Algorithms and Lazarus

Structural Elements: MODULARITY

Modules in Lazarus are code segments that are re-usable. In a visual system every event-handler is a module (either a procedure or a function).

Procedures are encapsulated actions, Functions are similar but are designed to return with an answer as well. In a visual system there are two DIFFERENT types of modules - this that are aware of form components and those that aren't. When making modules it is important to determine (first) whether the module is going to need to play with/use a form component - if it does, it needs to be written a special way.

Parameters in a module are ways to communicate with it while it is running. Many standard event handlers come equipped with a sender parameter. This parameter corresponds to the object (a button or other visual component) that triggered the module. Smart event handlers know what triggers them and does things relative to the initiator of the action.

Typically there are 2 types of modules - those that are FORM AWARE and those that are not. They differ markedly in what they can do with visual projects.

Form Aware Modules.

Modules (procedures or functions that are DECLARED in the FORM CLASS DEFINITION) and then DEFINED in the body of a UNIT are form aware. Such modules can act directly on form components because they are part of the form's parentage. Some have a sender, some do not but all are introduced (or declared) in for CLASS definition at the top of the unit (see procedure initialise in the code below:)

type
  Tform1 = class(TForm)
    Label1: TLabel;
    Label2: TLabel;
    procedure initialise;
    procedure FormCreate(Sender: TObject);
    procedure newClick(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

The module is defined later as:

Procedure TForm1.initialise;
begin
{instructions go here}

end;

Form-aware modules are very useful for encapsulating tasks that need to be done more than once - code recycling is a good thing, code repetition (ie. typing the same code more than once) is generally not.