Receiving Unfiltered Messages¶

Backendless Real-Time Messaging delivers messages to listeners added to a channel. A listener is a callback function which is invoked by Backendless to deliver a message to the client application. Depending on the API used to add a listener, it can receive either all messages published into the channel or a filtered subset of messages. The filtering options are explained in greater detail in the Receiving Filtered Messages section of this guide.

Receiving String Messages¶

Adding a listener to receive messages as string objects. Messages should be published as string objects):

Channel channel = await Backendless.messaging.subscribe("MyChannel");

channel.addMessageListener((String stringMessage) {
  print("Received string message: $stringMessage");
});

Receiving Dictionary/Map messages¶

Adding a listener to receive messages as dictionary/map objects. Messages can be published as JSON objects, complex types or disctionary/map objects:

Channel channel = await Backendless.messaging.subscribe("MyChannel");

channel.addMessageListener((Map message) {
  print("Received message as a map: $message");
});

Receiving custom type/class messages¶

Adding a listener to receive messages as instances of a custom type (the Person class). Published messages must be instances of the class, map/dictionary objects or JSON objects with the same structure as the class:

Channel channel = await Backendless.messaging.subscribe("MyChannel");

channel.addMessageListener((Person message) {
  print("Received message as a Person object: $message");
});

General signature¶

A general signature for receiving messages as instances of a custom type:

// channel object must be obtained via a subscription call, for example:
// Channel channel = await Backendless.messaging.subscribe("MyChannel");
channel.addMessageListener<T>(void callback(T response));

Receiving messages in Backendless internal format¶

Adding a listener to receive message in the Backendless internal format:

Channel channel = await Backendless.messaging.subscribe("MyChannel");

channel.addMessageListener((PublishMessageInfo message) {
    print("Published message - ${message.message}");
    print("Publisher ID - ${message.publisherId}");
    print("Message headers - ${message.headers}");
    print("Message subtopic ${message.subtopic}");
});