TJsonTextReader - extract object

[msd 5]

Hello, 

 

I found that Delphi has a nice classes for JSON objects TJsonTextReader & TJsonTextWriter and I have one quesion for the TJsonTextReader.

How to extract only search node from bottom JSON sample (or columns node or colors node, I need only one node for processing out of whole JSON)

{
   "columns":[
      {
         "column":"AccountID",
         "caption":"Account",
         "width":"100"
      },
      {
         "column":"Name",
         "caption":"Account Name",
         "width":"250"
      },
      {
         "column":"IsActive",
         "caption":"A",
         "width":"30"
      }
   ],
  "search":[
      {
         "column":"Account",
         "rules":"Asc"
      },
      {
         "column":"Name",
         "rules":"Asc;Part"
      }
   ],
  "colors":[
      {
         "column":"Account",
         "rules":"Red;White"
      },
      {
         "column":"Name",
         "rules":"Blue;Black"
      }
   ]
}

Thanks for the help and assistance in the advance 🙂

[Remy Lebeau 1393]

Using TJSONTextReader, you can parse the document like this:

uses
  ..., System.Generics.Collections, System.JSON.Types, System.JSON.Readers;
  
type
  TColumnNode = record
    Column: string;
    Caption: string;
    Width: string;
  end;

  TSearchNode = record
    Column: string;
    Rules: string;
  end;

  TColorNode = record
    Column: string;
    Rules: string;
  end;

  TReadState = (
    None,
    InTopObject,
    InColumnsProp,
    InColumnsArray,
    InColumnObject,
    InSearchProp,
    InSearchArray,
    InSearchObject,
    InColorsProp,
    InColorsArray,
    InColorObject
  );

var
  JSON: string;
  Columns: TList<TColumnNode>;
  Search: TList<TSearchNode>;
  Colors: TList<TColorNode>;
  ColumnNode: TColumnNode;
  SearchNode: TSearchNode;
  ColorNode: TColorNode;
  TextReader: TTextReader;
  JSONReader: TJSONTextReader;
  State: TReadState;
  PropertyName: string;
begin
  Columns := nil;
  Search := nil;
  Colors := nil;

  JSON := '...';

  try
    Columns := TList<TColumnNode>.Create;
    Search := TList<TSearchNode>.Create;
    Colors := TList<TColorNode>.Create;

    TextReader := TStringReader.Create(JSON);
    try
      JSONReader := TJSONTextReader.Create(TextReader);
      try
        State := TReadState.None;
        while JSONReader.Read do
        begin
          case JSONReader.TokenType of
            TJsonToken.StartObject:
            begin
              case State of
                TReadState.None:
                  State := TReadState.InTopObject;
                TReadState.InColumnsArray:
                begin
                  ColumnNode := Default(TColumnNode);
                  State := TReadState.InColumnObject;
                end;
                TReadState.InSearchArray:
                begin
                  SearchNode := Default(TSearchNode);
                  State := TReadState.InSearchObject;
                end;
                TReadState.InColorsArray:
                begin
                  ColorNode := Default(TColorNode);
                  State := TReadState.InColorObject;
                end;
              end;
            end;
            TJsonToken.EndObject:
            begin
              case State of
                TReadState.InTopObject:
                  State := TReadState.None;
                TReadState.InColumnObject:
                begin
                  Columns.Add(ColumnNode);
                  State := TReadState.InColumnsArray;
                end;
                TReadState.InSearchObject:
                begin
                  Search.Add(SearchNode);
                  State := TReadState.InSearchArray;
                end;
                TReadState.InColorObject:
                begin
                  Colors.Add(ColorNode);
                  State := TReadState.InColorsArray;
                end;
              end;
            end;
            TJsonToken.StartArray:
            begin
              case State of
                TReadState.InColumnsProp:
                  State := TReadState.InColumnsArray;
                TReadState.InSearchProp:
                  State := TReadState.InSearchArray;
                TReadState.InSearchProp:
                  State := TReadState.InColorsArray;
              end;
            end;
            TJsonToken.EndArray:
            begin
              case State of
                TReadState.InColumnsArray,
                TReadState.InSearchArray,
                TReadState.InColorsArray:
                  State := TReadState.InTopObject;
              end;
            end;
            TJsonToken.PropertyName:
            begin
              PropertyName := JSONReader.Value.AsString;
              case State of
                TReadState.InTopObject:
                begin
                  if PropertyName = 'columns' then 
                    State := TReadState.InColumnsProp
                  end
                  else if PropertyName = 'search' then 
                    State := TReadState.InSearchProp
                  end
                  else if PropertyName = 'colors' then 
                    State := TReadState.InColorsProp
                  else
                    JSONReader.Skip;
                end;
                TReadState.InColumnObject:
                begin
                  if PropertyName = 'column' then
                    CurrColumn.Column := JSONReader.ReadAsString
                  else if PropertyName = 'caption' then
                    CurrColumn.Caption := JSONReader.ReadAsString
                  else if PropertyName = 'width' then
                    CurrColumn.Width := JSONReader.ReadAsString
                  else
                    JSONReader.Skip;
                end;
                TReadState.InSearchObject:
                begin
                  if PropertyName = 'column' then
                    CurrSearch.Column := JSONReader.ReadAsString
                  else if PropertyName = 'rules' then
                    CurrSearch.Rules := JSONReader.ReadAsString
                  else
                    JSONReader.Skip;
                end;
                TReadState.InColorObject:
                begin
                  if PropertyName = 'column' then
                    CurrColor.Column := JSONReader.ReadAsString
                  else if PropertyName = 'rules' then
                    CurrColor.Rules := JSONReader.ReadAsString
                  else
                    JSONReader.Skip;
                end;
              else
                JSONReader.Skip;
              end;
            end;
          end;
        end;
      finally
        JSONReader.Free;
      end;
    finally
      TextReader.Free;
    end;

