Delayed scrolling / delayed OnChange?

If the user keeps either key pressed and the change of the item (ComboBox.OnChange) takes a long(ish) time an annoying delay will be noticed.

As a "work around" I would like to react to the change of the ItemIndex only after a short period of time, e.g. 100 ms.

Here's an example. Written in D2, but technique should work OK in D1 too. Just a simple form with a combo and a label. You probably should consider using Yield in addition to the call to Application.ProcessMessages, to avoid slowing down the PC when the forms message queue is empty.

unit Unit1;

interface

uses

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

Dialogs,

StdCtrls;

const

// Just some message constant

PM_COMBOCHANGE = WM_USER + 8001;

// 500 ms

CWantedDelay = 500;

type

TForm1 = class(TForm)

ComboBox1: TComboBox;

Label1: TLabel;

procedure ComboBox1Change(Sender: TObject);

private

procedure PMComboChange(var message : TMessage); message PM_COMBOCHANGE;

public

end;

var

Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.ComboBox1Change(Sender: TObject);

begin

PostMessage(Handle, PM_COMBOCHANGE, 0, 0);

end;

procedure TForm1.PMComboChange(var message : TMessage);

const

InProc : BOOLEAN = FALSE;

StartTick : LONGINT = 0;

begin

if InProc then begin

// Update the starting time for the delay

StartTick := GetTickCount;

end else begin

// We're in the loop

InProc := TRUE;

// Initial starting time

StartTick := GetTickCount;

// Wait until wanted time has elapsed.

// If proc gets called again, starting time will change

while GetTickCount - StartTick < CWantedDelay do Application.ProcessMessages;

// Increment a counter, just for illustration of when to do the actual OnChange work

Label1.Caption := IntToStr ( StrToIntDef ( Label1.Caption, 0 ) + 1);

// We're finished with the loop

InProc := FALSE;

end;

end.