How to respond to changes in fields of object properties in Delphi

Question

In Delphi 7, descend a new component from TGraphicControl, and add a TFont property, implement the paint method to write some string using the TFont property. Install the component.

At design time when you change the TFont property using the property dialog, it will be reflected in your component instantaneously. But when you change individual properties of TFont like Color or Size, your component will not be repainted until you hover over it.

How do I correctly handle changes in fields of object properties?

Answer 1

Assign an event handler to the TFont.OnChange event. In the handler, Invalidate() your control to trigger a repaint. For example:

type
  TMyControl = class(TGraphicControl)
  private
    FMyFont: TFont;
    procedure MyFontChanged(Sender: TObject);
    procedure SetMyFont(Value: TFont);
  protected
    procedure Paint; override;
  public
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;
  published
    property MyFont: TFont read FMyFont write SetMyFont;
  end;

constructor TMyControl.Create(AOwner: TComponent);
begin
  inherited;
  FMyFont := TFont.Create;
  FMyFont.OnChange := MyFontChanged;
end;

destructor TMyControl.Destroy;
begin
  FMyFont.Free;
  inherited;
end;

procedure TMyControl.MyFontChanged(Sender: TObject);
begin
  Invalidate;
end;

procedure TMyControl.SetMyFont(Value: TFont);
begin
  FMyFont.Assign(Value);
end;

procedure TMyControl.Paint;
begin
  // use MyFont as needed...
end;