How are Delphi units automatically added when a component is added to a form? [duplicate]

Question

If I add a TXMLDocument to a form from the IDE, the units Xml.XMLDoc, Xml.xmldom, Xml.XMLIntf, Xml.Win.msxmldom are automatically added (on save/compile), how does the IDE know to add these units. I understand why/how XMLDoc is added (it contains TXMLDocument) but what about the others.

Additionally if I change the DOMVendor from MSXML to ADOM XML v4, Xml.adomxmldom is automatically added (on the next compile). At this point I can remove Xml.Win.msxmldom without it being added back automatically. How does the IDE know this based on a component property?

I have two reasons for asking this question, firstly curiosity, but secondly I'm cleaning up the uses section of a large number of units (hundreds). The project uses DevExpress, and it adds heaps of additional files to the uses - for instance add a TcxSpinEdit then cxSpinEdit, cxGraphics, cxControls, cxLookAndFeels, cxLookAndFeelPainters, cxContainer, cxEdit, cxTextEdit, cxMaskEdit are added. I'm wanting to minimize the uses clause where controls have been removed from forms (but their units remain in the uses) and thus need to understand the process whereby they are added better.

Answer 1

Components can arrange that their presence in the designer forces specific units to be added to the unit's uses clause. They do so by calling RegisterSelectionEditor to register their TSelectionEditor sub-classes. These sub-classes override TSelectionEditor.RequiresUnits and there specify the units which must be added.

For instance:

uses
  DesignEditors;
....
type
  TMySelectionEditor = class(TSelectionEditor)
  public
    procedure RequiresUnits(Proc: TGetStrProc); override;
  end;

procedure TMySelectionEditor.RequiresUnits(Proc: TGetStrProc);
begin
  Proc('MyUnit');
end;

procedure Register;
begin
  RegisterSelectionEditor(TMyComponent, TMySelectionEditor);
end;