Author: Tomas Rutkauskas

I have a frame with two panels on it (right and left). The left panel is a pretty 
standard treeview. The right panel is a representation of the data record for the 
selected node in the tree view. The right panel is dynamic, that is it is 
constructed on the fly at run time as the users have the ability to define what 
fields they wish to show and what the labels are for those fields.

I need the right panel to change the treeview node text when the record in the 
right panel is posted. The general consensus is that it'd best be done with a 
callback, but I've never done one before. Can anyone give me a simple example of 
how to do this?

Answer:

What you are describing is a Mediator Pattern. In essence, you set up a class which 
is aware of your two (or more) components. Set up properties to record changes and 
then in the appropriate event handler, simply assign the new values. The Mediator 
is solely responsible for synchronizing the two.

Very simple example (yours will be slightly more complex, but surprisingly probably 
won't have too much more code):

1   TEditBoxMediator = class
2   private
3     FIsChanging: boolean;
4     FFirstBox: TEdit;
5     FSecondBox: TEdit;
6     function GetText: string;
7     procedure SetText(Value: string);
8   public
9     property Text: string read GetText write SetText;
10    constructor Create(const FirstBox, SecondBox: TEdit);
11  end;
12  
13  constructor TEditBoxMediator.Create(const FirstBox, SecondBox: TEdit);
14  begin
15    inherited Create;
16    FFirstBox := FirstBox;
17    FSecondBox := SecondBox;
18    FIsChanging := False;
19  end;
20  
21  function TEditBoxMediator.GetText: string;
22  begin
23    Result := FFirstBox.Text;
24  end;
25  
26  procedure TEditBoxMediator.SetText(Value: string);
27  begin
28    if FIsChanging then
29      Exit;
30    FIsChanging := True;
31    if FFirstBox.Text <> Value then
32      FFirstBox.Text := Value;
33    if FSecondBox.Text <> Value then
34      FSecondBox.Text := Value;
35    FIsChanging := False;
36  end;
37  
38  procedure TForm1.Create {...}
39  begin
40    FEditBoxMediator := TEditBoxMediator.Create(Edit1, Edit2);
41  end;
42  
43  procedure TForm1.Edit1Change(Sender: TObject);
44  begin
45    FEditBoxMediator.Text := Edit1.Text;
46  end;


and so on.


The idea is that the mediator handles the changes (and uses the internal flag to 
prevent an endless
loop).