Custom converter for TJsonMarshal in Delphi 11

MattBaech
September 26, 2023
115 views
1 vote
2 Answers

I am trying to register a custom converter to avoid the inherited properties in TObjectList – mainly FListHelper and FOwnsObjects. But I cannot get the custom converter to register, and the documentation shows no real examples.

(No this is not a duplicate of: How to hide "ownsObjects" and "listHelper" TObjectList's properties from a Json using Delphi (Rest.JSON)?)

I am trying to get a converter Registered – but it seems to never run.

I have wrapped it in a custom class of mine that looks like this:

TMyJsonConverter = class(TJsonConverter)
  class function JsonConvert(ObjectToConvert:TObject): string;
  private
    type
      TListOfObjectInterceptor = class(TJSONInterceptor)
        function ObjectsConverter(Data: TObject; Field:string): TListOfObjects; override;
    end;
  end;


function TMyJsonConverter.TListOfObjectInterceptor.ObjectsConverter(Data: TObject; Field:string): TListOfObjects;
begin
  raise Exception.Create('converter found');
end;

class function TMyJsonConverter.JsonConvert(ObjectToConvert: TObject): string; 
begin
  var customConverter := TMyJsonConverter.TListOfObjectInterceptor.Create();
  var otherConverter := TMyJsonConverter.Create;
  var marshaller := TJSONMarshal.Create(otherconverter);

  marshaller.RegisterConverter(
    ObjectToConvert.ClassType,
    '*',
    customConverter.ObjectsConverter
  );

  var json := marshaller.Marshal(ObjectToConvert);

  try
    exit(json.ToString);
  finally
    marshaller.Free;
  end;
end;

I have tried to register the types of TObjectConverter, TObjectsConverter, TTypeObjectsConverter but I never seem to get into the conversion function. I can see that the call to register does register the converter, but when I marshal the JSON, it does not find the custom converter again.

Here is a sample structure that highlights the issue, I want to marshal TMySampleDTO as JSON:

type
  TEmployee = class
  public
    Id: Integer;
    Name: string;
  end;
  TEmployeeList = class(TObjectList<TEmployee>);

  TWorktime = class
  public
    EmployeeId: Integer;
    DepartmentId: Integer;
    StartTime: TDateTime;
    StopTime: TDateTime;
  end;
  TWorktimeList = class(TObjectList<TWorktime>);

  TDepartment = class
  public
    Id: Integer;
    Address: string;
    Employees: TEmployeelist;
  end;
  TDepartmentList = class(TObjectList<TDepartment>);

  TMySampleDTO = class
  public
    Departments: TDepartmentList;
    Worktimes: TWorktimeList;
    Employees: TEmployeeList;
  end;

UPDATE: I got the Converter to run, apparently even though Embarcadero defined the const FIELD_ANY as '*', it doesn’t run if you don’t specify the exact fieldname, in my case FListHelper. This raises the next issue though, I also have to give the exact type, as it doesn’t check for inheritance. So if my object structure has properties derived from TObjectList<T> all those lists will be serialized as objects with a list as a property.

Answers

Chosen as BEST ANSWER

Here is something that works - for some reason it doesnt work when the top level object is a list, but I can live with that for now. It uses RTTI and some questionable methods for finding Lists in an object, but it works for me - somewhat.

If ObjectToConvert is a TList<T> descendant, it will still return json as an object with the property listHelper, containing the elements of the list.

But only for the top level object

If anyone can figure out why the converter doesn't work for the top list (outermost json value), feel free to tell me why, and I will add it to solution.

procedure FindObjectListsInHierarchy(obj: TObject; var result: TList<TClass>);

  function IsListDescendant(clazz:TClass) : boolean;
  begin
    result := false;
    while clazz <> nil do
    begin
      if clazz.ClassName.StartsWith('TList<') then
         exit(true);
      clazz := clazz.ClassParent;
    end;
  end;

begin
  if not assigned(result) then
    Result := TList<TClass>.Create;

  var ctx := TRttiContext.Create;
  var objType := ctx.GetType(obj.ClassType);

  for var prop in objType.GetProperties do
  begin
    if not ((prop.Visibility = mvPublic) or (prop.Visibility = mvPublished)) then
      continue;

    if prop.PropertyType.TypeKind in [tkClass, tkDynarray] then
    begin
      if IsListDescendant(TRttiInstanceType(Prop.Parent).MetaclassType) then
      begin
        if not result.Contains(obj.ClassType) then
          Result.Add(obj.ClassType);

