Print Page - Mosquitto MQTT and Lazarus: how do you receive payload?

Programming => General => Topic started by: Awesome Programmer on January 26, 2018, 06:12:32 pm

Title: Mosquitto MQTT and Lazarus: how do you receive payload?
Post by: Awesome Programmer on January 26, 2018, 06:12:32 pm

:)
I am really liking this MQTT protocol and am looking to use it in my current project. However, I am having an issue.

I wrote a mini test program to test Mosquitto Broker (MQTT). So far, I am able to publish a topic and send its payload and receive the payload on the other end, but I can't seem to read or receive a payload for a topic into my test program. All I can do is SUBSCRIBE to a TOPIC. I thought MQTT broker communicates with publisher and subscribers back and forth or two ways.

Code: Pascal [Select][+]

...
uses MQTT;
...
var
    MQTTClient2:TMQTTClient;
...
 
procedure TForm1.BCButton3Click(Sender: TObject);
begin
  MQTTClient2.Publish('Emergency_Stop','1');
end;
 
procedure TForm1.BCButton4Click(Sender: TObject);
begin
  MQTTClient2.OnPublish:=@myPublish;
  MQTTClient2.Subscribe('Emergency_Stop');
end;
 
procedure TForm1.FormCreate(Sender: TObject);
begin
    MQTTClient2 := TMQTTClient.Create('localhost',1883);
    MQTTClient2.Connect;
end;
 
procedure TForm1.Destroy;
begin
    MQTTClient2.Disconnect;
    MQTTClient2.Free;
end;
 
procedure TForm1.myPublish(sender:TObject; topic, payload:string);
begin
     Memo1.Lines.Add(topic+':'+payload);
end;
 

So, how do you setup your program to receive a topic and a payload from the Mosquitto Broker? I assume you need to have some sort of event for this. Any help will be greatly appreciated.

Title: Re: Mosquitto MQTT and Lazarus: how do you receive payload?
Post by: jamie on January 26, 2018, 11:06:39 pm

I don't know that component but from judging from the names you are using I would say
there must be a "MQTTServer" object in there somewhere?

the client usually connects to a server and if you want to go the other way, you need a server on
your end..

Title: Re: Mosquitto MQTT and Lazarus: how do you receive payload?
Post by: kupferstecher on January 27, 2018, 01:17:39 am

What library did you take? When I looked for MQTT a half year before, there were like three branches of a Lazarus-MQTT-Library (don't remember the name). And all had some different example code. The receival looked a bit complicated, one example used a statemachine to send frequent messages to tell the broker that the client is still there in case there was no recent data transfer. But I didn't follow it any further, so I can't give any example code. So the question also is what the library already covers and what has to be done in user code.

Edit: I was refering to the following code:
https://github.com/ZiCog/mqtt-free-pascal/blob/master/examples/embeddedApp/embeddedApp.pas

Title: Re: Mosquitto MQTT and Lazarus: how do you receive payload?
Post by: Awesome Programmer on February 01, 2018, 04:54:21 pm

:) :) :) :) :D :D :D :D :D
Yahoooooooooooooo!!!!!!!

I just figured out how to receive payloads from MQTT protocol. All the payloads is received within OnPublish event. Anytime any clients publish topics, it is received in your onpublish event. :)

AWESOME!!!!!!!

Thank you fellow programmers. :)

Title: Re: Mosquitto MQTT and Lazarus: how do you receive payload?
Post by: Mi-Ki on October 26, 2020, 07:08:06 am

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