VCL Fundamentals

33.1 The VCL Architecture

VCL = Windows-only, a native Win32/Win64 wrapper. It's fast, mature, and has a huge third-party ecosystem.

TObject
└── TPersistent (streaming, Assign)
    └── TComponent (Owner, Name, Tag, published properties, design-time support)
        ├── TControl (visual: Left, Top, Width, Height, Align, Anchors, Visible, Enabled)
        │   ├── TGraphicControl (no window handle: TLabel, TShape, TImage, TSpeedButton)
        │   └── TWinControl (has an HWND: focus, children)
        │       └── TCustomControl → TButton, TEdit, TMemo, TPanel, TListView, TForm...
        └── Non-visual components (TTimer, TDataSource, TFDConnection, TActionList, TOpenDialog)

33.2 Forms and Common Controls

TButton, TLabel, TEdit, TMemo, TCheckBox, TRadioButton, TListBox, TComboBox, TPanel, TGroupBox, TRadioGroup, TScrollBar, TMainMenu, TPopupMenu, TActionList

TBitBtn, TSpeedButton, TMaskEdit, TStringGrid, TDrawGrid, TImage, TShape, TScrollBox, TSplitter, TLabeledEdit, TColorBox, TValueListEditor, TControlBar

TPageControl, TTabControl, TTreeView, TListView, TRichEdit, TProgressBar, TTrackBar, TUpDown, TDateTimePicker, TMonthCalendar, TStatusBar, TToolBar, THeaderControl, TImageList TTimer, TPaintBox, TMediaPlayer

TOpenDialog, TSaveDialog, TFontDialog, TColorDialog, TPrintDialog, TFindDialog, TTaskDialog, TFileOpenDialog

TDBGrid, TDBEdit, TDBNavigator, TDBText, TDBComboBox, TDBLookupComboBox, TDBMemo, TDBImage

TCardPanel, TSearchBox, TSplitView, TToggleSwitch, TActivityIndicator, TRelativePanel, TStackPanel, TNumberBox, TEdgeBrowser (WebView2), TVirtualImageList + TImageCollection (HighDPI)

33.3 Event Handling and Form Lifecycle

Form events in order: OnCreate → OnShow → OnActivate → OnPaint ... OnCloseQuery → OnClose (Action: caFree, caHide, caMinimize, caNone) → OnDestroy.

Creating forms dynamically:

Remove auto-created forms in Project → Options → Forms, and create them on demand to save memory and startup time.

procedure TMainForm.FormCreate(Sender: TObject);
begin
  Caption := 'Customer Manager';
  cboCity.Items.AddStrings(['Pune', 'Mumbai', 'Delhi']);
  cboCity.ItemIndex := 0;
end;

procedure TMainForm.btnAddClick(Sender: TObject);
begin
  if Trim(edtName.Text) = '' then
  begin
    MessageDlg('Please enter a name.', mtWarning, [mbOK], 0);
    edtName.SetFocus;
    Exit;
  end;
  lstCustomers.Items.Add(Format('%s (%s)', [edtName.Text, cboCity.Text]));
  edtName.Clear;
  StatusBar1.SimpleText := Format('%d customers', [lstCustomers.Count]);
end;

procedure TMainForm.FormCloseQuery(Sender: TObject; var CanClose: Boolean);
begin
  CanClose := MessageDlg('Exit?', mtConfirmation, [mbYes, mbNo], 0) = mrYes;
end;

procedure TMainForm.FormKeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
begin
  if (Key = VK_F5) then RefreshData;           // KeyPreview := True on the form
end;

var Dlg := TSettingsForm.Create(nil);
try
  Dlg.UserName := CurrentUser;
  if Dlg.ShowModal = mrOk then CurrentUser := Dlg.UserName;
finally
  Dlg.Free;
end;

33.4 Actions, Menus and Layout

TActionList / TActionManager: centralize commands (Caption, ShortCut, ImageIndex, OnExecute, OnUpdate for enabling/disabling). Link actions to buttons and menu items.

Layout: Align (alTop, alClient...), Anchors, Constraints, Margins/AlignWithMargins, TGridPanel, TFlowPanel, TRelativePanel, TStackPanel. HighDPI (per-monitor v2) with TVirtualImageList.

33.5 Frames and Data Modules

TFrame: reusable composite UI pieces (like user controls) designed visually and dropped on forms.

TDataModule: a non-visual container for database connections, queries, actions, and business components, shared between forms. It separates data access from the UI.