Rtti data manipulation and consistency in Delphi 2010

Question

Has anyone an idea, how I can make TValue using a reference to the original data? In my serialization project, I use (as suggested in XML-Serialization) a generic serializer which stores TValues in an internal tree-structure (similar to the MemberMap in the example).

This member-tree should also be used to create a dynamic setup form and manipulate the data. My idea was to define a property for the Data:

TDataModel <T> = class
  {...}
  private
    FData : TValue;
    function GetData : T;
    procedure SetData (Value : T);
  public
    property Data : T read GetData write SetData;
end;

The implementation of the GetData, SetData Methods:

procedure TDataModel <T>.SetData (Value : T);

begin
FData := TValue.From <T> (Value);
end;

procedure TDataModel <T>.GetData : T;

begin
Result := FData.AsType <T>;
end;

Unfortunately, the TValue.From method always makes a copy of the original data. So whenever the application makes changes to the data, the DataModel is not updated and vice versa if I change my DataModel in a dynamic form, the original data is not affected. Sure I could always use the Data property before and after changing anything, but as I use lot of Rtti inside my DataModel, I do not realy want to do this anytime.

Perhaps someone has a better suggestion?

Answer 1

A TValue is designed to hold any kind of data in a very compact form, it's not designed to emulate an "pointer". Take a look in the RTTI.pas unit: TValue is a RECORD that only has one data member of the type TValueData. TValueData itself is itself a variant record.

Looking at TValueData you will see how it does NOT hold anything but the minimum amount of data: There's no way to know where that TValue came from.

The solution: Don't hold a TValue in your structures, replace it with a pair of TRttiField + an TObject. When you need the TValue use TRttiField.GetValue(Instance), when you want to set a value use TRttiField.SetValue(Instance, AValue:TValue).

Answer 2

Thanks Cosmin for your help, the solution is not to save a TValue in the structure but use a Pointer to the data and use the GetValue, SetValue methods of a field or property.

So here is how I solved it in my generic class:

TDataModel <T> = class
  private
    FType     : PTypeInfo;
    FInstance : Pointer;
  public 
    constructor Create (var Data : T);
    procedure ShowContent;
  end;

constructor TDataModel <T>.Create (var Data : T);

begin
FType := TypeInfo(T);
if FType.Kind = tkClass then
  FInstance := TObject (Data)
else if FType.Kind = tkRecord then
  FInstance := @Data;
end;

procedure TDataModel <T>.ShowContent;
var 
  Ctx   : TRttiContext;
  Field : TRttiField;

begin
for Field in Ctx.GetType (FType).GetFields do
  Writeln (Field.GetValue (FInstance).ToString);
end;

Now you can either change fields using the DataModel through or using the original record class.