Author: Tomas Rutkauskas

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):

1   interface
2   
3   procedure ShowForm;
4   procedure CloseForm;
5   
6   implementation
7   
8   var
9     Form: TForm1;
10  
11  procedure ShowForm;
12  begin
13    if (Form <> nil) then
14      Form.Show
15    else
16    begin
17      Form := TForm1.Create(Application);
18      Form.Show;
19    end;
20  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!

21  procedure CloseForm;
22  begin
23    Form.Free;
24    Form := nil;
25  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:
26  
27  function IsFormCreated(Form: TCustomForm): Boolean;
28  var
29    i: Integer;
30  begin
31    Result := False;
32    for i := 0 to Screen.FormCount - 1 do
33    begin
34      if Screen.Forms[i] = Form then
35      begin
36        Result := True;
37        Break;
38      end;
39    end;
40  end;



Solve 3:

Know whether a form already exist before I dynamically create it
41  
42  function IsFormOpen(const FormName: string): Boolean;
43  var
44    i: Integer;
45  begin
46    Result := False;
47    for i := Screen.FormCount - 1 downto 0 do
48      if (Screen.Forms[i].Name = FormName) then
49      begin
50        Result := True;
51        Break;
52      end;
53  end;
54  
55  //First check, if the Form (here Form2) is open. If not, create it. 
56  
57  procedure TForm1.Button1Click(Sender: TObject);
58  begin
59    if not IsFormOpen('Form2') then
60      Form2 := TForm2.Create(Self);
61  
62    Form2.Show
63  end;
64  
65  { For MDI Children }
66  
67  function IsMDIChildOpen(const AFormName: TForm; const AMDIChildName: string): 
68  Boolean;
69  var
70    i: Integer;
71  begin
72    Result := False;
73    for i := Pred(AFormName.MDIChildCount) downto 0 do
74      if (AFormName.MDIChildren[i].Name = AMDIChildName) then
75      begin
76        Result := True;
77        Break;
78      end;
79  end;
80  
81  //First check, if the MDI Child is open. If not, create it. 
82  
83  procedure TForm1.Button2Click(Sender: TObject);
84  begin
85    if not IsMDIChildOpen(Form1, 'MyMDIChild') then
86      MyMDIChild := TMyMDIChild.Create(Self);
87    MyMDIChild.Show;
88    MyMDIChild.BringToFront;
89  end;