HandleMessageExample (Delphi)

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Description

The following code demonstrates background processing using the OnIdle event and using the HandleMessage method to allow messages or background processing to get through. Note: You must add MyIdleHandler to the Form1 methods.

Code

var
{ Global variables to show the order of events }
  XPos, YPos, Delta: integer;

{ Utility procedure to display messages }
{ Add it at the beginning of the implementation section. }
procedure StatusMsg(
  MyForm : TForm1; Canvas : TCanvas; Message : string; Bkg : Boolean);
begin
  if not bkg then
    Canvas.Font.Style := [fsBold]; {Foreground messages are in bold type. }
  Canvas.TextOut(XPos, YPos, Message);
  if not bkg then
    Canvas.Font.Style := [];
  { Change Xpos and YPos to prepare for the next message. }
  Ypos := Ypos + Delta;
  if YPos >= MyForm.ClientHeight - 10 then
  begin
    YPos := 10;
    Xpos := Xpos + 180;
  end;
  if (Xpos >= MyForm.ClientWidth - 100) then
  begin
    if (Canvas.Font.Color = clRed) then
      Canvas.Font.Color := clBlack
    else
      Canvas.Font.Color := clRed;
    Xpos := 10;
  end;
end;

{ This is the form�s OnCreate event handler. }
{ It initializes global variables and sets the OnIdle 
event handler. }
procedure TForm1.FormCreate(Sender: TObject);
begin
  Button1.Caption := 'Do not yield';
  Button2.Caption := 'Handle Message';
  Application.OnIdle:= MyIdleHandler;
  XPos := 10;
  YPos := 10;
  Delta := Abs(Canvas.Font.Height) + 1;
 end;

{
This is the OnIdle event handler. It is set in the form�s 
OnCreate event handler, so you need to only add it as a private
method of the form. Usually, it would perform 
background processing for the application. This one simply
writes a message to let you know when it is there.
procedure TForm1.MyIdleHandler(Sender: TObject; var Done: Boolean);
begin
  StatusMsg(
    Form1, Canvas, 'This represents a background process.', True);
end;

{
Set this method as the OnClick event handler of Button1. It
performs a calculation without yielding to other messages or
idle time.
procedure TForm1.Button1Click(Sender: TObject);
var
  I, J, X, Y: Word;
begin
  StatusMsg(
    Form1, Canvas, 'The synchronous handler is starting', False);
  I := 0;
  J := 0;
  while I < 10 do
  begin
    Randomize;
    while J < 10 do
    begin
      Y := Random(J);
      Inc(J);
    end;
    X := Random(I);
    Inc(I);
  end;
  StatusMsg(
    Form1, Canvas, 'The synchronous handler is done', False);
end;

{
Set this method as the OnClick event handler of Button2. It
performs a calculation, but calls HandleMessage to allow idle
time or asynchronous message processing.
procedure TForm1.Button2Click(Sender: TObject);
var
  I, J, X, Y: Word;
begin
 StatusMsg(
    Form1, Canvas, 'The asynchronous handler is starting', False);
  I := 0;
  J := 0;
  while I < 10 do
  begin
    Randomize;
    while J < 10 do
    begin
      Y := Random(J);
      Inc(J);
    end;
    X := Random(I);
    Inc(I);
    { yield to OnIdle or other messages }
    Application.HandleMessage;
  end;
  StatusMsg(
    Form1, Canvas, 'The asynchronous handler is done', False);
end;

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