how to make an event handler exchange message(s) with other event handler(s)

Question

suppose i write a button1click event handler which when called(ie when button1 clicked) keeps on adding 1 to variable counter:

var 
i : counter;
begin
while 1 = 1 do
inc(i);
end;

now i want to add another button on my form and name it ''stop'' which will abruptly exit button1click(after its called) or stop it from adding 1 to counter, how do i do that?

Answer 1

1. Put Application.ProcessMessages inside the while. This allows other controls to process and respond to the events.
2. Create a global variable "Stop".
3. In stop-button event handler, set Stop to true.
4. Inside your while, check for the value of Stop and break if it is true.

But, it is not very good practice to do stuff like this. All things you do in form should be reponses to some events, so if you want to do some periodical things, you should use a Timer or something like that.

Answer 2

You can put your while in a separate thread. Simple TThread will be enough, but there are also special threading libraries like AsyncCalls or OmniThreadLibrary.

Answer 3

Hopefully this will provide some ideas as to how to adapt a timer based approach for your needs:

type
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    Timer1: TTimer;  // with Interval = 1 (for example) and Enabled = FALSE
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure Timer1Execute(Sender: TObject);
  private
    fCounter: Integer;
  end;



procedure TForm1.Button1Click(Sender: TObject);
begin
  Timer1.Enabled := TRUE;
end;


procedure TForm1.Button2Click(Sender: TObject);
begin
  Timer1.Enabled := FALSE;
end;


procedure TForm1.Timer1Execute(Sender: TObject);
begin
  Inc(fCounter);
end;

If your needs are more sophisticated then a TThread based approach may be more appropriate. But whatever you do, do NOT resort to Application.ProcessMessages it will only cause you grief !