[SOLVED] Runtime component creation

[pcurtis]

So, we can create a runtime component as such

Code: Pascal  [Select][+][-]

1. var
2.   MyComp: TDBGrid;
3.  
4. begin
5.   MyComp:= TDBGrid.Create(Self);
6.   ......
7.  

How can I create a runtime component when the only thing
known is the classname?

So, for example, I could create a TLabel, then a TScrollbox and so on.

How to define MyComp? and how to create the component from a string holding the classname?

All components are visual.

[pcurtis]

No.
Pseudo code ...
var
  MyComp: array of TUniversalBaseComponent;

  MyComp[0] := 'TLabel'.Create(nil);
  MyComp[1] := 'TMemo'.Create(nil);
  MyComp[2] := 'TDBGrid'.Create(nil);
  .....
  .....
  MyComp[0].Free;
  MyComp[1].Free;
  MyComp[2].Free;

[pcurtis]

Close but

Var
  J:TClass;

.....
  J := Tmemo;  <--

--- in script land.

 If J.ClassName = StringFromScript('Tmemo') Then myComponentArray[?] := J.Create;

How can I tell "J" to be a TMemo from a string that holds 'TMemo' or a TScrollbox if the string contains 'TScrollbox'?

Or do I have to do a If .. Then for each component type? There are a lot of them and there new ones all the time.

[pcurtis]

Maybe this makes it a little clearer

Code: Pascal  [Select][+][-]

1. var
2.   ComponentNames: Array[0..10] of String;
3.   Components: Array[0..10] of TClass;
4.  
5.   ComponentNames[0]:= 'TLabel';
6.   ComponentNames[1]:= 'TDBGrid';
7.   ComponentNames[2]:= 'TSomeComponent';
8.  
9.   for i:= 0 to 2 do
10.     Components[i]:= ComponentNames[i].Create; <-- Here

Something like
StringToClass(ComponentNames).Create

[Handoko]

Write a new function and use case statement:

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, StdCtrls;
9.  
10. type
11.  
12.   { TForm1 }
13.  
14.   TForm1 = class(TForm)
15.     Button1: TButton;
16.     procedure Button1Click(Sender: TObject);
17.   private
18.     function CreateClassFromName(const S: string): TControl;
19.   end;
20.  
21. var
22.   Form1: TForm1;
23.  
24. implementation
25.  
26. {$R *.lfm}
27.  
28. { TForm1 }
29.  
30. function TForm1.CreateClassFromName(const S: string): TControl;
31. begin
32.   case S.ToUpper of
33.     'TLABEL':  Result := TLabel.Create(Self);
34.     'TBUTTON': Result := TButton.Create(Self);
35.     else
36.       Result := nil;
37.   end;
38.   if Assigned(Result) then
39.     Result.Parent := Self;
40.   Assert(Result <> nil, 'Wrong class name for CreateClassFromName.');
41. end;
42.  
43. procedure TForm1.Button1Click(Sender: TObject);
44. begin
45.   with CreateClassFromName('TButton') do
46.   begin
47.     Left    := 120;
48.     Top     := 50;
49.     Caption := 'Button';
50.   end;
51.   with CreateClassFromName('TLabel') do
52.   begin
53.     Left    := 60;
54.     Top     := 80;
55.     Caption := 'A new button has been created.';
56.   end;
57.   Button1.Enabled := False;
58. end;
59.  
60. end.

~edit~
Because the result is TControl, I think it will be better to name it CreateControlFromName.

[pcurtis]

Thanks Hanoko, but

  case S.ToUpper of
    'TLABEL':  Result := TLabel.Create(Self);
    'TBUTTON': Result := TButton.Create(Self);
    else
      Result := nil;
  end;

Is what I am trying to avoid. The types of components is large, and is constantly growing. The list of cases would become unmanageable. There must be a way.
When the IDE reads a *.lfm file component names are just strings.
Problem is I don't see how.

[PascalDragon]

When the IDE reads a *.lfm file component names are just strings.
Problem is I don't see how.

As long as your components are registered correctly so that they could also be handled with a form you can retrieve them using Classes.FindClass (which will throw an exception if the class isn't found) or Classes.GetClass (which doesn't throw an exception but returns Nil). Please note that this will only work for classes that descend from TPersistent (any TComponent descendant and thus any LCL control does that). If you need to register a descendant of TPersistent that isn't yet registered you need to use Classes.RegisterClass.

[avk]

It seems that, if desired, a similar system of registration can be done only for controls.

[pcurtis]

Thanks all.