Fast DDS之Subscriber

Subscriber扮演容器的角色，里面可以有很多DataReaders，它们使用Subscriber的同一份SubscriberQos配置。Subscriber可以承载不同Topic和数据类型的DataReader对象。

Subscriber

SubscriberQos

默认Qos配置可以通过DomainParticipant实例的get_default_subscriber_qos()函数获取。

SubscriberListener

Subscriber的状态改变会触发SubscriberListener的回调函数调用。用户可以通过继承实现自定义回调。新增callback：on_data_on_readers()

创建Subscriber

通过DomainParticipant的create_subscriber()函数创建，SubscriberQos参数是必需的，SubscriberListener和StatusMask参数可选。

// 创建DomainParticipant实例participant
Subscriber* subscriber_with_default_qos = participant->create_subscriber(SUBSCRIBER_QOS_DEFAULT);
if (nullptr == subscriber_with_default_qos) {
    // Error
    return;
}

基于Profile创建Subscriber：用于标识subscriber的字符串名字参数是必需的，listener和StatusMask是可选的

Subscriber* subscriber_with_profile = participant->create_subscriber_with_profile("subscriber_profile");
if (nullptr == subscriber_with_profile) {
    // Error
    return;
}

删除Subscriber：需要先删除Subscriber中的所有实体（DataReaders），再调用 delete_subscriber() 删除Subscriber

// Delete the entities the subscriber created
if (subscriber->delete_contained_entities() != ReturnCode_t::RETCODE_OK) {
    // Subscriber failed to delete the entities it created
    return;
}
// Delete the Subscriber
if (participant->delete_subscriber(subscriber) != ReturnCode_t::RETCODE_OK) {
    // Error
    return;
}

DataReader

DataReader只归属于创建它的Subscriber，每个DataReader在它创建支持绑定到单一的Topic上，所以Topic必须是在DataReader创建之前就已经创建。可以通过DataReader::read_next_sample()或DataReader::take_next_sample()函数获取pub端写入的数据。
DataReaderQos
DataReaderListener用于监听DataReader状态的改变。有以下callback成员函数：

• on_data_available
• on_subscription_matched
  创建一个DataReader：绑定到要传输数据的Topic和DataReaderQos是必需的参数，DataReaderListener和StatusMask是可选的

DataReader* data_reader_with_default_qos = subscriber->create_datareader(topic, DATAREADER_QOS_DEFAULT);
if (nullptr == data_reader_with_default_qos) {
    // Error
    return;
}

基于Profile创建DataReader
使用自定义的PayloadPool创建DataReader（为什么要使用自定义的PayloadPool？）

// A DataReaderQos must be provided to the creation method
DataReaderQos qos;

// Create PayloadPool
std::shared_ptr<CustomPayloadPool> payload_pool = std::make_shared<CustomPayloadPool>();
DataReader* data_reader = subscriber->create_datareader(topic, qos, nullptr, StatusMask::all(), payload_pool);
if (nullptr == data_reader) {
    // Error
    return;
}

删除DataReader
同理，删除DataReader之前需要删除所有属于DataReader的实体（QueryConditions）

// Delete the entities the DataReader created
if (data_reader->delete_contained_entities() != ReturnCode_t::RETCODE_OK) {
    // DataReader failed to delete the entities it created.
    return;
}

// Delete the DataReader
if (subscriber->delete_datareader(data_reader) != ReturnCode_t::RETCODE_OK) {
    // Error
    return;
}

SampleInfo

一个重要的数据结构，为每个DataReader提供了相关数据的元数据信息，包括：

• 数据样本的状态，如是否被修改或是否被读走
• 数据样本的来源，如发布者的实例句柄或公开的实例句柄
• 数据样本的序列号，这对确保数据样本的顺序接收非常重要
• 数据样本的时间戳，即样本何时被写入或修改
• 数据样本是否有效，无效的数据样本通常表示实例的生命周期已经结束

读取数据

通过reading或taking接收和消费DataReader读到的数据。reading和taking的这些函数实现都是一样的：

• DataReader::read_next_sample / DataReader::take_next_sample
• DataReader::read(), DataReader::read_instance(), DataReader::read_next_instance() / DataReader::take(), DataReader::take_instance(), DataReader::take_next_instance()：获取符合特定条件的samples的集合

通过callback获取数据

两个Listener的callbacks，可以自定义listener继承自DataReaderListener：

• on_data_available()
• on_data_readers()

通过启动一个等待线程处理

1. 异步

// Create a DataReader
DataReader* data_reader =
        subscriber->create_datareader(topic, DATAREADER_QOS_DEFAULT);
if (nullptr == data_reader)
{
    // Error
    return;
}

// Prepare a wait-set to wait for data on the DataReader
WaitSet wait_set;
StatusCondition& condition = data_reader->get_statuscondition();
condition.set_enabled_statuses(StatusMask::data_available());
wait_set.attach_condition(condition);

// Create a data and SampleInfo instance
Foo data;
SampleInfo info;

//Define a timeout of 5 seconds
eprosima::fastrtps::Duration_t timeout (5, 0);

// Loop reading data as it arrives
// This will make the current thread to be dedicated exclusively to
// waiting and reading data until the remote DataWriter dies
while (true) {
    ConditionSeq active_conditions;
    if (ReturnCode_t::RETCODE_OK == wait_set.wait(active_conditions, timeout)) {
        while (ReturnCode_t::RETCODE_OK == data_reader->take_next_sample(&data, &info)) {
            if (info.valid_data) {
                // Do something with the data
                std::cout << "Received new data value for topic "
                          << topic->get_name() << std::endl;
            } else {
                // If the remote writer is not alive, we exit the reading loop
                std::cout << "Remote writer for topic "
                          << topic->get_name() << " is dead" << std::endl;
                break;
            }
        }
    } else {
        std::cout << "No data this time" << std::endl;
    }
}

2. 同步，通过DataReader::wait_for_unread_message() 函数等待直到数据到达或者时间超时

// Create a DataReader
DataReader* data_reader = subscriber->create_datareader(topic, DATAREADER_QOS_DEFAULT);
if (nullptr == data_reader) {
    // Error
    return;
}
// Create a data and SampleInfo instance
Foo data;
SampleInfo info;
//Define a timeout of 5 seconds
eprosima::fastrtps::Duration_t timeout (5, 0);

// Loop reading data as it arrives
// This will make the current thread to be dedicated exclusively to
// waiting and reading data until the remote DataWriter dies
while (true) {
    if (data_reader->wait_for_unread_message(timeout)) {
        if (ReturnCode_t::RETCODE_OK == data_reader->take_next_sample(&data, &info)) {
            if (info.valid_data) {
                // Do something with the data
                std::cout << "Received new data value for topic "
                          << topic->get_name() << std::endl;
            } else {
                // If the remote writer is not alive, we exit the reading loop
                std::cout << "Remote writer for topic "
                          << topic->get_name() << " is dead" << std::endl;
                break;
            }
        }
    } else {
        std::cout << "No data this time" << std::endl;
    }
}