Application.ProcessMessages hangs?

Question

my single threaded delphi 2009 app (not quite yet complete) has started to have a problem with Application.ProcessMessages hanging. my app has a TTimer object that fires every 100 ms to poll an external device. i use Application.ProcessMessages to update the screen when something changes so the app is still responsive.

one of these was in a grid OnMouseDown event. in there, it had an Application.ProcessMessages that essentially hung. removing that was no problem except that i soon discovered another Application.ProcessMessages that was also blocking.

i think what may be happening to me is that the TTimer is--in the app mode i'm currently debugging--probably taking too long to complete. i have prevented the TTimer.OnTimer event hander from re-entering the same code (see below):

procedure TfrmMeas.tmrCheckTimer(Sender: TObject);
begin
  if m_CheckTimerBusy then
    exit;

  m_CheckTimerBusy:=true;
  try
    PollForAndShowMeasurements;
  finally
    m_CheckTimerBusy:=false;
  end;
end;

what places would it be a bad practice to call Application.ProcessMessages? OnPaint routines springs to mind as something that wouldn't make sense.

any general recommendations?

i am surprised to see this kind of problem arise at this point in the development!

Answer 1

thank you all for your comments/suggestions.

here i made a test app that has a timer routine that takes longer than the interval it is set for. when i push button1, Application.ProcessMessages hangs. my solution for now is to disable the timer during the timer routine.

later we plan to put the "device communications" in a thread.

thank you! mp

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls, StdCtrls;

type
  TForm1 = class(TForm)
    Timer1: TTimer;
    Button1: TButton;
    Memo1: TMemo;
    procedure Timer1Timer(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
  memo1.Lines.Add('text 1');

  // this call blocks
  Application.ProcessMessages;

  memo1.Lines.Add('text 2');
end;

procedure TForm1.Timer1Timer(Sender: TObject);
var
  iTime:cardinal;
begin
  // fix by adding this:  timer1.Enabled:=false;

  iTime:=GetTickCount;
  while GetTickCount-iTime<200 do
    ;

  // fix by adding this:  timer1.Enabled:=true;
end;

end.

object Form1: TForm1
  Left = 0
  Top = 0
  Caption = 'Form1'
  ClientHeight = 286
  ClientWidth = 426
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  PixelsPerInch = 96
  TextHeight = 13
  object Button1: TButton
    Left = 8
    Top = 56
    Width = 75
    Height = 25
    Caption = 'Button1'
    TabOrder = 0
    OnClick = Button1Click
  end
  object Memo1: TMemo
    Left = 112
    Top = 8
    Width = 297
    Height = 233
    Lines.Strings = (
      'Memo1')
    TabOrder = 1
  end
  object Timer1: TTimer
    Interval = 100
    OnTimer = Timer1Timer
    Left = 200
    Top = 144
  end