How to check if a child window exists?

Question

I have a main form (MainForm) and a MDI child window (TFormChild). I want to create multiple TFormChild forms, but the first one must behave in a certain way so I need to detect if a TFormChild window already exists.

I use this code but it is not working:

function FindChildWindowByClass(CONST aParent: HWnd; CONST aClass: string): THandle;   
begin
  Result:= FindWindowEx(aParent, 0, PChar(aClass), NIL);
end;

I call it like this:

Found:= FindChildWindowByClass(MainForm.Handle, 'TFormChild')> 0;   

Answer 1

In a form, you can refer to the MDIChildCount and MDIChildren properties.

for example :

var
  i: integer;
begin
  for i:= 0 to MainForm.MDIChildCount-1 do
  begin
    if MainForm.MDIChildren[i] is TFormChild  then
    ...
  end;
  ...
end;

Answer 2

Call it like

Found:= FindChildWindowByClass(MainForm.ClientHandle, 'TFormChild')> 0;  

MDI child windows are children of the 'MDICLIENT', ClientHandle property of TCustomFrom holds the handle.

Answer 3

The best way to accomplish this is to have the form you want to open actually check to see if it already exists. To do so your, form must declare a class procedure. Declared as a class procedure, the proc can be called regardless of whether the form exists or not.

Add to your form's public section

class procedure OpenCheck;

then the procedure looks like this

Class procedure TForm1.OpenCheck;
var
f: TForm1;
N: Integer;
begin
   F := Nil;
   With Application.MainForm do
   begin
      For N := 0 to MDIChildCount - 1 do
      begin
         If MDIChildren[N] is TForm1 then
            F := MDIChildren[N] as TForm1;
      end;
   end;
   if F = Nil then //we know the form doesn't exist
      //open the form as the 1st instance/add a new constructor to open as 1st
   else
      //open form as subsequent instance/add new constructor to open as subsqt instance
end;

Add Form1's unit to your mdiframe's uses clause.

To open the form, call your class procedure, which in turn will call the form's constructor.

TForm1.OpenCheck;

One word of warning using class procedures, do not access any of the components/properties of the form. Since the form does not actually have to be instantiated, accessing them would produce an access violation/that is until you know F is not nil. Then you can use F. to access form components/properties.