[SOLVED] Can't access component property
lazarus / 21-General

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pcurtis · 2021-05-13 07:00

I am trying to design a component. When I try to set a property at
runtime it fails with "unit1.pas(40,26) Error: Argument cannot be
assigned to".
What's wrong?

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
mythreaddisplay; {$mode objfpc}{$H+} interface uses  Classes,
SysUtils, LResources, Forms, Controls, Graphics, StdCtrls, ExtCtrls,
ComCtrls, Dialogs, Buttons, Windows;type  TThreadInfo = record   
Position : Integer;    Caption : String;  end;   TMyThreadDisplay =
class(TCustomControl)  private    flblLabel : array[0..4] of TLabel; 
 fprbProgressbar : array[0..4] of TProgressBar;    fThreadInfo :
array[0..4] of TThreadInfo;    function GetThreadInfo(Index: Integer):
TThreadInfo;    procedure SetThreadInfo(Index: Integer; AValue:
TThreadInfo);  protected   public    constructor Create(AOwner :
TComponent); override;    property ThreadInfo[Index: Integer]:
TThreadInfo read GetThreadInfo write SetThreadInfo;  published   
property Align;  end; procedure Register; implementation function
TMyThreadDisplay.GetThreadInfo(Index: Integer): TThreadInfo;begin 
Result := fThreadInfo[Index];end; procedure
TMyThreadDisplay.SetThreadInfo(Index: Integer; AValue:
TThreadInfo);begin  if fThreadInfo[Index].Caption <> AValue.Caption
then    begin      fThreadInfo[Index].Caption := AValue.Caption;     
flblLabel[Index].Caption := AValue.Caption;    end;   if
fThreadInfo[Index].Position <> AValue.Position then    begin     
fThreadInfo[Index].Position := AValue.Position;     
fprbProgressbar[Index].Position := AValue.Position;   
end;end; constructor TMyThreadDisplay.Create(AOwner : TComponent);var 
iTEMP : Integer;begin  inherited Create(AOwner);   for iTEMP := 4
downto 0 do    begin      fprbProgressbar[iTEMP] :=
TProgressBar.Create(Self);       with fprbProgressbar[iTEMP] do      
begin          fThreadInfo[iTEMP].Position := 0;          Align :=
alTop;          Parent := Self;          Position :=
fThreadInfo[iTEMP].Position;        end;       flblLabel[iTEMP] :=
TLabel.Create(Self);        with flblLabel[iTEMP] do          begin  
        fThreadInfo[iTEMP].Caption := 'Thread ' + iTEMP.ToString;    
      Caption := fThreadInfo[iTEMP].Caption;            Align :=
alTop;            Parent := Self;            ParentFont := True;     
   end;      end;   Constraints.MinHeight := 205; 
Constraints.MinWidth := 100;  SetBounds(0, 0, 100, 205);end; procedure
Register;begin  RegisterComponents('Misc',
[TMyThreadDisplay]);end; end. 
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, ComCtrls, StdCtrls, 
mythreaddisplay; type   { TForm1 }   TForm1 = class(TForm)   
procedure FormCreate(Sender: TObject);  private   public   end; var 
Form1: TForm1;  MyDisplay : TMyThreadDisplay; implementation {$R
*.lfm} { TForm1 } procedure TForm1.FormCreate(Sender: TObject);begin 
MyDisplay := TMyThreadDisplay.Create(self);  MyDisplay.Left := 8; 
MyDisplay.Top := 8;  MyDisplay.Parent := Form1;  
MyDisplay.ThreadInfo[1].Caption := 'something';  // fails
hereend; end.

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PascalDragon · 2021-05-13 09:07

If you have a property that is of a record type you must change it as
a whole. This avoids people wondering why their code does not work,
because the getter only provides a copy (*).

So you need to use 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 var  info:
TThreadInfo;begin  info := MyDisplay.ThreadInfo[1];  info.Caption :=
'something';  MyDisplay.ThreadInfo[1] := info;end;
Alternatively you can use a class instead of a record for TThreadInfo
(you need to instantiate them correctly of course). Or your property
needs to a pointer to the TThreadInfo.

(*) That is because the compiler would essentially generate code 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 var  tmp:
TThreadInfo;begin  // MyDisplay.ThreadInfo[1].Caption := 'something'; 
tmp := MyDisplay.ThreadInfo[1];  tmp.Caption := 'something'
The tmp record would be a copy, not a reference.

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pcurtis · 2021-05-13 09:42

Thank you.