Author: Tomas Rutkauskas

I am writing a MDI application. In this application, I just want to create one 
instance of the MDI child form. That is, if the user clicks New the first time, a 
MDI child form is created, but on subsequent click, no MDI child will be created. 
How can I do this and how can I refer to the MDI child that I have created.

Answer:

I assume you know the method to create an instance of a MDIChild form, but I'm 
going to go through it just in case:

Create your form, set its formstyle property to fsMDIChild, remove it from the 
autocreate list in your Project|Options.
Use the following code to create the form at run-time (I'm using Button1.Click to 
create the event):


1   procedure TMainForm.Button1Click(Sender: TObject);
2   var
3     MyChildForm: TMDIChild {where TMDIChild is TNameOfYourForm}
4   begin
5     MyChildForm := TMDIChild.Create(Application);
6   end;



Now, to ensure that only 1 instance of a MDIChild is created change the above code 
to read:


7   begin
8     if MDIChildCount < 1 then
9       MyChildForm := TMDIChild.Create(Application);
10  end;



This code will ensure that only one MDIChild can be open at a given time. Remember 
to place Action:=caFree in the OnClose event handler of your MDIChild to free 
memory and resources when it's closed.

To access properties of your MDIChild (Even though there's only going to be 1 in 
existence) you must use the following type of code (Here an event of the MDIParent 
is going to disable a component on the MDIChild):


11  procedure TMainForm.TurnItOffClick(Sender: TObject);
12  begin
13    MyChildForm(ActiveMDIChild).Button1.Enabled := False;
14  end;



Now, you didn't state whether you only wanted 1 MDIChild form open in the parent form or only 1 of each particular type. If it's the latter, you'll need to access the classes of all your MDI children to see if an instance already exists.