Loading and Saving Custom Variant Values
Go Up to Copying and Clearing Custom Variants
By default, when the custom variant is assigned as the value of a published property, it is typecast to a string when that property is saved to a form file, and converted back from a string when the property is read from a form file. You can, however, provide your own mechanism for loading and saving custom variant values in a more natural representation. To do so, the TCustomVariantType descendant must implement the System.Classes.IVarStreamable interface from Classes.pas.
IVarStreamable defines two methods, StreamIn and StreamOut, for reading a value of a variant from a stream and for writing the variant's value to the stream. For example, TComplexVariantType, in the VarCmplx unit, implements the IVarStreamable methods as follows:
Delphi:
procedure TComplexVariantType.StreamIn(var Dest: TVarData; const Stream: TStream);
begin
with TReader.Create(Stream, 1024) do
try
with TComplexVarData(Dest) do
begin
VComplex := TComplexData.Create;
VComplex.Real := ReadFloat;
VComplex.Imaginary := ReadFloat;
end;
finally
Free;
end;
end;
procedure TComplexVariantType.StreamOut(const Source: TVarData; const Stream: TStream);
begin
with TWriter.Create(Stream, 1024) do
try
with TComplexVarData(Source).VComplex do
begin
WriteFloat(Real);
WriteFloat(Imaginary);
end;
finally
Free;
end;
end;
Note how these methods create a Reader or Writer object for the Stream parameter to handle the details of reading or writing values.