Check if a TForm has already been created

Problem/Question/Abstract:

How to check if a TForm has already been created

Answer:

Solve 1:

You just need to provide a little code to manage the lifetime of the form. Something like this (which assumes there should only ever be one instance of the TForm1 class):

interface

procedure ShowForm;
procedure CloseForm;

implementation

var
  Form: TForm1;

procedure ShowForm;
begin
  if (Form <> nil) then
    Form.Show
  else
  begin
    Form := TForm1.Create(Application);
    Form.Show;
  end;
end;

You can only check for nil the first time, because once you create and free the form it won't be nil again unless you make it nil in code!

procedure CloseForm;
begin
  Form.Free;
  Form := nil;
end;

Then simply call these help functions whenever you want to show or close the form.


Solve 2:

You can use the FindWindow API function, but for checking if a form has been created within an application, this is easier:

function IsFormCreated(Form: TCustomForm): Boolean;
var
  i: Integer;
begin
  Result := False;
  for i := 0 to Screen.FormCount - 1 do
  begin
    if Screen.Forms[i] = Form then
    begin
      Result := True;
      Break;
    end;
  end;
end;


Solve 3:

Know whether a form already exist before I dynamically create it

function IsFormOpen(const FormName: string): Boolean;
var
  i: Integer;
begin
  Result := False;
  for i := Screen.FormCount - 1 downto 0 do
    if (Screen.Forms[i].Name = FormName) then
    begin
      Result := True;
      Break;
    end;
end;

First check, if the Form (here Form2) is open. If not, create it.

procedure TForm1.Button1Click(Sender: TObject);
begin
  if not IsFormOpen('Form2') then
    Form2 := TForm2.Create(Self);

  Form2.Show
end;

{ For MDI Children }

function IsMDIChildOpen(const AFormName: TForm; const AMDIChildName: string): Boolean;
var
  i: Integer;
begin
  Result := False;
  for i := Pred(AFormName.MDIChildCount) downto 0 do
    if (AFormName.MDIChildren[i].Name = AMDIChildName) then
    begin
      Result := True;
      Break;
    end;
end;

First check, if the MDI Child is open. If not, create it.

procedure TForm1.Button2Click(Sender: TObject);
begin
  if not IsMDIChildOpen(Form1, 'MyMDIChild') then
    MyMDIChild := TMyMDIChild.Create(Self);
  MyMDIChild.Show;
  MyMDIChild.BringToFront;
end;