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Using ICS v9.3

1. Add IcsMQTTClient component

** if not receiving msg from any subscribe topic, set SSLCLISECURITY to sslclisecnone and AUTOSUBSCRIBE to TRUE

To connect:

  MQTTClient.Host := 'hostname';
  MQTTClient.Port := 1883;
  MQTTClient.Activate (true);

To subscribe:

  mqttclient.Subscribe('topic',qtAT_LEAST_ONCE);

*note, DO NOT subscribe immediately after activate. Give some time first, if want to do almost immediate add this setting in mqttclient:

  mqttclient.AutoSubscribe := true;

To Publish:

   MQTTClient.Publish('topic', msgstring , qtAT_LEAST_ONCE, true);

Taken from ICS demo, to show some info on the connection:

procedure TForm1.btnshowClick(Sender: TObject);
var
  j : integer;
  x : cardinal;
begin
  mmo1.Lines.Add ('');
  mmo1.Lines.Add('------ Client ' + string (MQTTClient.Parser.ClientID) + ' -------');
  mmo1.Lines.Add (format ('Username "%s" Password "%s"', [MQTTClient.Parser.Username, MQTTClient.Parser.Password]));
  mmo1.Lines.Add (format ('Keep Alive "%d" Retry Time "%d" Max Retries "%d"', [MQTTClient.Parser.KeepAlive, MQTTClient.Parser.RetryTime, MQTTClient.Parser.MaxRetries]));
  mmo1.Lines.Add (format ('Will Topic "%s" Message "%s" @ %s', [MQTTClient.Parser.WillTopic, MQTTClient.Parser.WillMessage, QosNames [MQTTClient.Parser.WillQos]]));
  mmo1.Lines.Add ('Subscriptions ----');
  for j := 0 to MQTTClient.Subscriptions.Count - 1 do
    begin
      x := cardinal (MQTTClient.Subscriptions.Objects[j]) and $03;
      if (cardinal (MQTTClient.Subscriptions.Objects[j]) shr 8) and $ff = $ff then
        mmo1.Lines.Add ('  "' + MQTTClient.Subscriptions[j] + '" @ ' + QOSNames[TMQTTQOSType (x)] + ' Acked.')
      else
        mmo1.Lines.Add ('  "' + MQTTClient.Subscriptions[j] + '" @ ' + QOSNames[TMQTTQOSType (x)]);
    end;
end;

Below is fully working sample:

unit main;
interface
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, OverbyteIcsMQTT, OverbyteIcsWndControl,
  OverbyteIcsSslX509Certs,McJSON;
type
  Tiotmqttsql = class(TForm)
    mmo: TMemo;
    mqttclient: TIcsMQTTClient;
    edtpublish: TEdit;
    btnpublish: TButton;
    slx509crts1: TSslX509Certs;
    icsmqtclnt1: TIcsMQTTClient;
    procedure FormShow(Sender: TObject);
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
    procedure mqttclientMsg(Sender: TObject; aTopic: UTF8String; const aMessage: String; aQos: TMQTTQOSType; aRetained: Boolean);
    procedure btnpublishClick(Sender: TObject);
    procedure mqttclientOnline(Sender: TObject);
    procedure mqttclientOffline(Sender: TObject; Graceful: Boolean);
    procedure mqttclientClientID(Sender: TObject;
      var aClientID: UTF8String);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  iotmqttsql: Tiotmqttsql;

const
   mqtttopic : string = 'topicname';
   mqtthost  : string = 'sample.com';

implementation

{$R *.dfm}

procedure Tiotmqttsql.FormShow(Sender: TObject);
begin
   mmo.lines.clear;
   mqttclient.AutoSubscribe := true;
   mqttclient.host := mqtthost;
   mqttclient.Port := 1883;
   mqttclient.KeepAlive := 10;
   mqttclient.Activate(true);
   mqttclient.Subscribe(mqtttopic,qtAT_LEAST_ONCE);
end;

procedure Tiotmqttsql.FormClose(Sender: TObject; var Action: TCloseAction);
begin
   mqttclient.Destroy;
end;

procedure Tiotmqttsql.mqttclientMsg(Sender: TObject; aTopic: UTF8String; const aMessage: String; aQos: MQTTQOSType; aRetained: Boolean);
begin
   mmo.Lines.Add(FormatDateTime('yyyy-MM-dd',now())+': '+aMessage+#13#10);
end;

procedure Tiotmqttsql.btnpublishClick(Sender: TObject);
begin
   mqttclient.Publish(mqtttopic,edtpublish.Text,qtAT_LEAST_ONCE,true);
end;

procedure Tiotmqttsql.mqttclientOnline(Sender: TObject);
begin
//   mmo.Lines.add('Online');
end;

procedure Tiotmqttsql.mqttclientOffline(Sender: TObject;  Graceful: Boolean);
begin
//   mmo.Lines.add('Offline');
end;

procedure Tiotmqttsql.mqttclientClientID(Sender: TObject; var aClientID: UTF8String);
begin
   mmo.Lines.add('Client ID: '+aClientID);
end;

end.