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Executing Events in components

Posted on 2001-06-21
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Last Modified: 2010-04-06
Hi, I've just recently launched myself into component-writing.  I have written a sample component with the objective of adding a new event, namely OnNewTransaction
which will be triggered whenever the OnTimer event of a timer is triggered.  Below are some portions of that class declaration :

type
  TRtStkData = class(TComponent)
  private
    FTimer            : TTimer;
    FOnNewTransaction : TNotifyEvent;
    FClose            : TStringList;
    procedure FTimerOnTimer(Sender:TObject);
    procedure SetClose(Value: TStringList);
    procedure AddItem;
  protected
  public
    constructor Create(AOwner : TComponent); override;
    destructor Destroy; override;
    procedure Show(index : integer);
    procedure Addnew;
  published
    property OnNewTransaction : TNotifyEvent read   FOnNewTransaction write FOnNewTransaction;
    property Close : TStringList read FClose write SetClose;
  end;


constructor TRtStkData.Create(AOwner : TComponent);
begin
  inherited Create(AOwner);
  FClose:=TStringList.Create;
  FTimer:=TTimer.Create(Self);
  FTimer.Interval:=2000;
  FTimer.Enabled:=false;
  FTimer.OnTimer:=FTimerOnTimer;
end;

procedure TRtStkData.SetClose(Value: TStringList);
begin
  FClose:=Value;
end;

procedure TRtStkData.add;
begin
  FClose.Add(IntToStr(random(100)));
end;

procedure TRtStkData.FTimerOnTimer(Sender:TObject);
begin
 Add;
 if Assigned(FOnNewTransaction) then
    FOnNewTransaction(Self);
end;


The class compiled successfully and the new component was installed successfully.  I've made a sample calling program into which I've added the new component.
I enabled the timer at one point of the calling program.The  OnNewTransaction event appeared in the Object inspector but that event is never triggered.  
When I traced the program I found out that  the problem is in the method FTimerOnTimer :

the statement Assigned(FOnNewTransaction)is always false.  

Can someone help me out?  
Below are some portions of the sample calling program:

procedure TForm1.FormCreate(Sender: TObject);
begin
  RtStkData1:=TRtStkData.Create(Self);
end;

procedure TForm1.RtStkData1NewTransaction(Sender: TObject);
begin
  Memo1.Lines.Add(RtStkData1.Close[RtStkData1.Close.Count-1]);
end;
0
Comment
Question by:TheBreeze
6 Comments
 
LVL 8

Expert Comment

by:TOndrej
ID: 6217128
I can't find in your code FTimer.Enabled := True;
0
 
LVL 1

Accepted Solution

by:
vorisek earned 75 total points
ID: 6217149
If you add the component from component palette
you need not (must not) create it in OnCreate.
So try to omit the code in TForm1.FormCreate.
Or you have to assigne OnNewTransaction manually in
TForm1.FormCreate :OnNewTransaction:=RtStkData1NewTransaction;
0
 
LVL 5

Expert Comment

by:Gwena
ID: 6217157
listening :-)
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LVL 3

Expert Comment

by:Stefaan
ID: 6217199
Hi,

One thing, did you add code to the Event Handler ? I mean did you double click on the OnNewTransaction event in the object inspector and write some code in that event handler ?

The following code :

if Assigned(FOnNewTransaction) then
   FOnNewTransaction(Self);

the assigned will only be true if you have added some code in the event handler of the OnNewTransaction event.

One last note, your component has a memory leak.  Why, well it is a good practice in Component Buidling to destroy everything you created yourself.  In the constructor you instantiate a TStringList and a TTimer.  Since the TTimer has an owner it will be cleaned up automatically when the Owner component gets destroyed.  The TStringList on the other hand doesn't have an owner and will not be destroyed automatically, thus it will stay in memory causing a memory leak.

You should override the destructor of your component and in your case destroy the TTimer and TStringList in the destructory of your component.  ( possibly you did it, but you didnt include the destructor in your code sample ).

Hoping that this will be of any help to you.

Best regards,


Stefaan
0
 

Author Comment

by:TheBreeze
ID: 6217207
Thank you vorisek!

I deleted the code inside the OnCreate
procedure of the calling program and it worked!

Thanks again!
0
 

Author Comment

by:TheBreeze
ID: 6223517
TOndrej,
I did enable the Timer but have not included it in the sample code.  

Stefaan,
I did all that you said in your comment, but again, have not included them in the sample code.  Sorry for that.  But I appreciate your help.  
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