Using TJSONEventReader for event-based JSON parsing

TJSONEventReader provides event-based JSON parsing similar to SAX parsing for XML. As the parser encounters different JSON elements, it fires corresponding events that you can handle to process the data.

This approach is memory-efficient for large JSON documents since it doesn't load the entire structure into memory. You only need to handle the events for the data you're interested in processing.

Basic usage involves:

1. Create a TJSONEventReader instance with JSON source
2. Assign event handlers for the JSON elements you want to process
3. Call Execute to start parsing

Here's a complete example that parses JSON and extracts specific values:

program JSONEventReaderDemo;

{$mode objfpc}{$H+}

uses
  Classes, SysUtils, jsonreader;

type
  TMyJSONProcessor = class
  private
    FPersonName: string;
    FPersonAge: Integer;
    FInAddress: Boolean;
    FCurrentKey: string;
  public
    procedure OnKeyName(Sender: TObject; const AKey: UTF8String);
    procedure OnStringValue(Sender: TObject; const AValue: UTF8String);
    procedure OnIntegerValue(Sender: TObject; const AValue: Integer);
    procedure OnStartObject(Sender: TObject);
    procedure OnEndObject(Sender: TObject);

    property PersonName: string read FPersonName;
    property PersonAge: Integer read FPersonAge;
  end;

procedure TMyJSONProcessor.OnKeyName(Sender: TObject; const AKey: UTF8String);
begin
  FCurrentKey := AKey;
  FInAddress := (AKey = 'address');
end;

procedure TMyJSONProcessor.OnStringValue(Sender: TObject; const AValue: UTF8String);
begin
  if not FInAddress and (FCurrentKey = 'name') then
    FPersonName := AValue;
end;

procedure TMyJSONProcessor.OnIntegerValue(Sender: TObject; const AValue: Integer);
begin
  if not FInAddress and (FCurrentKey = 'age') then
    FPersonAge := AValue;
end;

procedure TMyJSONProcessor.OnStartObject(Sender: TObject);
begin
  // Object started
end;

procedure TMyJSONProcessor.OnEndObject(Sender: TObject);
begin
  FInAddress := False;
end;

var
  Reader: TJSONEventReader;
  Processor: TMyJSONProcessor;
  JSONData: string;
begin
  JSONData := '{"name": "Alice Smith", "age": 28, "address": {"street": "123 Oak St", "city": "Springfield"}}';

  Processor := TMyJSONProcessor.Create;
  Reader := TJSONEventReader.Create(JSONData, []);
  try
    // Assign event handlers
    Reader.OnKeyName := @Processor.OnKeyName;
    Reader.OnStringValue := @Processor.OnStringValue;
    Reader.OnIntegerValue := @Processor.OnIntegerValue;
    Reader.OnStartObject := @Processor.OnStartObject;
    Reader.OnEndObject := @Processor.OnEndObject;

    // Parse the JSON
    Reader.Execute;

    // Display results
    Writeln('Person Name: ', Processor.PersonName);
    Writeln('Person Age: ', Processor.PersonAge);

  finally
    Reader.Free;
    Processor.Free;
  end;
end.

The event-based approach is ideal when you need to:

• Process large JSON files without loading them entirely into memory
• Extract specific values from complex JSON structures
• Transform JSON data during parsing
• Handle JSON streams in real-time