    // use Columns, Search, and Colors as needed...

  finally
    Columns.Free;
    Search.Free;
    Colors.Free;
  end;
end;

Obviously, take out whatever pieces you don't need.

 

But needless to say, using a Reader can keep memory usage to a minimum, but at the cost of complicating the parsing logic.  If your document is not very large, you might consider using a little extra memory in order to utilize the DOM-based framework instead, eg:

uses
  ..., System.Generics.Collections, System.JSON;
  
type
  TColumnNode = record
    Column: string;
    Caption: string;
    Width: string;
  end;

  TSearchNode = record
    Column: string;
    Rules: string;
  end;

  TColorNode = record
    Column: string;
    Rules: string;
  end;

var
  JSON: string;
  Columns: TList<TColumnNode>;
  Search: TList<TSearchNode>;
  Colors: TList<TColorNode>;
  ColumnNode: TColumnNode;
  SearchNode: TSearchNode;
  ColorNode: TColorNode;
  JSONValue: TJSONValue;
  JSONObject: TJSONObject;
  JSONArray: TJSONArray;
  ArrElement: TJSONValue;
begin
  Columns := nil;
  Search := nil;
  Colors := nil;

  JSON := '...';

  try
    Columns := TList<TColumnNode>.Create;
    Search := TList<TSearchNode>.Create;
    Colors := TList<TColorNode>.Create;

    JSONValue := TJSONObject.ParseJSONValue(JSON);
    if JSONValue <> nil then
    try
      JSONObject := JSONValue as TJSONObject;
      
      JSONArray := JSONObject.GetValue('columns') as TJSONArray;
      for ArrElement in JSONArray do
      begin
        with ArrElement as JSONObject do
        begin
          ColumnNode.Column := GetValue('column').Value;
          ColumnNode.Caption := GetValue('caption').Value;
          ColumnNode.Width := GetValue('width').Value;
          Columns.Add(ColumnNode);
        end;
      end;
      
      JSONArray := JSONObject.GetValue('search') as TJSONArray;
      for ArrElement in JSONArray do
      begin
        with ArrElement as JSONObject do
        begin
          SearchNode.Column := GetValue('column').Value;
          SearchNode.Rules := GetValue('rules').Value;
          Search.Add(SearchNode);
        end;
      end;

      JSONArray := JSONObject.GetValue('colors') as TJSONArray;
      for ArrElement in JSONArray do
      begin
        with ArrElement as JSONObject do
        begin
          ColorNode.Column := GetValue('column').Value;
          ColorNode.Rules := GetValue('rules').Value;
          Colors.Add(ColorNode);
        end;
      end;

    finally
      JSONValue.Free;
    end;

    // use Columns, Search, and Colors as needed...

  finally
    Columns.Free;
    Search.Free;
    Colors.Free;
  end;
end;

 

Edited December 24, 2021 by Remy Lebeau

[msd 5]

Hello Remy Lebeau,

 

Two very good ideas :-)

I plan to use JsonReader because of memory usage and it is fresh tech in RAD Studio, but you give me very good tip.

 

Thanks once again for the all advices...