How can I create a form that has a list box that I can perform an incremental search on?
There are a couple of ways to do incremental searching with list boxes. One way is hard and slow and involves going through the list item by item--the other approach is easy and fast. We are going to take the easy and fast option. For those of you who aren't familiar with incremental searching with list boxes, the concept is simple: A user types part of a string into an edit box, then the list box automatically selects one of its items that most closely matches the value typed by the user. For example, open up any topic search dialog in a Windows Help file. If you type into the edit box, the list will scroll to the value that most closely matches what you type.
Why is having this kind of capability useful? Because it's tedious to scroll through a list that has lots of items. Imagine if a list contained hundreds of unsorted items. Getting to the value you're looking for would take a long time if you only had the capability of scrolling through the list using the vertical scroll bar. But if you knew at least part of the value you're trying to find, entering it into an edit box and getting the item you want immediately is a much more attractive solution.
Let's delve into what you have to do make this solution work. First, here's the unit code for a sample form I produced:
unit uinclist;
interface
uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
StdCtrls;
type
TForm1 = class(TForm)
ListBox1: TListBox;
Edit1: TEdit;
procedure FormCreate(Sender: TObject);
procedure Edit1Change(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form1: TForm1;
implementation
{$R *.DFM}
procedure TForm1.FormCreate(Sender: TObject);
{This is a test string to load into the list box at runtime}
CONST ListStrings = 'United States'#13'Guatemala'#13'Mexico'#13+
'El Salvador'#13'Costa Rica'#13'Yucatan'#13+
'China'#13'Japan'#13'Thailand'#13'Switzerland'#13+
'Germany'#13'Lichtenstein'#13'Jamaica'#13'Greece'+
'Turkey'#13'Ireland'#13'United Kingdom'#13'Scotland'+
'Canada'#13'Uruguay'#13'Paraguay'#13'Cuba'#13+
'Spain'#13'Italy'#13'France'#13'Portugal'#13'New Zealand'#13+
'Austria'#13'Australia'#13'Philippines'#13'Korea'#13+
'Malaysia'#13'Tibet'#13'Nepal'#13'India'#13'Sri Lanka'#13+
'Pakistan'#13+'Saudi Arabia'#13'United Arab Emerates'#13'Iran'#13+
'Ukraine'#13'Belarus'#13+
'Chechen'#13'Yugoslavia'#13'Czechoslovakia'#13'Slovina'#13'Kazakhstan'#13+
'Egypt'#13'Morocco'#13'Macedonia'#13'Cyprus'#13'Finland'#13+
'Norway'#13'Sweden'#13'Denmark'#13'Netherlands'#13'Lithuania'#13;
begin
ListBox1.Items.SetText(ListStrings);
end;
procedure TForm1.Edit1Change(Sender: TObject);
var
S : Array[0..255] of Char;
begin
StrPCopy(S, Edit1.Text);
with ListBox1 do
ItemIndex := Perform(LB_SELECTSTRING, 0, LongInt(@S));
end;
end.
Form1 has two controls: a TEdit and a TListBox. Notice that during FormCreate, I loaded up the value of the list box with the huge string of countries. This step was only for testing purposes. How you load up your list is up to you. Now, the trick to making the incremental search is in the OnChange event of Edit1. I've used the Windows message LB_SELECTSTRING to perform the string selection for me. LB_SELECTSTRING is one of the members of the WinAPI list box message family (all preceeded by LB_) that manipulates all aspects of a list box object in Windows. The message takes two parameters: wParam, the index from which the search should start; and lParam, the address of the null-terminated string to search on. Since WinAPI calls require null-terminated strings, use either a PChar or an Array of Char to pass string values. Using an Array of Char is more advantageous if you know a string value won't exceed a certain length. You don't have to manually allocate and de-allocate memory with an Array of Char, as opposed to a PChar, which requires you to use GetMem or New and FreeMem to allocate and de-allocate memory.
In any case, to convert a Pascal string to a null-terminated string, just use StrPCopy to copy the contents of the Pascal string into the null-terminated string. Once that's done, all you have to do is pass the address of the null-terminated string into the wParam parameter of LB_SELECTSTRING using the @ symbol. When you use Perform to execute the LB_SELECTSTRING message, the message will return the item index of the matching list item. All that's left to do is assign the ItemIndex property of the list box to the return value of the message. The net result is that the list box will scroll to and select the list element that was found.
TListBox has several messages which you can handle and perform in
Delphi. If you bring up the help system and do a topic search, enter LB_ in the edit box, and peruse the list of messages.
