什么是透明（和Windows主题有关系），研究TLable和TPanel是两个好例子

在controls.pas单元里只有判断，没有赋值，所以一直不是很明白。于是在stdCtrls.pas里找了几个例子，直观加深一下印象：

constructor TCustomLabel.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  ControlStyle := ControlStyle + [csReplicatable];
  Width := 65;
  Height := 17;
  FAutoSize := True;
  FShowAccelChar := True;
  { The "default" value for the Transparent property depends on
    if you have Themes available and enabled or not. If you have
    ever explicitly set it, that will override the default value. }
  if ThemeServices.ThemesEnabled then
    ControlStyle := ControlStyle - [csOpaque]
  else
    ControlStyle := ControlStyle + [csOpaque]; // 默认走这里
end;

function TCustomLabel.GetTransparent: Boolean;
begin
  Result := not (csOpaque in ControlStyle);
end;

procedure TCustomLabel.SetTransparent(Value: Boolean);
begin
  if Transparent <> Value then
  begin
    if Value then
      ControlStyle := ControlStyle - [csOpaque] 
    else
      ControlStyle := ControlStyle + [csOpaque];
    Invalidate;
  end;
  FTransparentSet := True;
end;

constructor TCustomListBox.Create(AOwner: TComponent);
const
  ListBoxStyle = [csSetCaption, csDoubleClicks, csOpaque];
begin
  inherited Create(AOwner);
  if NewStyleControls then
    ControlStyle := ListBoxStyle else
    ControlStyle := ListBoxStyle + [csFramed];
  Width := 121;
  Height := 97;
  TabStop := True;
  ParentColor := False;
  FAutoComplete := True;
  FItems := TListBoxStrings.Create;
  TListBoxStrings(FItems).ListBox := Self;
  FCanvas := TControlCanvas.Create;
  TControlCanvas(FCanvas).Control := Self;
  FItemHeight := 16;
  FBorderStyle := bsSingle;
  FExtendedSelect := True;
  FOldCount := -1;
end;

再在ExtCtrls.pas里找几个素材：

procedure TImage.PictureChanged(Sender: TObject);
var
  G: TGraphic;
  D : TRect;
begin
  if AutoSize and (Picture.Width > 0) and (Picture.Height > 0) then
    SetBounds(Left, Top, Picture.Width, Picture.Height);
  G := Picture.Graphic;
  if G <> nil then
  begin
    if not ((G is TMetaFile) or (G is TIcon)) then
      G.Transparent := FTransparent;
        D := DestRect;
    if (not G.Transparent) and (D.Left <= 0) and (D.Top <= 0) and
       (D.Right >= Width) and (D.Bottom >= Height) then
      ControlStyle := ControlStyle + [csOpaque]
    else  // picture might not cover entire clientrect
      ControlStyle := ControlStyle - [csOpaque];
    if DoPaletteChange and FDrawing then Update;
  end
  else ControlStyle := ControlStyle - [csOpaque];
  if not FDrawing then Invalidate;
end;

constructor TCustomPanel.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  ControlStyle := [csAcceptsControls, csCaptureMouse, csClickEvents,
    csSetCaption, csOpaque, csDoubleClicks, csReplicatable];
  { When themes are on in an application default to making
    TCustomPanel's paint with their ParentBackground }
  if ThemeServices.ThemesEnabled then
    ControlStyle := ControlStyle + [csParentBackground] - [csOpaque];
  Width := 185;
  Height := 41;
  FAlignment := taCenter;
  BevelOuter := bvRaised;
  BevelWidth := 1;
  FBorderStyle := bsNone;
  Color := clBtnFace;
  FFullRepaint := True;
  UseDockManager := True;
end;

procedure TCustomPanel.SetParentBackground(Value: Boolean);
begin
  { TCustomPanel needs to not have csOpaque when painting
    with the ParentBackground in Themed applications }
  if Value then
    ControlStyle := ControlStyle - [csOpaque]
  else
    ControlStyle := ControlStyle + [csOpaque];
  FParentBackgroundSet := True;
  inherited;
end;