压缩OLEVARIANT数据

TCLIENTDATASET.DATA, TCLIENTDATASET.DELTA, TDATASETPROVIDER.DATA，它们的DATA属性的类型都是OLEVARIANT。

中间层和客户端之间通过OLEVARIANT来传递数据集的数据。

为了提高数据传输的效率，发送方在传输之前可以先压缩数据，然后把压缩的数据发送给接收方。

接收方收到发送方送来的压缩数据，先进行数据解压。

使用开源的ZLIB进行数据的压缩和解压。XE2已经自带此控件，需要引用system.zlib单元。

原理：将OLEVARIANT数据流化，然后对流进行压缩，还原成OLEVARIANT以后再发送。

procedure StreamToVariant(Stream: TStream; var V: OLEVariant);
var
  P : Pointer;
begin
  try
    V := VarArrayCreate ([0, Stream.Size - 1], varByte);
    P := VarArrayLock (V);
    Stream.Position := 0;
    Stream.Read (P^, Stream.Size);
    VarArrayUnlock (V);
  except
    Exit;
  end;
end;

procedure VariantToStream(const V: OLEVariant; Stream: TStream);
var
  P: Pointer;
begin
  try
    Stream.Position := 0;
    Stream.Size := VarArrayHighBound (V, 1) - VarArrayLowBound (V, 1) + 1;
    P := VarArrayLock (V);
    Stream.Write (P^, Stream.Size);
    VarArrayUnlock (V);
    Stream.Position := 0;
  except
    Exit;
  end;
end;

function CompressData(V: OleVariant): OleVariant;
var
  M, M0: TMemoryStream;
begin
  try
    M := TMemoryStream.Create;
    M0 := TMemoryStream.Create;
    try
      if V = Null then exit;
      VariantToStream(V,M);
      M.Position := 0;
      ZCompressStream(M, M0);
      StreamToVariant(M0, V);
    finally
      M.Free;
      M0.Free
    end;
    Result := V;
  except
    Exit;
  end;
end;

function DeCompressData(V: OleVariant): OleVariant;
var
  M, M0: TMemoryStream;
begin
  try
    M := TMemoryStream.Create;
    M0 := TMemoryStream.Create;
    try
      if V = Null then exit;
      VariantToStream(V,M);
      M.Position := 0;
      ZDeCompressStream(M, M0);
      StreamToVariant(M0, V);      
    finally
      M.Free;
      M0.Free
    end;
    Result := V;
  except
    Exit;
  end;
end;

http://www.cnblogs.com/hnxxcxg/archive/2012/02/26/2368612.html