Delphi MQTT Client — Connect to Any MQTT 3.1 / 5.0 Broker

uses
  sgcWebSocket, sgcWebSocket_Classes, sgcWebSocket_Types, sgcWebSocket_Protocols,
  sgcWebSocket_Protocol_MQTT_Client, sgcWebSocket_Protocol_MQTT_Message;

var
  WSClient: TsgcWebSocketClient;
  MQTT: TsgcWSPClient_MQTT;
begin
  WSClient := TsgcWebSocketClient.Create(nil);
  WSClient.Specifications.RFC6455 := False; // plain MQTT over TCP
  WSClient.Host := 'broker.hivemq.com';
  WSClient.Port := 1883;
  WSClient.WatchDog.Enabled := True;

  MQTT := TsgcWSPClient_MQTT.Create(nil);
  MQTT.Client  := WSClient;
  MQTT.MQTTVersion := mqtt5;
  MQTT.LastWillTestament.Enabled := True;
  MQTT.LastWillTestament.Topic   := 'devices/sensor-01/status';
  MQTT.LastWillTestament.Text    := 'offline';
  MQTT.LastWillTestament.QoS     := mtqsAtLeastOnce;
  MQTT.LastWillTestament.Retain  := True;

  MQTT.OnMQTTConnect := MQTTConnect;
  MQTT.OnMQTTPublish := MQTTPublish;

  WSClient.Active := True;
end;

procedure TForm1.MQTTConnect(Connection: TsgcWSConnection;
  const Session: Boolean; const ReasonCode: Integer;
  const ReasonName: string;
  const ConnectProperties: TsgcWSMQTTCONNACKProperties);
begin
  MQTT.Subscribe('sensors/+/temperature', mtqsAtLeastOnce);

  // Retained “online” message
  MQTT.Publish('devices/sensor-01/status',
    'online', mtqsAtLeastOnce, True);
end;

procedure TForm1.MQTTPublish(Connection: TsgcWSConnection;
  aTopic, aText: string;
  PublishProperties: TsgcWSMQTTPublishProperties);
begin
  Memo1.Lines.Add(aTopic + ' = ' + aText);
end;