Used only once	Inline Temp
Conditional with type	if Obj is TSubClass then	Replace Conditional with Polymorphism
Message chain	A.B.C.D.Execute	Law of Demeter / Introduce Method
Comment explaining what	The code should explain itself	Rename + Extract Method

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✂️ Extract Method

Extract purposeful blocks of code into named methods.

//❌ BEFORE — long method without separation of responsibilities
procedure TOrderService.PlaceOrder(AOrder: TOrder);
var
  LTotal: Currency;
  LDiscount: Currency;
  LItem: TOrderItem;
begin
  // Calcula total
  LTotal := 0;
  for LItem in AOrder.Items do
    LTotal := LTotal + (LItem.UnitPrice * LItem.Quantity);

  //Apply discount
  if AOrder.Customer.IsVip then
    LDiscount := LTotal * 0.10
  else if LTotal > 500 then
    LDiscount := LTotal * 0.05
  else
    LDiscount := 0;

  LTotal := LTotal - LDiscount;

  //Validates stock
  for LItem in AOrder.Items do
  begin
    if LItem.Quantity > LItem.Product.StockQty then
      raise EInvalidOrderException.CreateFmt(
        'Estoque insuficiente: %s', [LItem.Product.Name]);
  end;

  AOrder.TotalAmount := LTotal;
  FRepository.Save(AOrder);
end;

//✅ AFTER — each responsibility in its own method
procedure TOrderService.PlaceOrder(AOrder: TOrder);
begin
  ValidateStock(AOrder);
  AOrder.TotalAmount := CalculateTotalWithDiscount(AOrder);
  FRepository.Save(AOrder);
end;

function TOrderService.CalculateSubtotal(AOrder: TOrder): Currency;
var LItem: TOrderItem;
begin
  Result := 0;
  for LItem in AOrder.Items do
    Result := Result + (LItem.UnitPrice * LItem.Quantity);
end;

function TOrderService.CalculateDiscount(AOrder: TOrder; ASubtotal: Currency): Currency;
const
  VIP_DISCOUNT_RATE     = 0.10;
  BULK_DISCOUNT_RATE    = 0.05;
  BULK_DISCOUNT_MINIMUM = 500;
begin
  if AOrder.Customer.IsVip then
    Result := ASubtotal * VIP_DISCOUNT_RATE
  else if ASubtotal > BULK_DISCOUNT_MINIMUM then
    Result := ASubtotal * BULK_DISCOUNT_RATE
  else
    Result := 0;
end;

function TOrderService.CalculateTotalWithDiscount(AOrder: TOrder): Currency;
var LSubtotal: Currency;
begin
  LSubtotal := CalculateSubtotal(AOrder);
  Result := LSubtotal - CalculateDiscount(AOrder, LSubtotal);
end;

procedure TOrderService.ValidateStock(AOrder: TOrder);
var LItem: TOrderItem;
begin
  for LItem in AOrder.Items do
    if LItem.Quantity > LItem.Product.StockQty then
      raise EInvalidOrderException.CreateFmt(
        'Estoque insuficiente: %s', [LItem.Product.Name]);
end;