Author: Tomas Rutkauskas

ListBox.Items.Add is slow and flickers

Answer:

Adding a (larger) group of entries to a ListBox is very slow, because after every 
"items.add" call the ListBox is repainted.

There are two ways to overcome this: 

Use the Windows message WM_SETREDRAW (see Win32.hlp for details). The VCL provides 
two methods for this: BeginUpdate and EndUpdate. I would assume that this is faster 
than solve #2. 
Read the strings in a temporary TStringList object. Maybe you already have such a 
list - in this case you should use your existing list. Then use the Assign method 
to transfer the whole list.

Solve 1:
1   
2   procedure TForm1.Button1Click(Sender: TObject);
3   var
4     i: integer;
5   begin
6     ListBox1.Items.BeginUpdate;
7     for i := 1 to maxitems do
8       ListBox1.Items.add(IntToStr(i));
9     ListBox1.Items.EndUpate;
10  end;



Solve 2:
11  
12  procedure TForm1.Button2Click(Sender: TObject);
13  var
14    i: integer;
15    tmp: tstringlist;
16  begin
17    tmp := TStringList.Create;
18    for i := 1 to maxitems do
19      tmp.add(inttostr(i));
20    ListBox1.Items.Assign(tmp);
21    tmp.Free;
22  end;