How to get the published properties of an persistent object

How to get the published properties of an persistent object

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 :

 

  PTypeInfo = ^TTypeInfo;
  TTypeInfo = record
    Kind: TTypeKind;
    Name: ShortString;
  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.

  PPropInfo = ^TPropInfo;
  TPropInfo = packed record
    PropType: PPTypeInfo;
    GetProc: Pointer;
    SetProc: Pointer;
    StoredProc: Pointer;
    Index: Integer;
    Default: Longint;
    NameIndex: SmallInt;
    Name: ShortString;
  end;

To clarify it, please take a look at this example :

function GetFontSize(Obj:TPersistent):Integer;
{
in this Procedure we want to get the pPropInfo-pointer - pointing
on the Font-Property from an arbitrary TPersistent-Class.
The return-value in this instance will be the font-size ( if the font
property exists , if not -> the return value will be -1 )
}
var PropInfo:PPropInfo;
begin
  RESULT:=-1;
  // Get the PPropInfo-Pointer for Font of the TPersistent obj
  PropInfo:=GetPropInfo(Obj,'Font');
  // At first we will find out if the property FONT exists
  if PropInfo=nil then
    EXIT; // The Property doesn't exists
  {
    TFont is not an ordinal-Type - therefore will have to control if
    Typekind of the TypeInfo-Class is set to tkClass
  }
  if PropInfo.PropType^.Kind <>tkClass then
    EXIT; // property isn't a tkClass type
  {
    now, we now that the TypeKind of die PropInfo-pointer is a class .
    last but not least we will use the GetObjectProp, the return-value
    of this function is a TObject. Subsequently, we will use this object as
    a TFont to get the Size value.
  }
  RESULT:=( (GetObjectProp(Obj,PropInfo)) as TFont).Size;
end;

 

But to get the complete list of all properties of a TPersistent-Class we will need the pPropList-Type . This type is a simple pointer-array and the magic key to all Property-Informations and their structures.

Take a look at this :

 

procedure TForm1.Button1Click(Sender: TObject);

const tkOrdinal = [tkEnumeration,tkInteger,tkChar,tkSet,tkWChar]; //Filter

begin
{
  in this method of the mainform-class we are seeking for all ordinal-type
  properties of the edit1-component. The from the GetPropertyList method
  returned list of all properties will be written into the Listbox1. You can
  replace the obj parameter with an arbitrary TObject ( but usually TPersistent
  objects ).
  For another filter please take a look at the TTypeKinds-set.
}
  GetPropertyList(Edit1,ListBox1.Items,tkOrdinal);
end;

 

procedure GetPropertyList(Obj:TObject ;List: TStrings;Filter: TTypeKinds);
var PropList:pPropList;
    count,i : Integer;
begin
  List.Clear;
  // Here we'll get the count of the given properties, ...
  Count := GetPropList(Obj.ClassInfo, Filter, nil);
  // ...and create room for the PropList,...
  GetMem(PropList,Count * SizeOf(PPropInfo));
  // ...get the Proplist-Data,...
  GetPropList(Obj.ClassInfo, Filter, PropList);
  // ...and write the property-names into the StringList
  for i:=0 to Count -1 do
    List.Add(Proplist[i].Name);
end;

 

That's it ---- actually easy , or?

best regards

Boris Benjamin Wittfoth