Dynamic Inheritance

[rcloutie]

Hello,

I'm building a VCL based on TListBox control. I want to setup a brand new property called CheckBox (Boolean) which will flip from TListBox based inheritance to TCheckListBox based inheritance.

TNewList = class(TListBox);
turns to
TNewList = class(TCheckListBox);

Is it possible to do so?

Thanks

 

[Griffyn]

No way. You'd be better off setting up your component as a TCheckListBox, and modifying the drawing behaviour so that your Checkbox property influences what the list looks like.

 

[rcloutie]

Ok no dynamic inheritance. No problem to set up component as TCheckListBox. Now, how to modify the drawing behavior (hide the CheckBox section within the ListBox)?

Thanks

 

[Griffyn]

Set the Style property to lbOwnerDrawFixed. Then attach to the OnDrawItem event. You then write code in your custom DrawItem method to draw each item the way you want.

Alternatively, you may want to use a TPageControl instead, set up two pages, one with a TListBox, the other with a TCheckListBox, hide the TPageControl tabs and simply flip between the two pages as needed.

 

[rcloutie]

Ok, and any idea of the code to write within OnDrawItem?

 

[Griffyn]

sure. start with this code block:

procedure TForm1.CheckListBox1DrawItem(Control: TWinControl; Index: Integer Rect: TRect; State: TOwnerDrawState);
begin
  with TCheckListBox(Control).Canvas do
  begin
    // do stuff here to draw on the canvas
  end;
end;

Use the help file to find out how to use the Canvas property. The first one you should check out is the TextOut method. You can change colours, font styles, and you can draw lines and shapes on the canvas. Daunting at first, but fun

This OnDrawItem method gets called each time an item needs to be drawn. Use the variables passed to the OnDrawItem method. Index will be the Item index that is to be drawn, and Rect will be a TRect variable of the space on the canvas that this item can occupy. Use the help file to see what TOwnerDrawState is about.