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Bizarre problem with Trackbar & Combo box

Posted on 1998-01-05
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Last Modified: 2010-04-04
I have a trackbar and a combobox on a form. In the onchange event of the trackbar I clear the combobox and add some items to it. However, this causes the marker on the trackbar to zoom up to the mouse when it is clicked on the trackbar. If I remove the clear it is ok (but I get a build up of items in the combobox). If I don't do the clear, but delete the individual items I get the same behaviour. As both the trackbar and the combobox are windows common controls I cant track them past the points where the messages are sent to the appropriate handles.

Can anyone help me here?

I am using Delphi 3.02 on NT 4 SP3.

The source for the .DPR, .DFM and .PAS files are below.

program Project3;

uses
  Forms,
  Unit3 in 'Unit3.pas' {Form1};

{$R *.RES}

begin
  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.Run;
end.

--------------------------------------------------------------

object Form1: TForm1
  Left = 244
  Top = 115
  Width = 783
  Height = 540
  Caption = 'Form1'
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'MS Sans Serif'
  Font.Style = []
  PixelsPerInch = 96
  TextHeight = 13
  object TrackBar1: TTrackBar
    Left = 80
    Top = 24
    Width = 617
    Height = 41
    Orientation = trHorizontal
    PageSize = 1
    Frequency = 1
    Position = 0
    SelEnd = 0
    SelStart = 0
    TabOrder = 0
    TickMarks = tmBottomRight
    TickStyle = tsAuto
    OnChange = TrackBar1Change
  end
  object ComboBox1: TComboBox
    Left = 312
    Top = 80
    Width = 369
    Height = 21
    ItemHeight = 13
    TabOrder = 1
    Text = 'ComboBox1'
  end
end

---------------------------------------------------------

unit Unit3;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms,
Dialogs,
  StdCtrls, ComCtrls, Menus;

type
  TForm1 = class(TForm)
    TrackBar1: TTrackBar;
    ComboBox1: TComboBox;
    procedure TrackBar1Change(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.TrackBar1Change(Sender: TObject);
begin
with combobox1 do
begin
clear;
Items.Add('Start super');          {0}
Items.Add('Start spiral');         {1}
Items.Add('Start equal');          {2}

Items.Add('Start arc');            {3}
Items.Add('Start max');            {4}
Items.Add('End max');              {5}
Items.Add('End arc');              {6}
Items.Add('End equal');            {7}
Items.Add('End spiral');           {8}
Items.Add('End super');            {9}
Items.Add('User super');           {10}
end;
end;

end.
0
Comment
Question by:rwilson032697
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6 Comments
 
LVL 12

Author Comment

by:rwilson032697
ID: 1355332
Adjusted points to 300
0
 
LVL 2

Expert Comment

by:mvz121697
ID: 1355333
I do not understand what the problem is. Your code does work here as i should expect.
What do you mean by ' zoom up to the mouse' ?

0
 
LVL 5

Expert Comment

by:JimBob091197
ID: 1355334
(To mvz:
There is a problem:  Normally when you click on a TrackBar, the positions changes ONE step towards the mouse cursor, but with rwilson's code the TrackBar's position changes MANY steps, all the way to the mouse cursor!)

I once had the same problem, and it does indeed seem to be a bug in Windows common controls.  I got around it by using a TTimer (!!) with an interval of about 1/10th of a second.

The problem appears to be caused when the combobox's items are cleared in the TComboBoxStrings.Clear method if there are 2 or more items in the combo box's list.  The CB_RESETCONTENT message sent to the combo box to clear its items "appears" to be the problem???

JB
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LVL 5

Expert Comment

by:JimBob091197
ID: 1355335
Mmm…  Before I go accusing Microsoft of buggy common controls, maybe I should get my facts straight.  I just wrote the same app in VB 5 and it works fine...
0
 
LVL 2

Accepted Solution

by:
mvz121697 earned 1200 total points
ID: 1355336
Yep, now i see what the problem is.
It seems it relates to problems with mouse-capturing.

A workaround is :

procedure TForm1.TrackBar1Change(Sender: TObject);
begin
// ADD NEXT LINE
if GetKeyState(VK_LBUTTON) >= 0 then begin
  Combobox1.Clear;
  Combobox1.Items.Add('Test 1');
  Combobox1.Items.Add('Test 2');
End;


Greetings from Holland,
MvZ
0
 
LVL 12

Author Comment

by:rwilson032697
ID: 1355337
You cracked it mvz.

As far as I can see it fixes the problem perfectly. The fact you call it a word-around is not a problem.

Thanks!

PS: I am curious how you discovered this fix?
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