DelphiBasics
Dynamic
Directive
Allows a class method to be overriden in derived classes
  Function|Procedure declaration; Dynamic;
Description
The Dynamic directive allows a class method to be override (replaced) by a same named method in a derived class.
 
You would mark a function or procedure as Dynamic when you happily allow a programmer who creates a class based on your class to replace its functionality.
 
For example, you might allow a base class to paint a canvas in white, but allow a derived class to paint a picture on the canvas instead. Here, the Dynamic directive is allowing the code to be extended, to be enriched.
 
Dynamic may be followed by the Abstract directive. This modifies the effect of the Dynamic directive. It means that the current class must not code the method - it is there only as a placeholder to remind and ensure that derived classes implement it.
Notes
Dynamic is semantically equivalent to Virtual. The former is optimised for memory, the latter for speed.
Related commands
Abstract Defines a class method only implemented in subclasses
Class Starts the declaration of a type of object class
Function Defines a subroutine that returns a value
Procedure Defines a subroutine that does not return a value
Overload Allows 2 or more routines to have the same name
Override Defines a method that replaces a virtual parent class method
Virtual Allows a class method to be overriden in derived classes
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Example code : Implementing abstract dynamic class methods
// Full Unit code.
// -----------------------------------------------------------
// You must store this code in a unit called Unit1 with a form
// called Form1 that has an OnCreate event called FormCreate.

unit Unit1;

interface

uses
  Forms, Dialogs, Classes, Controls, StdCtrls, SysUtils;

type
  // Define a base TPolygon class :
  // This class is a traingle if 3 sides, square if 4 sides ...
  TPolygon = class
  private
    sideCount  : Integer;  // How many sides?
    sideLength : Integer;  // How long each side?
    shapeArea  : Double;   // Area of the polygon
  protected
     procedure setArea; Dynamic; Abstract;  // Must be implemented in child
    property count  : Integer read sideCount;
    property length : Integer read sideLength;
    property area   : Double  read shapeArea;
    constructor Create(sides, length : Integer);
  end;

  // Define triangle and square descendents
  TTriangle = class(TPolygon)
  protected
    procedure setArea; override;   // Override the abstract method
  end;

  TSquare = class(TPolygon)
  protected
    procedure setArea; override;   // Override the abstract method
  end;

  // Define the form class used by this unit
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation
{$R *.dfm} // Include form definitions

// Create the TPolygon object
constructor TPolygon.Create(sides, length : Integer);
begin
  // Save the number and length of the sides
  sideCount := sides;
  sideLength := length;

  // Set the area using the abstract setArea method :
  // This call will be satisfied only by a subclass
  setArea;
end;

// Implement the abstract setArea parent method for the triangle
procedure TTriangle.setArea;
begin
  // Calculate and save the area of the square
  shapeArea := (sideLength * sideLength) / 2;
end;

// Implement the abstract setArea parent method for the square
procedure TSquare.setArea;
begin
  // Calculate and save the area of the square
  shapeArea := sideLength * sideLength;
end;

// Main line code
procedure TForm1.FormCreate(Sender: TObject);
var
  triangle : TTriangle;
  square   : TSquare;
begin
  // Create a triangle and a square
  triangle := TTriangle.Create(3, 10);
  square   := TSquare.Create(4, 10);

  // Show the areas of our polygons:
  ShowMessageFmt('Triangle area = %f',[triangle.area]);
  ShowMessageFmt('Square   area = %f',[square.area]);
end;

end.
   Triangle area = 50.0
   Square   area = 100.0
 
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