TTreeView - Change Expand and Collapse Image? - delphi

Question

Is it possible using the standard TTreeView to change the Expand and Collapse Image?
I don't mean Node images, I mean the little arrows next to Nodes that have children, like so:
Ideally I would like the arrows to show as + and - Symbols, like the Delphi component structure tree:
If it is possible to change this, how would I go about doing it?
Working Demo based on David's Answer
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, ComCtrls, Themes, uxTheme;
type
TForm1 = class(TForm)
procedure FormCreate(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
type
TMyTreeView = class(TTreeView)
protected
procedure CreateWnd; override;
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
{ TMyTreeView }
procedure TMyTreeView.CreateWnd;
begin
inherited;
if ThemeServices.Enabled and CheckWin32Version(6, 0) then
SetWindowTheme(Handle, nil, nil);
end;
procedure TForm1.FormCreate(Sender: TObject);
var
MyTree: TMyTreeView;
Node: TTreeNode;
begin
MyTree := TMyTreeView.Create(nil);
with MyTree do
begin
Parent := Self;
Height := 100;
Width := 100;
Left := 30;
Top := 30;
Node := Items.Add(nil, 'Item');
Items.AddChild(Node, 'Item');
Node := Items.AddChild(Node, 'Item');
Items.AddChild(Node, 'Item');
end;
end;
end.
The Result:

Answer 1

Tree views in post-Vista Windows have two alternative themes. The theme that you are wanting to avoid is known as the explorer theme. You want to use the standard theme. A control has to opt-in to get the explorer theme. It does so via the SetWindowTheme API. The VCL tree view control calls this to opt-in. It does so at the end of its CreateWnd method.
You can revert to the standard theme by undoing the change like this:
type
TMyTreeView = class(TTreeView)
protected
procedure CreateWnd; override;
end;
procedure TMyTreeView.CreateWnd;
begin
inherited;
if StyleServices.Enabled and TOSVersion.Check(6) and StyleServices.IsSystemStyle then
SetWindowTheme(Handle, nil, nil);
end;
This code is written for XE2. If you have an earlier Delphi then I think you want it like this:
if ThemeServices.Enabled and CheckWin32Version(6, 0) then
SetWindowTheme(Handle, nil, nil);

Answer 2

I addition to Davids answer. Put the following code in some extra unit and add it in the uses after the ComCtrls unit. That way you can use the standard TTreeView and change the theme whenever you like. Or register it in your own package if you like.
type
TTreeView = class(ComCtrls.TTreeView)
private
procedure SetExplorerTheme(const Value: Boolean);
public
property ExplorerTheme: Boolean write SetExplorerTheme;
end;
procedure TTreeView.SetExplorerTheme(const Value: Boolean);
begin
if ThemeServices.ThemesEnabled and CheckWin32Version(6, 0) then
if Value then
SetWindowTheme(Handle, 'Explorer', nil)
else
SetWindowTheme(Handle, nil, nil);
end;
In never Delphi versions you could also use a class helper to avoid the extra inheritance.