this->Hide() works only once - delphi

Question

My environment: C++ Builder XE4
I am working on hiding/showing one application from other application, both built using XE4.
Two project
UnitShow: to show/hide UnitHide
UnitHide: to be shown/hidden from UnitShow
The code for UnitShow is as follows:
void __fastcall TFormShow::B_showClick(TObject *Sender)
{
HWND hwnd = GetDesktopWindow();
hwnd = FindWindowEx(hwnd, NULL, L"TFormHide", NULL);
ShowWindow(hwnd, SW_SHOWNORMAL);
}
The code for UnitHide is as follows:
void __fastcall TFormHide::B_hideClick(TObject *Sender)
{
this->Hide();
}
With above, I tried followings.
On UnitHide, click B_hide >> UnitHide is hidden
On UnitShow, click B_show >> UnitHide is shown
On UnitHide, click B_hide >> UnitHide is not hidden
I expected that the 2nd B_hide button push will hide UnitHide.
What is the cause of this problem?

Answer 1

I tried followings.
void __fastcall TFormHide::B_hideClick(TObject *Sender)
{
bool isShowing = this->Showing;
#if 1
this->Hide();
#else
ShowWindow(this->Handle, SW_HIDE);
#endif
}
When I use this->Hide(), after shown by UnitShow, the isShowing is false. With this, it seems this->Hide() does not work (after the 2nd time).
On the other hand, using ShowWindow(this->Handle, SW_HIDE); the isShowing is true always. With this, hiding after the 2nd time works.
For this->Hide(), it seems that I have to use this->Show() from other application, which currently I do not know how to do.

Answer 2

Use FindWindow function instead of FindWindowEx as following:
procedure TfShow.btnShowClick(Sender: TObject);
var
hWindow:Hwnd;
begin
hWindow := FindWindow('TFormHide', nil);
ShowWindow(hWindow, SW_SHOWNORMAL);
end;
Regarding the UnitHide you can call self.hide in your button
procedure TfHide.btnHideClick(Sender: TObject);
begin
self.Hide; {Hide the hidden form }
//or
ShowWindow(Self.Handle, SW_HIDE);
end;
You can do the same calls in the C++
Update
as Remy mentions in his answer
So you can use the Following:
Define custom messages in the both applications
const
Show_MESSAGE = WM_USER + 1;
Hide_MESSAGE = Show_MESSAGE + 1;
In the HideUnit Define a handler for the message
private
{ Private declarations }
procedure WMShow(var Message: TMessage); message Show_MESSAGE;
procedure WMHide(var Message: TMessage); message Hide_MESSAGE;
....
implementation
procedure TfHide.WMShow(var Message: TMessage);
begin
Self.Show;
end;
procedure TfHide.WMHide(var Message: TMessage);
begin
Self.Hide;
end;
Sending the message form the ShowUnit > Showbtn
procedure TfShow.btnShowClick(Sender: TObject);
var
hWndX : HWND;
begin
hWndX := FindWindow('TfHide',nil);
if hWndX <> 0 then begin
SendMessage(hWndX, Show_MESSAGE, 0, 0);
end
else MessageBox(0, 'Window not found', 'Msg', 0);
end;
Hiding the Form from hidden form
procedure TfHide.btnHideClick(Sender: TObject);
begin
SendMessage(Handle, Hide_MESSAGE, 0, 0);
end;