How to create an object by its "string" class name ?
lazarus / 64-FPC development

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izd · 2017-03-07 17:40

Hi everybody ! 

I would like to found out an easy way to create an object by its class
name from a string.

It should look 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 var  obj:
TObject;   obj := BuildObject( 'TButton', Self );
Of course, the NOT easy way is to cast the class name...

nb: For now, my interest is about controls (cf. Self)

Thank you in advance

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Thaddy · 2017-03-07 17:57

This is not a scripting language but probably the RTTI lovers   will
give you an elaborate   answer on how exactly you can do that.    (And
it's a bad idea)

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marcov · 2017-03-07 18:08

Afaik VCL classes are registered, so something like

getclass('TButton').create(selfl);

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izd · 2017-03-07 18:11

Thank you mates 

@marcov: Unfortunately, its not works...

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Thaddy · 2017-03-07 18:29

They are only registered when used somewhere. Otherwise you have to
register them by hand. (code reduction)

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izd · 2017-03-07 18:33

This is what I thought after searching the web...

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sky_khan · 2017-03-07 19:29

Notice that, this is not thread-safe but it provides you a way to use
classes you want, without interfering streaming of components

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
ControlReg; {$mode objfpc}{$H+} interface uses  Classes, SysUtils,
Controls, fgl; type  TClassMap = specialize
TFPGMap<string,TControlClass>;   { TControlRegistry }  
TControlRegistry = class(TClassMap)  public    procedure
RegisterControl(aClass:TControlClass);    procedure
RegisterControls(aClassArr : array of TControlClass);    function
CreateControl(const aClassName:string;aOwner:TComponent):TControl; 
end;    function ControlRegistry :
TControlRegistry; implementationuses  rtlconsts; var  FReg :
TControlRegistry; function ControlRegistry : TControlRegistry;begin 
if FReg=nil then    FReg :=TControlRegistry.Create; 
Result:=FReg;end; { TControlRegistry } procedure
TControlRegistry.RegisterControl(aClass: TControlClass);begin 
Add(aClass.ClassName,aClass);end; procedure
TControlRegistry.RegisterControls(aClassArr: array of
TControlClass);var  aClass : TControlClass;begin  for aClass in
aClassArr do    RegisterControl(aClass);end; function
TControlRegistry.CreateControl(const aClassName: string;  aOwner:
TComponent): TControl;var  Ndx : Integer;begin 
Ndx:=IndexOf(aClassName);  if Ndx>=0 then   
Result:=Data[Ndx].Create(aOwner)  else    raise
EClassNotFound.CreateFmt(SClassNotFound,
[AClassName]);end; initializationfinalization  FReg.Free;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 uses
ControlReg; procedure TForm1.Button1Click(Sender: TObject);begin  with
ControlRegistry.CreateControl('TEdit',Self) do   
Parent:=Panel1;end; constructor TForm1.Create(aOwner:
TComponent);begin  inherited Create(aOwner); 
ControlRegistry.RegisterControls([TMemo,TButton,TEdit,TLabel]);end;

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Remy Lebeau · 2017-03-07 19:34

Quote from: marcov on March 07, 2017, 07:08:56 pm
Afaik VCL classes are registered, so something like

getclass('TButton').create(selfl);

GetClass() returns a general purpose TPersistentClass (aka 'class of
TPersistent').  You have to type-cast it in order to call the virtual
TComponent.Create() constructor:

Code: [Select]var
  Btn: TButton;
begin
  Btn := TButton(TComponentClass(GetClass('TButton')).Create(Self));
end;

Alternatively:

Code: [Select]type
  TButtonClass = class of TButton;

var
  Btn: TButton;
begin
  Btn := TButtonClass(GetClass('TButton')).Create(Self);
end;

Just make sure the desired classes are registered at runtime, either
by the RTL itself when initializing packages, or explicitly in code:

Code: [Select]RegisterClass(TDesiredControlHere);

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bylaardt · 2017-03-07 22:16

No magic:
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 function
BuildControl(Const
ObjectClass:String;ObjectOwner:TComponent):TwinControlbegin  case
LowerCase(ObjectClass) of    'tbutton' :
result:=TButton.create(ObjectOwner);    'tlabel'  :
result:=TLabel.create(ObjectOwner);    'tedit'   :
result:=TEdit.create(ObjectOwner);    'tmemo'   :
result:=TMemo.create(ObjectOwner);    // and so on  else     raise
Exception.create(ObjectClass+' not allowed in BuilControl');  end;end;

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izd · 2017-03-08 18:18

So.....no magic....(sic)

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 function
BuildControl( const pClassName: String ): TObject;begin  case
pClassName of    'TButton'  : Result := TButton.Create( nil );   
.../... and so on .../...  end;end;
Thank you all friends 

ps: if someone found out the way to do so, I'll take it

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Ñuño_Martínez · 2017-03-09 10:33

May be you can use an array of a record to store the classes. 
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   TYPE  {
Any component. }    ClassType = CLASS OF TComponent;   { Stores class
information. }    TClassListItem = RECORD      Name: STRING;     
TheClass: ClassType;    END;   VAR    ClassList: ARRAY [...] OF
TClassListItem;   BEGIN  { Add class information. }   
ClassList[0].Name := 'TButton';    ClassList[0].TheClass :=
TButton;...  { Searchs "ClassName" and creates "TheObject" }    FOR
Ndx := LOW (ClassList) TO HIGH (ClassList) DO      IF ClassName =
ClassList[Ndx].ClassName THEN        TheObject :=
ClassList[Ndx].TheClass.Create (SELF)  END; I didn't test actually so
may be it doesn't work at all.

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marcov · 2017-03-09 11:26

If you use LCL you might as well use the fact that such array/tlist
already exist, see Remy's answer.

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Ñuño_Martínez · 2017-03-11 17:42

Quote from: marcov on March 09, 2017, 12:26:08 pmIf you use LCL you
might as well use the fact that such array/tlist already exist, see
Remy's answer.
 I see.  Didn't knew that.