DDS笔记: hello world解析

fast dds 2.2.x中hello world代码总结：

1） 核心逻辑流程

获取DomainParticipantFactory -> 创建participant -> 注册sub-pub type -> 创建topic（顺序可延后） -> 创建publisher / subscriber -> 创建writer / reader -> write / read / WaitSet.wait

2）除获取factory外，创建其他对象之前都要准备好对应qos、listener、mask等参数。

3）subscriber有2种模式：

（1）listener callback

创建subscriber时传入listener参数，通过listener回调事件和数据（on_subscription_matched, on_data_available）。

（2）wait set

创建subscriber后，通过WaitSet来关联多个Condition，监听相应各个Condition的变化。

关联reader的status condition，监听topic的订阅和数据更新

关联stop condition，监听程序停止动作。

1. publisher app主要流程
   1）创建factory
   2）创建participant
   3）创建publisher
   4）创建topic
   5）创建writer
   6）发布数据

	// 1
	auto factory = DomainParticipantFactory::get_instance();
	// 2 listener null, mask none
	auto participant = factory->create_participant_with_default_profile(null_listener, mask);
	assert(participant);
	// 注册关联pub-sub-type
	pub_sub_type.register_type(participant);

	// 3 listener null, mask none
	auto publisher = pariticipant->create_publisher(pub_qos, null_listener, mask);
	assert(publisher);

	// 4
	auto topic = participant->create_topic(topic_name, topic_type, topic_qos);
	assert(topic);
	
	// 5 listener, mask all
	auto writer = publisher->create_datawriter(topic, writer_qos, listener, mask);
	assert(writer);

	// 6
	Helloworld hello{...};
	writer->write(&hello);

3. ListenerSubscriberApp主要流程
   1）创建participant factory
   2）创建participant
   3）创建subscriber
   4）创建topic
   5）创建data reader
   6）data callback

	// 1
	auto factory = DomainParticipantFactory::get_instance();

	// 2 listener null, mask none
	auto participant = factory->create_participant_with_default_profile(null_listener, mask);
	assert(participant);
	pub_sub_type.register_type(participant);

	// 3 listener null, mask none
	auto subscriber = participant->create_subscriber(sub_qos, null_listener, mask);
	assert(subscriber);

	// 4
	auto topic = participant->create_topic(topic_name, topic_type, topic_qos);
	assert(topic);

	// 5 listener, mask all
	auto reader = subscriber->create_datareader(topic, reader_qos, listener, mask);
	assert(reader);

	// 6 data callback
	class DataReaderListener {
		void on_data_available(DataReader *reader); // <<<---
		void on_subscription_matched(DataReader *reader, status); // <<<---
		void on_requested_deadline_missed(DataReader *reader, status);
		void on_liveness_changed(DataReader *reader, status);
		void on_sample_rejected(DataReader *reader, status);
		void on_sample_lost(DataReader *reader, status);
		void on_requested_incompatible_qos(DataReader *reader, status);
	};
	
	void on_subscription_matched(DataReader *reader, SubscriptionMatchedStatus& status) {
		// check matched count
	}

	void on_data_available(Datareader *reader) {
		// read and process data
		Helloworld hello;
		SampleInfo info;
		reader->take_next_sample(&hello, &info);
	}

3. WaitsetSubscriberApp main process
   1）创建factory
   2）创建participant
   3）创建subscriber
   4）创建topic
   5）创建data reader
   6）WaitSet attach Condition
   7）WaitSet wait Condition

	// 1
	auto factory = DomainParticipantFactory::get_instance();
	
	// 2 listener null, mask none
	auto participant = factory->create_participant_with_default_profile(null_listener, mask);
	assert(participant);	
	pub_sub_type.register_type(participant);

	// 3 listener null, mask none
	auto subscriber = participant->create_subscriber(sub_qos, null_listener, mask)
	assert(subscriber);
	
	// 4
	auto topic = participant->create_topic(topic_name, topic_type, topic_qos);
	assert(topic);

	// 5 null listener, mask all
	auto reader = subscriber->create_datareader(topic, sub_qos, null_listener, mask);
	assert(reader);

	// 6
//	.hpp: 	WaitSet wait_setp;
//			GuardCondtion terminate_condition;

	// ---->>> ***************
	// 这里是关键，把wait和reader的status condition关联起来，监听reader的变化
	// 同时处理terminate_condition，监听stop事件
	wait_set.attach_condition(reader->get_statuscondition());
	wait_set.attach_condition(terminate_condition);

	// 7
	ConditionSeq triggered_conditions;
	wait_set.wait(triggered_conditions, timeout);
	for(Condition* cond : triggered_conditions) {
		auto *status_cond = dynamic_cast<StatusCondition>(cond);
		assert(status_cond);

		Entity *entity = status_cond.get_entity();
		StatusMask changed_status = entity->get_status_changes();

		if (changed_status.is_active(StatusMask::subscriptioin_matched()) {
			// check matched count
		}

		if (changed_status.is_active(StatusMask::data_available())) {
			// read and process data
			SampleInfo info;
			Helloworld hello;
			reader->take_next_sample(&hello, &info);
		}
	}