detect if a variable has Changed
lazarus / 62-General

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Weitentaaal · 2021-10-19 09:19

Hello ,

is this Somehow possible ?

I know i can use Getter and Setter or define an old Var to store old
Value.

Is there a better approach ?

Thanks in advance

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Mr.Madguy · 2021-10-19 09:37

Flag - is obvious solution:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TMyClass.SetValue(AValue:TValue);//Setterbegin  if AValue <> FValue
then begin    FValue := AValue;    FValueChanged := True;//Flag   
OnValueChanged(Self, AValue);//Or event  end;end;

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howardpc · 2021-10-19 09:45

Outside of a class you can use a stand-alone property.
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i unit
Unit1; {$mode objfpc}{$H+} interface uses  Forms, Dialogs, Spin; type 
TForm1 = class(TForm)    AlertSpinEdit: TSpinEdit;    procedure
AlertSpinEditChange(Sender: TObject);  end;    procedure
SetAlertIfChanged(aValue: Integer);   function GetAlertIfChanged:
Integer;   procedure Alert;   property AlertIfChanged: Integer read
GetAlertIfChanged write SetAlertIfChanged; var  Form1:
TForm1; implementation var  alertInt: Integer = 50; procedure
SetAlertIfChanged(aValue: Integer);begin  if aValue <> alertInt then
begin    alertInt := aValue;    Alert;  end;end; function
GetAlertIfChanged: Integer;begin  Exit(alertInt);end; procedure
Alert;begin  ShowMessageFmt('AlertIfChanged is now
%d',[AlertIfChanged]);end; {$R *.lfm} { TForm1 } procedure
TForm1.AlertSpinEditChange(Sender: TObject);begin  AlertIfChanged :=
(Sender as TSpinEdit).Value;end; end.

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white_zombie · 2021-10-19 10:25

Depending on what you want to do, you can implement an observer
pattern. I think FPC supports this since version 2.6.4

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Weitentaaal · 2021-10-19 11:14

Quote from: white_zombie on October 19, 2021, 12:25:05 pmDepending on
what you want to do, you can implement an observer pattern. I think
FPC supports this since version 2.6.4

I think thats an overkill for a Single Variable ?

Quote from: howardpc on October 19, 2021, 11:45:33 amOutside of a
class you can use a stand-alone property.
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i unit
Unit1; {$mode objfpc}{$H+} interface uses  Forms, Dialogs, Spin; type 
TForm1 = class(TForm)    AlertSpinEdit: TSpinEdit;    procedure
AlertSpinEditChange(Sender: TObject);  end;    procedure
SetAlertIfChanged(aValue: Integer);   function GetAlertIfChanged:
Integer;   procedure Alert;   property AlertIfChanged: Integer read
GetAlertIfChanged write SetAlertIfChanged; var  Form1:
TForm1; implementation var  alertInt: Integer = 50; procedure
SetAlertIfChanged(aValue: Integer);begin  if aValue <> alertInt then
begin    alertInt := aValue;    Alert;  end;end; function
GetAlertIfChanged: Integer;begin  Exit(alertInt);end; procedure
Alert;begin  ShowMessageFmt('AlertIfChanged is now
%d',[AlertIfChanged]);end; {$R *.lfm} { TForm1 } procedure
TForm1.AlertSpinEditChange(Sender: TObject);begin  AlertIfChanged :=
(Sender as TSpinEdit).Value;end; end.
Isn't this the same when i use a property for a Variable like that
(pseudo Code):


Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  TMyClass =
class private   int fx;    procedure SetX(aValue: Integer);   function
GetX: Integer;   procedure xChanged public   property x:Integer read
Getx write SetX;end; implementation procedure TMyClass .SetX(AVAlue:
Integer);begin   fx:= AVAlue;   xChanged();end;  procedure TMyClass
GetX(): Integer;begin   Result:= fX;end; 
i hope u get what i was trying to say 

Quote from: Mr.Madguy on October 19, 2021, 11:37:00 amFlag - is
obvious solution:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TMyClass.SetValue(AValue:TValue);//Setterbegin  if AValue <> FValue
then begin    FValue := AValue;    FValueChanged := True;//Flag   
OnValueChanged(Self, AValue);//Or event  end;end; 
Isn't this the same as above too ? 

Btw. i need to fire an event just like TEdit has OnChange.

I did not want to create a property and use its setter / getter for
this, or create a whole new value just to store the old value.
Something like i assign a Handler to a Value and if the Value is
changing the Handler "handles" the new Value.

Thanks anyway for your suggestions

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MarkMLl · 2021-10-19 11:21

@Weitentaaal: I've held off commenting up to this point, since I was
hoping that either you or somebody else would throw light on the
fundamental question: when you say "variable", do you mean a "classic
Pascal" unmanaged integer (etc.) in memory, rather than a field of a
class?