        for var item in TList(Obj) do
          FindObjectListsInHierarchy(item, result);
      end;

      if prop.PropertyType.TypeKind = tkClass then
        FindObjectListsInHierarchy(prop.GetValue(obj).AsObject, result);
    end;
  end;
end;

function TMyJsonInterceptor.TypeObjectsConverter(Data: TObject): TListOfObjects;
begin
  var list := TList<TObject>(Data);
  Result := TListOfObjects(list.List);
  SetLength(Result, list.Count);
end;

function TMyJsonConverter.JsonConvert(ObjectToConvert: TObject): string;
begin
  var interCeptor := TMyJsonInterceptor.Create;
  var marshaller := TJSONMarshal.Create;
  try
    var listObjects: TList<TClass> := nil;
    try
      FindObjectListsInHierarchy(ObjectToConvert, listObjects);
      for var o in listObjects do
      begin
        marshaller.RegisterJSONMarshalled(o, 'FOwnsObjects', false);
        marshaller.RegisterConverter(o, interCeptor.TypeObjectsConverter);
      end;
    finally
      listObjects.Free;
    end;

    var json := marshaller.Marshal(ObjectToConvert);
    result := json.ToString;
    json.Free;
  finally
    marshaller.Free;
    interceptor.Free;
  end;
end;

(Edit)

- PeterWolf
- September 25, 2023 at 5:21 pm
- 0 votes
0
The documentation for TTypeMarshaller.RegisterConverter lacks any detailed information leaving you no option but to study Delphi RTL source code. At first look we can divide all overloaded versions of the method into 3 groups:
1. An overload with 3 parameters with last parameter of type TConverterEvent – registers a converter, which performs conversion based on its ConverterType property. This property read-only, but its value is set along with setting any of its conversion delegates (*Converter properties).
2. Overloads with 3 parameters with last parameter of type T...Converter – registers a converter for FieldName (2nd parameter) within class type clazz (1st parameter). Let’s call it a "field converter".
3. Overloads with 2 parameters with last parameter of type TType...Converter – registers a converter for class type clazz (1st parameter). This basically calls the first overload with FieldName set to '*', which is reserved for type converters.
In your sample code you are registering a field converter for field with name * within class type of an instance provided as an argument to your TMyJsonConverter.JsonConvert method. That explains why your conversion routine is not invoked.

Furthermore you create an instance TListOfObjectInterceptor in order to pass a reference to its ObjectsConverter method to TTypeMarshaller.RegisterConverter method. This way you leak this TListOfObjectInterceptor instance. Interceptors should be used in combination with JsonReflect attribute.

The correct way to register a field converter is:
```
function ListFieldConverter(Data: TObject; Field: string): TListOfObjects;
var
  RttiContext: TRttiContext;
  List: TList<TObject>;
begin
  List := TList<TObject>(RttiContext.GetType(Data.ClassInfo).GetField(Field).GetValue(Data).AsObject);
  Result := TListOfObjects(List.List);
  SetLength(Result, List.Count); // makes unique copy
end;

{ ... }

marshaller.RegisterConverter(TMySampleDTO, 'Employees', ListFieldConverter);
marshaller.RegisterConverter(TMySampleDTO, 'Departments', ListFieldConverter);
marshaller.RegisterConverter(TDepartment, 'Employees', ListFieldConverter);
marshaller.RegisterConverter(TMySampleDTO, 'Worktimes', ListFieldConverter);
```
The correct way to register a type converter is:
```
function ListTypeConverter(Data: TObject): TListOfObjects;
var
  List: TList<TObject>;
begin
  List := TList<TObject>(Data);
  Result := TListOfObjects(List.List);
  SetLength(Result, List.Count); // makes unique copy
end;

{ ... }

marshaller.RegisterConverter(TEmployeeList, ListTypeConverter);
marshaller.RegisterConverter(TDepartmentList, ListTypeConverter);
marshaller.RegisterConverter(TWorktimeList, ListTypeConverter);
```
As you can see, this is as much work as using JsonReflect attributes. In theory you could register converters dynamically based on RTTI, but this can be cumbersome due to nested types and inheritance.

To sum that up, even though Delphi JSON library provides ways to hook into the marshalling process, it’s not very flexible and supports only very basic scenarios (see also recent question Spring4D Nullable JSON serialization).

The last thing I’d like to point out is that you should pay more attention to conventions. Fields of records/classes should be prefixed with F by convention. Delphi JSON library honors that and removes those F‘s in process of serialization. Serializing type TData = class Foo: string; end; to JSON yields: {"oo":""}.
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