Allow users to specify PubSub's subscriber callback about spring-cloud-gcp HOT 2 CLOSED

That's not correct.
The PubSubInboundChannelAdapter is an start of the integration flow, which is based on the message channel communication. So, letting end-user to inject his own callback eliminates the PubSubInboundChannelAdapter purpose.

The current solution is absolutely right where we receive a PubSub message, convert it into the SI Message and send to the configured channel. That is how SI works.

What you suggest makes sense outside of the SI. That is what we call MessageListenerContainer. The good sample of that is Spring Kafka: https://github.com/spring-projects/spring-kafka/tree/master/spring-kafka/src/main/java/org/springframework/kafka/listener
and the Inbound Channel Adapter for it: https://github.com/spring-projects/spring-integration-kafka/blob/master/src/main/java/org/springframework/integration/kafka/inbound/KafkaMessageDrivenChannelAdapter.java

I find this PubSub Subscriber as pretty straightforward API that we even don't need that MessageListenerContainer (at least for now).
The good sample for similar approach is MqttPahoMessageDrivenChannelAdapter: https://github.com/spring-projects/spring-integration/blob/master/spring-integration-mqtt/src/main/java/org/springframework/integration/mqtt/inbound/MqttPahoMessageDrivenChannelAdapter.java. Where we don't have any containers and just subscribe to the topics (this.client.subscribe(topics, grantedQos);) and register ourselves as callback - this.client.setCallback(this);.

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That's great! Yes, that makes sense.

I filed this issue after a conversation with @viniciusccarvalho yesterday, but I might have failed to understand what was actually being talked.
We spoke about this in the scope of the auto/manual PubsubMessage ack/nack. However, it seems like this isn't a good way of getting there.

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