Discuss: Make hasMessageAvailable to interface Consumer.

[horizonzy]

Now the method hasMessageAvailable just is announced in ConsumerImpl and MultiConsumerImpl. If we want to check is there message available, we should transfer Consumer to ConsumerImpl or MultiConsumerImpl, then invoke hasMessageAvailable like below:

            if (consumer instanceof ConsumerImpl<RubbishInfo>) {
                return ((ConsumerImpl<RubbishInfo>) consumer).hasMessageAvailable();
            } else if (consumer instanceof MultiTopicsConsumerImpl<RubbishInfo>) {
                return ((MultiTopicsConsumerImpl<RubbishInfo>) consumer).hasMessageAvailable();
            }

Maybe we can make method hasMessageAvailable to Consumer.

[shibd]

What are the scenarios for consumers to use hasMessageAvailable?

[Technoboy-]

What are the scenarios for consumers to use hasMessageAvailable?

I think it's the same with adding getLastMessageId(#1066). BTW, there is a case here : https://github.com/apache/pulsar/pull/1066#discussion_r162751622

[horizonzy]

What are the scenarios for consumers to use hasMessageAvailable?

Sorry for delay reply. Not the method hasMessageAvailable is defined in both ConsumerImpl and MultiConsumerImpl.

https://github.com/apache/pulsar/blob/aabd5d020543210921f10648caf6720adc41d651/pulsar-client/src/main/java/org/apache/pulsar/client/impl/ConsumerImpl.java#L2188

https://github.com/apache/pulsar/blob/aabd5d020543210921f10648caf6720adc41d651/pulsar-client/src/main/java/org/apache/pulsar/client/impl/MultiTopicsConsumerImpl.java#L811

The behavior is same. So I think the method should upper to parent file BaseConsumer or Consumer.

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