Message Poller

[elia-sysdesign]

Hello,

Sorry for the intrusion.

Just wondering what would be the best way to implement a polling mechanism (after having subscribed to a topic) that would trigger a callback whenever a new message is received.
So far I only found examples that would receive a single message and then disconnect from the broker.
I need to implement an endpoint that would maintain a connection with the broker and that would permanently subscribe to a specific topic and keep receiving messages on said topic.

Thanks in advance.
Elia

[redboltz]

You can call the next recv() to get meeage().
Here is client_cli.cpp code:

async_mqtt/tool/client_cli.cpp

Lines 808 to 851 in 984e81f

	// Recv loop
	while (true) {
	yield ep_.recv(*this);
	if (pv) {
	pv.visit(
	am::overload {
	[&](am::v3_1_1::publish_packet const& p) {
	std::cout << color_recv;
	std::cout << p << std::endl;
	std::cout << " payload:";
	for (auto const& e : p.payload()) {
	std::cout << am::json_like_out(e);
	}
	std::cout << std::endl;
	std::cout << color_none;
	},
	[&](am::v5::publish_packet const& p) {
	std::cout << color_recv;
	std::cout << p << std::endl;
	std::cout << " payload:";
	for (auto const& e : p.payload()) {
	std::cout << am::json_like_out(e);
	}
	std::cout << std::endl;
	std::cout << color_none;
	},
	[](auto const& p) {
	std::cout << color_recv;
	std::cout << p << std::endl;
	std::cout << color_none;
	}
	}
	);
	}
	else {
	std::cout << color_recv;
	std::cout
	<< "recv error:"
	<< pv.get<am::system_error>().what()
	<< std::endl;
	std::cout << color_none;
	return;
	}
	}

It uses stackless coroutine. If you use callback approach, then call recv() the last part of recv() callback instead of the loop.

NOTE: It is not a polling, it is still event driven.

[redboltz]

If you still have the problem, feel free to reopen the issue.

[elia-sysdesign]

Thank you for the quick response.
I will give it a go and let you know.