Delphi properties with single getter and setter

Question

I'm trying to implement a configuration file class wrapper and would be easier using a single function to get and a single function to set values to properties.

The code bellow is a minimum version of what I'm trying to achieve.

Any help will be welcome.

unit Config;

interface

uses Rtti;

type
  Group = class(TCustomAttribute)
  strict private
    FName: string;

  public
    constructor Create(const Name: string);

    property Name: string read FName;
  end;

  IConfig = class
  protected
    function GetString: string;
    procedure SetString(const Value: string);
  end;

  TConfig = class(IConfig)
  public
    [Group('Person')]
    property Name: string read GetString write SetString;
    [Group('Person')]
    property City: string read GetString write SetString;
  end;

implementation

{ Group }

constructor Group.Create(const Name: string);
begin
  FName := Name;
end;

{ IConfig }

function IConfig.GetString: string;
begin
  // Here I would need the following from the property that call this function:
  // * Property name
  // * Property attribute name

  // This kind of code will not work, because it loop through all available properties
  (*
    var
      ctx: TRttiContext;
      objType: TRttiType;
      Prop: TRttiProperty;
    begin
      ctx := TRttiContext.Create;
      objType := ctx.GetType(Obj.ClassInfo);
      for Prop in objType.GetProperties do begin
        if Prop.GetClassType is TClassBase then
          // do something special with base class properties
        else
          // standard functionality on all other properties
      end;
    end;
  *)
end;

procedure IConfig.SetString(const Value: string);
begin
  // Need the same as above
end;

end.

Answer 1

Property getters and setters do not know which property is calling them. The only way for a shared getter/setter to know that is to use the index specifier, eg:

unit Config;

interface

uses Rtti;

type
  Group = class(TCustomAttribute)
  strict private
    FName: string;

  public
    constructor Create(const Name: string);

    property Name: string read FName;
  end;

  IConfig = class
  protected
    function GetString(Index: Integer): string;
    procedure SetString(Index: Integer; const Value: string);
  end;

  TConfig = class(IConfig)
  public
    [Group('Person')]
    property Name: string index 0 read GetString write SetString;
    [Group('Person')]
    property City: string index 1 read GetString write SetString;
  end;

implementation

{ Group }

constructor Group.Create(const Name: string);
begin
  FName := Name;
end;

{ IConfig }

function IConfig.GetString(Index: Integer): string;
begin
  case Index of
    0: begin // Name
    ...
    end;
    1: begin // City
      ...
    end;
    ...
  end;
end;

procedure IConfig.SetString(Index: Integer; const Value: string);
begin
  // same as above
end;

end.

If the getter/setter needs to know the property name, you can use RTTI to find the property that has the corresponding index value, and if found then you would also have access to its attributes, eg:

function GetPropNameAndGroup(Cls: TClass; PropIndex: Integer; var PropName, GroupName: String): Boolean;
var
  Ctx: TRttiContext;
  Prop: TRttiProperty;
  Attr: TCustomAttribute;
begin
  PropName := '';
  GroupName := '';
  Ctx := TRttiContext.Create;
  for Prop in Ctx.GetType(Cls).GetProperties do
  begin
    if (Prop as TRttiInstanceProperty).Index = PropIndex then
    begin
      PropName := Prop.Name;
      for Attr in Prop.GetAttributes do
      begin
        if Attr is Group then
        begin
          GroupName := Group(Attr).Name;
          Break;
        end;
      end;
      Break;
    end;
  end;
  Result := (PropName <> '') and (GroupName <> '');
end;

function IConfig.GetString(Index: Integer): string;
var
  PropName, GroupName: string;
begin
  if GetPropNameAndGroup(ClassType, Index, PropName, GroupName) then
  begin
    //...
  end;
end;

procedure IConfig.SetString(Index: Integer; const Value: string);
var
  PropName, GroupName: string;
begin
  if GetPropNameAndGroup(ClassType, Index, PropName, GroupName) then
  begin
    //...
  end;
end;