Android Question Sending Text via MQTT - [SOLVED SORT OF]

JMB · Feb 6, 2021

Hi there... I have a piece of code that transmits a string via MQTT:

B4X:

    Dim m As String = "Test Message"
    mqtt.Publish("TextComment", Serializator.ConvertObjectToBytes(m))

And I have a piece of code that responds to this:

B4X:

Private Sub mqtt_MessageArrived (Topic As String, Payload() As Byte)
    Log("Message arrived: " & Topic)
    Log(BytesToString(Payload, 0, Payload.Length, "utf8"))
End Sub

Problem is, I don't get "Test message" back - I get a load of characters. How do I use the B4XSerializator to send a string and then how do I decode it?

Thank you.

JMB

Peter Lewis · Feb 6, 2021

This should work

B4X:

Send
mqttObj.Publish("chat",raf.ConvertObjectToBytes(sendmessage))


Receive

Dim m As string = raf.ConvertBytesToObject(Payload)

JMB · Feb 6, 2021

I love this forum!

Thank you Peter, for such a quick and help response.

JMB

JMB · Feb 6, 2021

Hmmm.

How would I receive this message in B4R? I have tried converting it using this:

B4X:

    Dim obj As Object = Serializator.ConvertBytesToObject(Payload)
    Dim s As String = obj
    Log(s)

where Serializator is a B4RSerializator, but it doesn't work...

In fact, I am a bit confused as to how I would SEND a text message through MQTT from B4R to B4A... I keep getting strings of characters.

Confused.

JMB

OliverA · Feb 7, 2021

Don't serialize (In B4J/A/i). Use a String's GetBytes method to turn the string into bytes. Then
1) In your first example, BytesToString should work
2) In B4R, just keep it as a byte array (from what I can tell, Strings in B4R are just byte arrays. I'm going from my interpretation of this: https://www.b4x.com/android/forum/threads/strings-and-bytes.66729/)
Note: Looking at the link in #2, looks like unicode is not really supported. So you may consider sticking to ASCII when using GetBytes when creating the byte arrays for B4R and for BytesToString when receiving strings from B4R.
Disclaimer: I have 0 experience with B4R, so all this may just be bah humbug

JMB · Feb 7, 2021

I have figured this out - or at least here is one way to make it work...

To send from B4A in a format that can be read by B4R:

Code:

mqtt.Publish("4RELAYBOARD", serializator.ConvertArrayToBytes(Array("TEST MESSAGE")))

To decode this message in B4R use:

Code:

Private Sub mqtt_MessageArrived (Topic As String, Payload() As Byte)

    Dim be(20) As Object 'used as a storage buffer. Can be a global variable
  
    'A Message has been received from the MQTT server
    Log("Message Received!!")
  
    Dim MQTTMessagePayload() As Object = Serializator.ConvertBytesToArray(Payload, be)
    Dim Message As String = MQTTMessagePayload(0)

    Log("MQTT message is: ", Message)
  
End Sub

To send something back from B4R to B4A use:

Code:

mqttClient.Publish("THETOPIC", Serializator.ConvertArrayToBytes(Array("My Message")))

JMB · Feb 7, 2021

I have just read OliverA's comments - I shall have a go with GetBytes in B4A...

UPDATE: that doesn't appear to work, but thank you for the suggestion.

Android Question Sending Text via MQTT - [SOLVED SORT OF]

JMB

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