[SOLVED] Passing data back and forth between forms
lazarus / 16-Databases

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HopefulGuy · 2021-05-09 05:00

I guess I'm too used to Delphi. I have a program I'm working on with
multiple forms. Form A has a button who's onClick event calls Form B.
So naturally the unit file for Form A uses the unit file for Form B.
BUT, Form B needs to change properties on Form A. In Delphi I'd just
include Form A's unit in Form B. But Lazarus/FreePascal doesn't seem
to want to let me do that. So how do I access a property of From A
from Form B? I tried referencing Self.Parent, but the compiler doesn't
recognize the properties in form A that way. I can't typecast as Form
A because in order to do that Form B would need to know the format of
Form A, which brings me right back to where I started.

Help?

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fpcoder · 2021-05-09 05:47

To avoid circular reference, add Form A's unit to Form B's
implementation uses instead of the interface part.

Then you reference form A's properties with frmA.SomeProperty.

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  frmB:
TfrmB; implementation uses  uFormA; {$R *.lfm} { TfrmB }

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HopefulGuy · 2021-05-09 08:37

Thanks! That's exactly what I needed!

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MarkMLl · 2021-05-09 08:42

You might still find that you have a circular reference in the
form/class definition. You can fix that using a class helper, yell if
you need an example.

MarkMLl

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wp · 2021-05-09 11:04

I try to avoid such a situation from the beginning. When a form
defines settings needed by another form then I never acces the form to
read the settings but declare a variable in a third unit, and this
unit is to be used by both forms.

Example (untested):
Suppose Form1 contains a Combobox in which a particular font size is
selected, and this font size will be needed by another Form2.

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
Globals;interface var  GlobalFontSize: Integer =
10; implementation end. ----------------------------- unit
Unit1; ... implementation uses  Unit2, Globals;   // Use the
implementation "uses" whenever possible. procedure
TForm1.Combobox1Change(Sender: TObject);begin  if Combobox1.ItemIndex
> -1 then    // Store selected font size in global variable   
GlobalFontSize := StrToInt(Combobox1.Items[Combobox1.ItemIndex]);end; 
------------------------------ unit Unit2; ... implementation uses 
Globals; procedure TForm2.UpdateFontSize;begin  // do something with
GlobalFontSize  // Read the font size from the global variable rather
than from Form1's combobox.end;
I know global variables are usually considered to be bad programming
style. But here a global variable is a true solution to the circular
reference issue which sooner or later may become unresolvable.

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MarkMLl · 2021-05-09 11:58

Quote from: wp on May 09, 2021, 01:04:21 pmI know global variables are
usually considered to be bad programming style. But here a global
variable is a true solution to the circular reference issue which
sooner or later may become unresolvable.

I think the "considered harmful" designation is unsupportable as long
as Lazarus insists on representing each form by a global variable.

MarkMLl

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jcmontherock · 2021-05-09 16:22

Another way:

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;    Interface             uses Unit2;    implementation    ... 
  var Font1: TFont;        procedure TForm1.FormCreate(Sender:
TObject);    begin      PFont :=  @Font1;      ......    end;    
===================================================    unit Unit2;   
implementation     var PFont: ^TFont;      // address of Font1 in
Unit1           procedure TForm2.UpdateFontSize;    var fontSize:
Single;    begin      fontSize := PFont^.Size;      .......     
PFont^.Size := 9;      .......    end;