My suspicion is that if you do, neither FPC nor any other conventional
language has the capability of flagging a state-change without
wrapping the variable in something: either compile-time complexity
(the variable becomes a field in an object, and is then accessed via a
property and a setter) or run-time complexity probably involving
either an interpretive runtime or alternatively processor-specific
debugging capabilities.

MarkMLl

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Weitentaaal · 2021-10-19 11:46

Quote from: MarkMLl on October 19, 2021, 01:21:11 pm@Weitentaaal: I've
held off commenting up to this point, since I was hoping that either
you or somebody else would throw light on the fundamental question:
when you say "variable", do you mean a "classic Pascal" unmanaged
integer (etc.) in memory, rather than a field of a class?

My suspicion is that if you do, neither FPC nor any other conventional
language has the capability of flagging a state-change without
wrapping the variable in something: either compile-time complexity
(the variable becomes a field in an object, and is then accessed via a
property and a setter) or run-time complexity probably involving
either an interpretive runtime or alternatively processor-specific
debugging capabilities.

MarkMLl

Sorry my fault,

i ment classic Pascal Variable (Integer, String, boolean etc)

 in the following example i have a "Variable" : "fX". if "fX" is
changing i want to fire an event. Thats why  i created Property "X" to
use the getter and setter of "X". This way i can fire an Event
("xChanged") in the setter methode
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  TMyClass =
class private   int fx;    procedure SetX(aValue: Integer);   function
GetX: Integer;   procedure xChanged public   property x:Integer read
Getx write SetX;end; implementation procedure TMyClass .SetX(AVAlue:
Integer);begin   if AValue <> fx then begin //forgott this      fx:=
AVAlue;      xChanged();   end;end;  procedure TMyClass GetX():
Integer;begin   Result:= fX;end;

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balazsszekely · 2021-10-19 12:29

@Weitentaaal

Try something like this:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i unit
uDetectChange; {$mode ObjFPC}{$H+} interface uses  Classes,
SysUtils; type     { TDetectChange }  TOnVariableChanged =
procedure(Sender: TObject; AX: Integer) of object;  TDetectChange =
class  private    FX: Integer;    FOnVariableChanged:
TOnVariableChanged;    procedure SetX(aValue: Integer);  public   
constructor Create(AX: Integer);    destructor Destroy; override; 
public    property X: Integer read FX write SetX;    property
OnVariableChanged: TOnVariableChanged read FOnVariableChanged write
FOnVariableChanged;  end; implementation constructor
TDetectChange.Create(AX: Integer);begin  FX := AX;end; destructor
TDetectChange.Destroy;begin  //  inherited;end; procedure
TDetectChange.SetX(aValue: Integer);begin  if AValue <> FX then 
begin    FX := AValue;    if Assigned(FOnVariableChanged) then     
FOnVariableChanged(Self, FX);  end;end; end. 
then:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i unit
Unit1; {$mode objfpc}{$H+} interface uses  Classes, SysUtils, Forms,
Controls, Graphics, Dialogs, StdCtrls,  uDetectChange; type   { TForm1
}   TForm1 = class(TForm)    Button1: TButton;    procedure
Button1Click(Sender: TObject);    procedure FormCreate(Sender:
TObject);    procedure FormDestroy(Sender: TObject);  private 
 FDetectChange: TDetectChange;   procedure OnVariableChange(Sender:
TObject; AX: Integer);  public   end; var  Form1:
TForm1; implementation{$R *.lfm} { TForm1 } procedure
TForm1.FormCreate(Sender: TObject);begin  FDetectChange :=
TDetectChange.Create(3);  FDetectChange.OnVariableChanged :=
@OnVariableChange;end; procedure TForm1.FormDestroy(Sender:
TObject);begin  FDetectChange.Free;end; procedure
TForm1.OnVariableChange(Sender: TObject; AX: Integer);begin 
ShowMessage('Variable X has changed, new value: ' +
IntToStr(AX));end; procedure TForm1.Button1Click(Sender:
TObject);begin  FDetectChange.X := 15;end;  end.

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MarkMLl · 2021-10-19 12:32

Quote from: Weitentaaal on October 19, 2021, 01:46:07 pmi ment classic
Pascal Variable (Integer, String, boolean etc)

I thought you did, hence that it was necessary (eventually) to ask for
clarification. So what you're effectively doing there is what I
referred to as a compile-time wrapper... I don't know if any sort of
runtime wrapper using e.g. debugging registers could be implemented
portably and that sort of thing has notoriously poor OS support.

MarkMLl

p.s. That callback should probably be protected by if Assigned().

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Weitentaaal · 2021-10-19 13:23

"p.s. That callback should probably be protected by if Assigned()."
Thanks for the Hint.

Thanks to all of your Reply's, i guess i will keep the Properties.