How to combine reading and writing of properties in one procedure or function to simplify XML serialization

Question

I want to write and read some properties of an object to an XML file.

Therefore I would like to use a procedure which bundles the writing and reading so that I don't have to repeat the list of XML node paths and names (one for writing and one for reading):

type
  TXMLFileIO=class
  public
    FReadFromXMLFile: Boolean; 
    procedure WriteReadInteger(const APathOfTheParentNode: string; const ANodeName: string; var AValue: Integer);
    //other declarations
  end;

procedure TXMLFileIO.WriteReadInteger(const APathOfTheParentNode: string; const ANodeName: string; var AValue: Integer);
begin
  if FReadFromXMLFile then
  begin
    AValue:=GetXMLNodeIntegerValue(APathOfTheParentNode, ANodeName);
  end
  else
  begin
    AddXMLIntegerNode(APathOfTheParentNode, ANodeName, AValue);
  end;
end;

This will not work since I cannot pass a property as var parameter in var AValue: Integer

I could use a function like

function TXMLFileIO.WriteReadInteger(const APathOfTheParentNode: string; const ANodeName: string; const AValue: Integer): Integer;
begin
  if FReadFromXMLFile then
  begin
    Result:=GetXMLNodeIntegerValue(APathOfTheParentNode, ANodeName);
  end
  else
  begin
    AddXMLIntegerNode(APathOfTheParentNode, ANodeName, AValue);
    Result:=AValue;
  end;
end;

and call it like

  MyXMLFileIO.FReadFromXMLFile := true; {or false depending if we want to read or write}
  MyIntegerProperty1 := MyXMLFileIO.WriteReadInteger(Path1, Name1, MyIntegerProperty1);
  MyIntegerProperty2 := MyXMLFileIO.WriteReadInteger(Path2, Name2, MyIntegerProperty2);
  etc.

but this will set the property also when I write (which seems not elegant to me because it might cause problems depending on the setter method of the property).

Is there another better way to do this?

Is `WriteReadInteger` not supposed to be the method of a class? From your code it seems the provided `aValue` acts as a default value. I do not understand where the `ReadFromXLFile` comes from either. — H.Hasenack, Jul 06 '20 at 12:37
Thanks for the feedback. I edited the question. Is it more understandable now? — user3384674, Jul 06 '20 at 13:38
I have the impression you are kind-a looking for `ReadInteger(const aPath,aName:string; const aDefaultValue:integer=0):integer;` which would be a lot simpler to implement and would not require to modify the XML file to get the value. I have a similar solution for getting values from the registry, and therefore also have a `WriteInteger(const aPath,aName:string;const aValue:integer;const aDefaultValue:integer=0):` counterpart that actually removes the value when the provided `aValue` equals the `aDefaultValue`. Keeping read and write separate is usually a good idea. — H.Hasenack, Jul 06 '20 at 14:54
Basicaly reading/writing an property value are two distinct different actions so a such you need to handle them. But when you have access to the object definition itself, you could add ‘attributes’ to your properties to specify you ‘path’ in the xml you need to read/write to and the use rtti to write generic code for reading/writing. But it would be nice to know your actual problem your trying to solve: are you for example trying to create a xml serializer to handle different object or creating an object that internally store it’s data into xml? — R. Hoek, Jul 20 '20 at 20:58

score 0 · Answer 1 · answered Jul 19 '20 at 23:49

Passing a var param in order to retrieve a value could lead to side effects, so not a good idea.

It would be better to create separate read and write for the property.

type
  TXMLFileIO=class
  private
    property Propvalue: integer read ReadInteger write WriteInteger;
  public
    FReadFromXMLFile: Boolean; 
    function ReadInteger(const APathOfTheParentNode: string; const ANodeName: string): integer;
    procedure WriteInteger(const APathOfTheParentNode: string; const ANodeName: string; var AValue: Integer);
    //other declarations
  end;

procedure TXMLFileIO.ReadInteger(const APathOfTheParentNode: string; const ANodeName: string): integer;
begin
  result:=GetXMLNodeIntegerValue(APathOfTheParentNode, ANodeName);
end;

procedure TXMLFileIO.WriteInteger(const APathOfTheParentNode: string; const ANodeName: string; var AValue: Integer);
begin
  AddXMLIntegerNode(APathOfTheParentNode, ANodeName, AValue);
end;

I cant make comments so posting as an answer.

How to combine reading and writing of properties in one procedure or function to simplify XML serialization

1 Answers1