Implementing MQTT Communication with AWS SDK on Android

Communicating with IoT Devices using AWS SDK for Android

The AWS SDK for Android offers a way to communicate with IoT devices through the MQTT protocol. MQTT, a lightweight protocol based on the publish/subscribe model, is implemented in the SDK via the AWSIotMqttManager class.

Subscribing to Topics

The method subscribeToTopic() is used to subscribe to specific topics and register callbacks for published messages. The following Kotlin code illustrates this process:

val topic = "your/topic"
val qos = 0

awsIotMqttManager.subscribeToTopic(topic, qos) { topic, message ->
    // Process the message when it is received
    val msg = String(message.payload, Charset.forName("UTF-8"))
    Log.d("MQTT message received", ": $topic, Message: $msg")
}

Error Handling: Disconnecting from Callback Method

Even when subscription and callbacks are functioning correctly, there may be instances where you need to close the connection due to specific reasons. Attempting to call the disconnect() method of AWSIotMqttManager from within the callback will result in an error:

"Disconnecting from the callback method is not allowed (32107)"

Solving Callback Issues with Kotlin Coroutines on Android

To solve this issue, you can utilize Kotlin's Coroutine feature on Android. Coroutines enable easy execution of asynchronous tasks and facilitate multithreading.

Including Coroutines Library in Your Project

To use coroutines for solving callback issues, first include kotlinx.coroutines library in your project.

Safely Closing Connection Using CoroutineScope Object

Create a CoroutineScope object within your callback and call disconnect() on AWSIotMqttManager within it. This allows you safely close connection from within your callback method and avoid errors.

import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.launch

// ...

// Subscribe to a topic and register a callback using subscribeToTopic()
val topic = "your/topic"
val qos = 0

awsIotMqttManager.subscribeToTopic(topic, qos) { topic, message ->
    // Process the message when it is received
    val msg = String(message.payload, Charset.forName("UTF-8"))
    Log.d("MQTT message received", "Topic: $topic, Message: $msg")

    // Handle connection closure (for example, when a specific message is received)
    CoroutineScope(Dispatchers.Default).launch {
        awsIotMqttManager.disconnect()
    }
}

Conclusion

By configuring your code in this way, you can safely close the connection from within the callback method. This prevents the error "Disconnecting from the callback method is not allowed (32107)" from occurring.