How to hide a MDI Child form in Delphi?

Question

How can I hide a MDIChild window in Delphi?

I'm using this code in FormClose() event of my MDI children, but it doesn't seem to work:

procedure TfrmInstrument.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  Action := caNone;
  ShowWindow(Handle, SW_HIDE);
  frmMainForm.MDIChildClosed(Handle);
end;

My child window is minimized instead of being hidden.

Answer 1

There is a protected procedure in TCustomForm defined as:

procedure TCustomForm.VisibleChanging;
begin
  if (FormStyle = fsMDIChild) and Visible and (Parent = nil) then
    raise EInvalidOperation.Create(SMDIChildNotVisible);
end;

Override it in your MDI child windows as:

procedure TMDIChildForm.VisibleChanging;
begin
  // :-P
end;

Here's a simple example

After reading Jeroen's comment, I tried another solution which works as well, but with a little flickering:

procedure TMDIChildForm.VisibleChanging;
begin
  if Visible then
    FormStyle := fsNormal
  else
    FormStyle := fsMDIChild;
end;

Maybe this works on all Windows versions.

PS: I didn't find any problem with the first solution on Windows 2k3SP2 x86 and Windows 7 Ultimate x86

Answer 2

You cannot hide an MDI child window. This is a Win32 limitation.