Delphi and JSON serialization with Spring collections

I am trying to include the Spring4D framework in my latest Delphi project. The collection functions for sorting and filtering are the main features.

However, I have great difficulty to serialize a JSON array to an IList<T> where T is a record.

When I use the default generic.collecions it all gets serialized perfectly. I'm using https://github.com/gruco0002/DelphiJSON for the JSON part because it's serializing records and the standard generic lists.

This is working when using the default generic collections:

result.Data := DelphiJSON<T>.Deserialize(response.Content, jsonSetting);

I hope I don't have to revert back to TDataSets. I have tried to instantiate the IList<T> and add the JSON array myself.

The System.JSON.Converters unit contains converters for standard generics - learn it to make your own converters. To convert the root array you must to add your converter to the serializer. To convert fields you can use the RTTI attribute [JsonConverter(<Your_Own_Converter_Class_Name>)] declared in the System.JSON.Serializers unit.

Converter example for IList<T>:

unit Unit1;

interface

uses
  System.SysUtils,
  System.JSON.Serializers,
  System.JSON.Converters,
  System.JSON.Writers,
  System.JSON.Readers,
  Spring.Collections,
  Spring;

type
  TJsonListConverter<V> = class(TJsonConverter)
  protected
    function CreateInstance: IList<V>; virtual;
  public
    procedure WriteJson(const AWriter: TJsonWriter; const AValue: TValue;
      const ASerializer: TJsonSerializer); override;
    function ReadJson(const AReader: TJsonReader; ATypeInf: PTypeInfo;
      const AExistingValue: TValue; const ASerializer: TJsonSerializer): TValue; override;
    function CanConvert(ATypeInf: PTypeInfo): Boolean; override;
  end;

implementation

uses
  System.Rtti,
  System.TypInfo,
  System.JSON.Utils,
  System.JSON.Types,
  System.JSONConsts;

function TJsonListConverter<V>.CanConvert(ATypeInf: PTypeInfo): Boolean;
begin
  Result := TJsonTypeUtils.IsAssignableFrom(ATypeInf, TypeInfo(IList<V>));
end;

function TJsonListConverter<V>.CreateInstance: IList<V>;
begin
  Result := TCollections.CreateList<V>;
end;

function TJsonListConverter<V>.ReadJson(const AReader: TJsonReader;
  ATypeInf: PTypeInfo; const AExistingValue: TValue;
  const ASerializer: TJsonSerializer): TValue;
var
  List: IList<V>;
  Arr: TArray<V>;
begin
  if AReader.TokenType = TJsonToken.Null then
    Result := nil
  else
  begin
    ASerializer.Populate(AReader, Arr);
    if AExistingValue.IsEmpty then
      List := CreateInstance
    else
      List := AExistingValue.AsType<IList<V>>;
    List.AddRange(Arr);
    Result := TValue.From(List);
  end;
end;

procedure TJsonListConverter<V>.WriteJson(const AWriter: TJsonWriter;
  const AValue: TValue; const ASerializer: TJsonSerializer);
var
  List: IList<V>;
begin
  if AValue.TryAsType(List) then
    ASerializer.Serialize(AWriter, List.ToArray)
  else
    raise EJsonException.Create(
      Format(SConverterNotMatchingType,
      [AValue.TypeInfo^.Name, PTypeInfo(TypeInfo(IList<V>))^.Name]));
end;

end.

Using:

uses
  System.JSON.Serializers,
  Spring.Collections,
  Spring,
  Unit1 in 'Unit1.pas';

type
  TMyRecord = record
    p1, p2, p3: integer;
  end;

resourcestring
  MyJson =
    '[{"p1":1,"p2":2,"p3":3},{"p1":4,"p2":5,"p3":6},{"p1":7,"p2":8,"p3":9}]';

begin
  // create a serializer
  var Serializer :=
    Shared.Make<TJsonSerializer>(
      TJsonSerializer.Create,
      procedure(const arg: TJsonSerializer)
      begin
        for var Converter in arg.Converters do Converter.Free;
      end
    );

  // add converter
  Serializer.Converters.Add(TJsonListConverter<TMyRecord>.Create);

  // deserialize
  var MyArray := Serializer.Deserialize<IList<TMyRecord>>(MyJson);

  // serialize
  Writeln(Serializer.Serialize(MyArray));

  Readln;
end.