Oktay Sancak's Programming Tips

Cancel a thread in the middle of execution

2005-01-28T15:20:00.000+02:00

Q: I got a little problem. I do work on an application which is going to download different stuff from the internet, and I got a little problem with the threads. I use some classes that can use up to 5 seconds to complete (like Connect), and it all runs after eachother (not a big loop for the entire thread, but a task, start to end). Therefore I can't use the easy, classic, Cancel := True, if cancel = True then Exit; way. I did try most procedures in TThread to cancel it, like Terminate, DoTerminate, Suspend, etc..., but none of them seems to cancel it. Is there some not too hard way to simply terminate/cancel, and then free the Thread without having to add extra If Cancel = True ..... code in the Execute procedure (I can't do that, because it doesn't run in a loop, and some of the procedures inside that thread takes time to do, and I can't do that If Cancel = True between each line of code).

Another extra question, what is the difference between Destroy, and Free when I free a component, or class (like TMemoryStream)? Which of them frees it fully (returns just as many bytes as it took when I created it)?

Answer :

Free method is doing the same work with Destroy but there is a little difference :

procedure TObject.Free;
begin
if Self <> nil then
Destroy;
end;

Free method is a procedure of TObject class. When you use TObject.Free, first of all it checks if this object nil or not then if it's not nil, it destroys the component. This protects you from to destroy a destroyed component and escape you from an error. This is the reason why Borland suggest you to use Free instead of Destroy. Because it's more secure.

When you free an object, it's not means that nil assigned to it. So most of the programmers using FreeAndNil(var Object) procedure, which has this code inside it :

procedure FreeAndNil(var Obj);
var
  Temp: TObject;
begin
  Temp := TObject(Obj);
  Pointer(Obj) := nil;
  Temp.Free;
end;

Take a look at this, first of all 'nil' assigned to the object and then this object frees.

You can design your thread class like this :

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,Dialogs, IdBaseComponent, IdComponent, IdTCPConnection, IdTCPClient;

type
  TForm1 = class(TForm)
    IdTCPClient1: TIdTCPClient;
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

  TMyThread = class(TThread)
  private
    FSocks: array[0..2] of TIdTCPClient;
  protected
    procedure Execute;
  public
    constructor Create(ConnIP: string; ConnPort);
    procedure StopConnections;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin

end;

{ TMyThread }

constructor TMyThread.Create(ConnIP: string; ConnPort);
var
  i: integer;
begin
  for i := 0 to 2 do
  begin
    FSocks[i] := TIdTCPClient.Create(nil);
    FSocks[i].Host := ConnIP;
    FSocks[i].Port := 381;
    FSocks[i].Connect;
end;
  inherited Create(True);
  FreeOnTerminate := True;
end;

procedure TMyThread.Execute;
begin
  // internet stuff here
  Terminate;
end;

procedure TMyThread.StopConnections;
var
  i: integer;
begin
  Suspend;
  for i := 0 to 2 do
  begin
    FSocks[i].Disconnect;
    FreeAndNil(FSocks[i]);
  end;
  Terminate;
end;

end.

Author: Oktay Sancak

For to stop your connections with this class, all you need to do is calling StopConnections. Then after to close all the connections, thread will terminate itself.

Preventing the application close

2004-10-04T06:28:00.000+03:00

Q: I want to prevent the user from closing the application. When windows closes I want the application to do some tasks then close and then windows close.

Answer:

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    btnCloseAllow: TButton;
    btnCloseDecline: TButton;
    procedure FormCreate(Sender: TObject);
    procedure btnCloseAllowClick(Sender: TObject);
    procedure FormCloseQuery(Sender: TObject; var CanClose: Boolean);
    procedure btnCloseDeclineClick(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  IsCloseAllowed: Boolean;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
  IsCloseAllowed := False;
end;

procedure TForm1.btnCloseAllowClick(Sender: TObject);
begin
  IsCloseAllowed := True;
end;

procedure TForm1.FormCloseQuery(Sender: TObject; var CanClose: Boolean);
begin
  CanClose := IsCloseAllowed;
end;

procedure TForm1.btnCloseDeclineClick(Sender: TObject);
begin
  IsCloseAllowed := False;
end;

end.

Author: Oktay Sancak

Communicate between Desktop and Web apps

2004-10-04T06:25:00.000+03:00

Q: How can i pass variables between a Delphi app and PHP form on the web ?

Answer:

- Delphi Side -

procedure TForm1.btnLoginClick(Sender: TObject);
var
  SL: TStringList;
  i: integer;
begin
  SL := TSTringList.Create;
  try
    SL.Values['username'] := 'JDJVR';
    SL.Values['password'] := 'mypass19';
    SL.Text := Http.Post('http://localhost/logintest.php', SL);

    if (SL.Text = 'OK') then
    begin
      Caption := 'User authorized..';
    end
    else begin
        Caption := 'User could not authorized';
    end;
  finally
    SL.Free;
  end;
end;

- PHP Side -

(i) check the username and password :

  $username = $_POST[username];
  $password = $_POST[password];

(ii) return the result with echo or print function

Author: Oktay Sancak

Is CheckBox's Item Enable or Disable ?

2004-10-04T06:19:00.000+03:00

Q: I want to be able to do :

for i := 0 to numberItems do
begin
if checklistbox.checked[i] = true then
// do something..
end;

but I cant find a property for the number of items in a Checklistbox !!

Answer :

var
  i: integer;
begin
  for i := 0 to CheckListBox1.Items.Count - 1 do
  begin
    if CheckListBox1.ItemEnabled[i] then
    begin
      // do something..
    end;
  end;
end;

Author: Oktay Sancak

Split String

2004-10-04T06:06:00.000+03:00

Q: I have this as a string
123:321

How can I split it into 2 strings by “:” character ?

123:321 into
String1 = 123
String2 = 321

Answer:

var
  StrL: TStringList;
begin
  StrL := TStringList.Create;
  try
    StrL.Delimiter := ‘:’;
    StrL.DelimitedText := ‘123:321';
    Caption := StrL.Strings[0];
  finally
    StrL.Free;
  end;
end;

Author: Oktay Sancak