[solved] Iteration of class fields

[Joanna from IRC]

Code: Pascal  [Select][+][-]

1. Myclass = class
2. Lbl:tlabel;
3. Edit:tedit;
4. Button:tbutton;
5. End;
6.  
7.  

Assuming that none of these are created until runtime . Is there an elegant way to iterate through them and create them? Yes I know i can iterate through controls list after creation. But is there a way to check class fields for nil and create them?

[Thaddy]

assigned()

[Joanna from IRC]

Assigned() Checkes if it’s nil I presume.
Is it possible to iterate through Class fields?

[Thaddy]

Assigned() Checkes if it’s nil I presume.
Is it possible to iterate through Class fields?

in your example, yes, provided that the container class is instantiated, you can use assigned() on the contained components.

[rvk]

Assuming that none of these are created until runtime . Is there an elegant way to iterate through them and create them? Yes I know i can iterate through controls list after creation. But is there a way to check class fields for nil and create them?

If you want this dynamically without knowing the names of the fields you would need RTTI.

Have a look at this topic. https://forum.lazarus.freepascal.org/index.php?topic=39791.0
It has some code example normally used for Delphi but maybe someone can translate it for FPC, it that is what you meant with your question.
(Back in 2018 when that topic was created RTTI wasn't yet implemented fully).

assigned()

How is this related to ITERATION of the class fieds. It's just checking if they exists but you would need to do this separate with each field. The question was... how can you ITERATE through them !!

[alpine]

Assuming that none of these are created until runtime . Is there an elegant way to iterate through them and create them? Yes I know i can iterate through controls list after creation. But is there a way to check class fields for nil and create them?

If you want this dynamically without knowing the names of the fields you would need RTTI.

Have a look at this topic. https://forum.lazarus.freepascal.org/index.php?topic=39791.0
It has some code example normally used for Delphi but maybe someone can translate it for FPC, it that is what you meant with your question.
(Back in 2018 when that topic was created RTTI wasn't yet implemented fully).

Perhaps a custom enumerator can be defined, based on the RTTI. But where is the point when each of them needs to be created by its own factory and then customized separately?

[rvk]

Perhaps a custom enumerator can be defined, based on the RTTI. But where is the point when each of them needs to be created by its own factory and then customized separately?

Creation should be possible through the classtype. Assuming some common elements like owner and parent you could initialize those too. Other fields could be customized on class basis.

But yes, it might be useful to have more info about the use-case.

[marcov]

(Back in 2018 when that topic was created RTTI wasn't yet implemented fully).

_published_ RTTI worked since the 199xs.  See Tcomponent.readcomponent* and its helpers outside of tcomponent in rtl/objpas/classes.

The new RTTI (unit) is for the extra RTTI for non published properties that was introduced with IIRC D2010, and turned on globally with Delphi XE 2.

[Thaddy]

Just in case Joanna indeed really creates the components dynamically: in that case the component will not be present in the component list. Example assumes a button and a form:

Code: Pascal  [Select][+][-]

1. function CreateUniqueName(const BaseName: string; Owner: TComponent): string;
2. var
3.   Counter: Integer;
4. begin
5.   Counter := 1;
6.   Writestr(Result,BaseName,Counter);
7.   while Assigned(Owner.FindComponent(Result)) do
8.   begin
9.     Writestr(Result,BaseName,Counter);
10.     Inc(Counter);
11.   end;
12. end;
13.  
14. procedure TForm1.Button1Click(Sender: TObject);
15. begin
16.    with tbutton.create(form1) do
17.    begin
18.      parent := form1;
19.      { strip the T from the classname }
20.      name :=CreateUniqueName(copy(classname,2,length(classname)), form1);
21.      align:=altop;
22.      visible :=true;
23.      OnClick  := @Button1Click;
24.    end;
25. end;

       

[rvk]

(Back in 2018 when that topic was created RTTI wasn't yet implemented fully).

_published_ RTTI worked since the 199xs.  See Tcomponent.readcomponent* and its helpers outside of tcomponent in rtl/objpas/classes.

The new RTTI (unit) is for the extra RTTI for non published properties that was introduced with IIRC D2010, and turned on globally with Delphi XE 2.

For Delphi 10+ something like this simply works. I Lazarus it doesn't produce anything.

Code: Pascal  [Select][+][-]

1. uses rtti;
2.  
3. type
4.   TMyClass = class
5.     Lbl: TLabel;
6.     Edit: tedit;
7.     Button: TButton;
8.   end;
9.  
10. procedure TForm1.FormCreate(Sender: TObject);
11. var
12.   context: TRttiContext;
13.   rType: TRttiType;
14.   Field: TRttiField;
15. begin
16.   context := TRttiContext.Create;
17.   rType := context.GetType(TMyClass.ClassInfo);
18.   for Field in rType.GetFields do
19.       Memo1.Lines.Add(Field.Name);
20. end;

Lbl
Edit
Button

So the question is... is this already possible with the rtti from Lazarus???

[Thaddy]

Of course and without RTTI, see my example above.

But with RTTI this should be possible in trunk with maybe small changes because the extended RTTI is similar to Delphi but not equal, 100% compatible.
I do not see anything in your code snippet that would not work.

[rvk]

Of course and without RTTI, see my example above.

I don't see how the class mentioned in the openingspost would have a componentcount or even an owner. It's just a class with fields/elements.

But you could argue why this is constructed like this... you could also use a record or something. So the actual use-case would be important to know.

Code: Pascal  [Select][+][-]

1. type
2.   TMyClass = class
3.     Lbl: TLabel;
4.     Edit: tedit;
5.     Button: TButton;
6.   end;

[Thaddy]

@RvK our posts crossed, you may examine my edit?

[rvk]

But with RTTI this should be possible in trunk with maybe small changes because the extended RTTI is similar to Delphi but not equal, 100% compatible.
I do not see anything in your code snippet that would not work.

I tried the code in Delphi and got the result below it.
Trying the same code in an empty project with a simple TMemo and pasting it in, results in an empty memo.

I use FPC and Lazarus trunk from 3 days ago.

[marcov]

So the question is... is this already possible with the rtti from Lazarus???

• Work in progress in FPC/trunk.
• Not production yet, don't know exact status.
• Might also require release engineering changes to actually enable it when compiling. You'll know when the EXEs double