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How to get the published properties of an persistent object Turn on/off line numbers in source code. Switch to Orginial background IDE or DSP color Comment or reply to this aritlce/tip for discussion. Bookmark this article to my favorite article(s). Print this article
02-Jun-03
Category
VCL-General
Language
Delphi 2.x
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Publisher:
DSP, Administrator
Reference URL:
DKB
			 Author: Boris Benjamin Wittfoth

How to get the published properties of an persistent object / Using the 
pPropInfo-Pointer and the RTTI of Delphi

Answer:

The TypeInfo unit of Delphi declares several types and functions that gives you 
easy access to the puplished properties of an object and other informations. 
You can obtain a list of the published properties of a class and get the name an 
type of each property. 

The TypeInfo funtion returns a pointer to a type information record. The TypInfo 
unit declares a real type, that is, a pointer to a TTypeInfo record : 

1   PTypeInfo = ^TTypeInfo;
2   TTypeInfo = record
3     Kind: TTypeKind;
4     Name: ShortString;
5   end;


The TTypeKind datatype describes the Datatype , returned by the GetTypeData 
function. 

TTypeKind = (tkUnknown, tkInteger, tkChar, tkEnumeration, tkFloat,
  tkString, tkSet, tkClass, tkMethod, tkWChar, tkLString, tkWString,
  tkVariant, tkArray, tkRecord, tkInterface, tkInt64, tkDynArray);
TTypeKinds = set of TTypeKind;

Well ... for our first step to access the objects published properties we need to 
use the PPropInfo-pointer. 

6   PPropInfo = ^TPropInfo;
7   TPropInfo = packed record
8     PropType: PPTypeInfo;
9     GetProc: Pointer;
10    SetProc: Pointer;
11    StoredProc: Pointer;
12    Index: Integer;
13    default: Longint;
14    NameIndex: SmallInt;
15    Name: ShortString;
16  end;
17  
18  //To clarify it, please take a look at this example : 
19  
20  function GetFontSize(Obj: TPersistent): Integer;
21  {
22  in this Procedure we want to get the pPropInfo-pointer - pointing
23  on the Font-Property from an arbitrary TPersistent-Class.
24  The return-value in this instance will be the font-size ( if the font
25  property exists , if not -> the return value will be -1 )
26  }
27  var
28    PropInfo: PPropInfo;
29  begin
30    RESULT := -1;
31    // Get the PPropInfo-Pointer for Font of the TPersistent obj
32    PropInfo := GetPropInfo(Obj, 'Font');
33    // At first we will find out if the property FONT exists
34    if PropInfo = nil then
35      EXIT; // The Property doesn't exists
36    {
37      TFont is not an ordinal-Type - therefore will have to control if
38      Typekind of the TypeInfo-Class is set to tkClass
39    }
40    if PropInfo.PropType^.Kind <> tkClass then
41      EXIT; // property isn't a tkClass type
42    {
43      now, we now that the TypeKind of die PropInfo-pointer is a class .
44      last but not least we will use the GetObjectProp, the return-value
45      of this function is a TObject. Subsequently, we will use this object as
46      a TFont to get the Size value.
47    }
48    RESULT := ((GetObjectProp(Obj, PropInfo)) as TFont).Size;
49  end;
50  
51  But to get the complete list of all properties of a TPersistent-class we will need 
52  the pPropList-type . This type is a simple pointer-array and the magic key to all 
53  property-Informations and their structures. 
54  
55  Take a look at this : 
56  
57  procedure TForm1.Button1Click(Sender: TObject);
58  
59  const
60    tkOrdinal = [tkEnumeration, tkInteger, tkChar, tkSet, tkWChar]; //Filter
61  
62  begin
63    {
64      in this method of the mainform-class we are seeking for all ordinal-type
65      properties of the edit1-component. The from the GetPropertyList method
66      returned list of all properties will be written into the Listbox1. You can
67      replace the obj parameter with an arbitrary TObject ( but usually TPersistent
68      objects ).
69      For another filter please take a look at the TTypeKinds-set.
70    }
71    GetPropertyList(Edit1, ListBox1.Items, tkOrdinal);
72  end;
73  
74  procedure GetPropertyList(Obj: TObject; List: TStrings; Filter: TTypeKinds);
75  var
76    PropList: pPropList;
77    count, i: Integer;
78  begin
79    List.Clear;
80    // Here we'll get the count of the given properties, ...
81    Count := GetPropList(Obj.ClassInfo, Filter, nil);
82    // ...and create room for the PropList,...
83    GetMem(PropList, Count * SizeOf(PPropInfo));
84    // ...get the Proplist-Data,...
85    GetPropList(Obj.ClassInfo, Filter, PropList);
86    // ...and write the property-names into the StringList
87    for i := 0 to Count - 1 do
88      List.Add(Proplist[i].Name);
89  end;


			
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