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Kamis, 07 Juli 2011

Incremental Searches with a TListbox

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 this. One's hard and slow, the other easy and fast (we're 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 of this, 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 creating a capability like this essential? Because it's tedious to scroll through a list that has lots of items. Imagine if a list contained hundreds of unsorted items. To get 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 work. First, here's the unit code for a sample form I produced:

unit uinclist;


Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,

TForm1 = class(TForm)
ListBox1: TListBox;
Edit1: TEdit;
procedure FormCreate(Sender: TObject);
procedure Edit1Change(Sender: TObject);
{ Private declarations }
{ Public declarations }

Form1: TForm1;


{$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+
'Turkey'#13'Ireland'#13'United Kingdom'#13'Scotland'+
'Spain'#13'Italy'#13'France'#13'Portugal'#13'New Zealand'#13+
'Malaysia'#13'Tibet'#13'Nepal'#13'India'#13'Sri Lanka'#13+
'Pakistan'#13+'Saudi Arabia'#13'United Arab Emerates'#13'Iran'#13+

procedure TForm1.Edit1Change(Sender: TObject);
S : Array[0..255] of Char;
StrPCopy(S, Edit1.Text);
with ListBox1 do
ItemIndex := Perform(LB_SELECTSTRING, 0, LongInt(@S));


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 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. Let's talk about the message.

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. It's more advantageous to use a an Array of Char 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 that 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 we have to do is pass the address of the null-terminated string into the wParam parameter of LB_SELECTSTRING, and that's done by using the @ symbol.

When we use Perform to execute the LB_SELECTSTRING message, the message will return the item index of the matching list item. Then 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.

There are several list box messages you can 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.

Copyright © 1995, 1996, 1997 Brendan V. Delumpa All Rights Reserved
Delphi Expert Eddie Shipman adds the following useful information:

This procedure can be applied to TComboBox by changing to this code:

procedure TForm1.ComboBox1Change(Sender: TObject);
S : Array[0..255] of Char;
StrPCopy(S, TComboBox(Sender).Text);
with ComboBox1 do
ItemIndex := Perform(CB_SELECTSTRING, 0, LongInt(@S));

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