Form Design & Events

Delphi uses a What You See Is What You Get (WYSIWYG) approach to UI design. When you run the application, the visual form you designed becomes the main window.

1. The Form Designer and Component Placement

The Form (TForm) is the container for all other visual components.

• Placement: To add a component, select it from the Component Palette and click on the form. The component is automatically named (e.g., Button1, Edit1).
• Form File (.dfm): The IDE automatically saves the component layout, size, position, and property values in the binary .dfm file. You do not edit this file directly.
• Unit File (.pas): The IDE automatically adds a field for the component into the form’s class declaration in the .pas unit.

2. The Object Inspector

The Object Inspector is the primary tool for customizing components at design time. It is split into two tabs:

A. Properties Tab

The Properties tab allows you to set the initial state and appearance of a component.

B. Events Tab

The Events tab lists all actions the component can respond to. When you double-click an empty event field (e.g., OnClick), the IDE does three things:

1. Generates Signature: Creates the method signature (header) in the form’s class declaration (interface).
2. Links Event: Assigns the new method to the event in the .dfm file.
3. Opens Code Editor: Jumps to the implementation section with the skeleton code block for you to fill in.

3. Event-Driven Programming Model

Delphi applications are event-driven. Execution is driven by external events (user clicks, system notifications, timers) rather than a linear sequence of code.

A. Event Handlers

An Event Handler is a procedure or function that runs automatically when a specific event occurs on a component.

All event handlers follow a standard signature, typically taking a Sender: TObject parameter.

// Procedure generated by the IDE when double-clicking the button's OnClick event
procedure TForm1.btnCalculateClick(Sender: TObject);
begin
  // This code executes only when the user clicks btnCalculate
  ShowMessage('Calculation started...');
end;

B. The Sender Parameter

The Sender parameter is a reference to the component that triggered the event. This is useful when multiple components share the same event handler.

// Example: Two buttons (btnYes, btnNo) sharing the same handler
procedure TForm1.QuestionButtonClick(Sender: TObject);
begin
  // Cast the generic TObject back to a TButton to access its specific properties
  if (Sender as TButton).Name = 'btnYes' then 
    ShowMessage('User said Yes')
  else 
    ShowMessage('User said No');
end;

C. Common Form Events

Forms also have key events that manage the application lifecycle: