[SOLVED] TImage TmouseEvent stackframe issue
lazarus / 21-General

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4nlegion · 2021-08-03 03:03

Hi, i have a problem with the stackframe of the OnMouseDown event of a
TImage created at runtime. Note the Timage.OnMouseDown assignment via
TMethod.
Upon MouseDown, MouseDownEvent is called with the wrong stackframe. It
appears to be off by sizeof(TMouseButton). Is my definition of
TMouseEvent incorrect? Regards.
Also, is there a recommended practice to preclude such a declaration
mismatch?

Details: Ubuntu 20.04, FPC version 3.2.0 [2020/07/07] for x86_64,
Lazarus: 2.0.12

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
MouseDownEvent(Sender: TObject; Button: TMouseButton; Shift:
TShiftState; X,Y: Integer);begin        if not (Sender is TImage) then
exit;        Form1.ListBox1.Items.AddText(format('[x,y]:
[%d,%d]',[x,y]));   // note incorrect [x,y] reportingend; procedure
TForm1.FormCreate(Sender: TObject);var        e:      TMethod;begin  
    img:=TImage.create(Form1);        img.Parent:=Form1;       
img.left:=32;        img.top:=32;        img.width:=400;       
img.height:=400;        img.Canvas.Brush.Color := clgreen;       
img.Canvas.Brush.style := bsSolid;       
img.canvas.FillRect(0,0,400,400);         e.Data:=img;       
e.Code:=@MouseDownEvent;        img.OnMouseDown:=TMouseEvent(e);end;

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lucamar · 2021-08-03 04:10

No, it isn't. Events are declared as "of object" which means that they
have to be declared/defined as methods of an object (usually, though
not always, of the form itself). Try something like:
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 {in the
interface section ...}   TForm1 = class(TForm)    {... other
declarations ...}    procedure ImageMouseDown(Sender: TObject; Button:
TMouseButton; Shift: TShiftState; X,Y: Integer);  end;  {then, in the
implementation section ...}  procedure TForm1.ImageMouseDown(Sender:
TObject; Button: TMouseButton; Shift: TShiftState; X,Y:
Integer);begin        if not (Sender is TImage) then exit;       
Form1.ListBox1.Items.AddText(format('[x,y]: [%d,%d]',[x,y]));   //
note incorrect [x,y] reportingend;
Any reason why you're using an assignment through TMethod? Normally
that's used only when you need to access the RTTI of an object's
methods.

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4nlegion · 2021-08-03 04:16

This was an example distilled from a larger unit. I am working on an
editor unit that handles many of it's own events and exhibits this
issue. I was (lucky?) that this smaller code showed the same symptom.

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4nlegion · 2021-08-03 04:40

I just read a different Forum Post
https://forum.lazarus.freepascal.org/index.php?topic=31306.0 and
Thaddy replied with:
"...procedure of object has a hidden self pointer..."

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
DoSomeThing(Dummy:Pointer;Sender:TObject);  begin   
writeln('Hello');  end;  
so changing the previous event handler to:

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
MouseDownEvent(Dummy:Pointer; Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X,Y: Integer);begin        if not (Sender is
TImage) then exit;        Form1.ListBox1.Items.AddText(format('[x,y]:
[%d,%d]',[x,y]));   // note NOW correct [x,y] reportingend;  
...works as intended.

Many thanks!

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4nlegion · 2021-08-03 06:55

In case anyone cares...

Combining lucamar's solution we have:

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+}interfaceuses        Classes, SysUtils,
Forms, Controls, Graphics,Dialogs, StdCtrls, ExtCtrls;type       
TImage1 = class(TImage)                procedure DoOnMouseDown(Sender:
TObject; Button: TMouseButton; Shift: TShiftState; X,Y: Integer);    
  end;         TForm1 = class(TForm)                ListBox1:
TListBox;                procedure FormCreate(Sender: TObject);      
private        public        end; var        Form1:  TForm1;       
img:    TImage; implementation{$R *.lfm}{ TForm1 }procedure
MouseDownEvent(Dummy:Pointer; Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X,Y: Integer);begin        if not (Sender is
TImage) then exit;        Form1.ListBox1.Items.AddText(format('[x,y]:
[%d,%d]',[x,y]));   // now is correct [x,y] reportingend; procedure
TImage1.DoOnMouseDown(Sender: TObject; Button: TMouseButton; Shift:
TShiftState; X,Y: Integer);begin        if not (Sender is TImage) then
exit;        Form1.ListBox1.Items.AddText(format('[x,y]:
[%d,%d]',[x,y]));   // now is correct [x,y] reportingend; procedure
TForm1.FormCreate(Sender: TObject);var        e:      TMethod;begin  
    img:=TImage.create(Form1);        img.Parent:=Form1;       
img.left:=32;        img.top:=32;        img.width:=400;       
img.height:=400;        img.Canvas.Brush.Color := clgreen;       
img.Canvas.Brush.style := bsSolid;       
img.canvas.FillRect(0,0,400,400);         e.Data:=img;       
e.Code:=@TImage1.DoOnMouseDown; //MouseDownEvent;       
img.OnMouseDown:=TMouseEvent(e);end; end.