Fast DDS之Publisher

数据结构

WriterProxyData，包含以下成员：

GUID_t m_guid;
RemoteLocatorList remote_locators_;  //!Holds locator information
InstanceHandle_t m_key;  //!GUID_t of the Writer converted to InstanceHandle_t
InstanceHandle_t m_RTPSParticipantKey;  //!GUID_t of the participant converted to InstanceHandle
string_255 m_typeName;  //!Type name
string_255 m_topicName;  //!Topic name
uint16_t m_userDefinedId;  //!User defined ID
uint32_t m_typeMaxSerialized;  //!Maximum size of the type associated with this Wrtiter, serialized.
TopicKind_t m_topicKind; //!Topic kind
GUID_t persistence_guid_;  //!Persistence GUID
TypeIdV1* m_type_id;  //!Type Identifier
TypeObjectV1* m_type;  //!Type Object
xtypes::TypeInformation* m_type_information;   //!Type Information
ParameterPropertyList_t m_properties;
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Publisher

Publisher是一直归属于DomainParticipant。

1. 通过DomainParticipant的create_publisher创建Publisher，参数PublisherQos是必需的，可以使用默认值PUBLISHER_QOS_DEFAULT。可选的参数：PublisherListener和StatusMask

Publisher* publisher_with_default_qos =
        participant->create_publisher(PUBLISHER_QOS_DEFAULT);
if (nullptr == publisher_with_default_qos) {
    // Error
    return;
}
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2. 基于Profile创建Publisher，参数PublisherQos是必需的，可以使用默认值PUBLISHER_QOS_DEFAULT。可选的参数：PublisherListener和StatusMask

// First load the XML with the profiles
DomainParticipantFactory::get_instance()->load_XML_profiles_file("profiles.xml");
// Create participant
// Create a Publisher using a profile and no Listener
Publisher* publisher_with_profile = participant->create_publisher_with_profile("publisher_profile");
if (nullptr == publisher_with_profile) {
    // Error
    return;
}
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3. 删除Publisher
   删除Publisher前需要先删除所有属于Publisher的实体（DataWriters），可以使用delete_contained_entities()。

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

// Delete the Publisher
if (participant->delete_publisher(publisher) != ReturnCode_t::RETCODE_OK) {
    // Error
    return;
}
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DataWriter

DataWriter归属于Publisher，对DataWriter来说，Publisher就是一个创建工厂。每个DataWriter在创建之处绑定到一个Topic，所以Topic必须先于DataWriter创建。
默认的DataWriterQos可以通过Publisher实例函数get_default_datawriter_qos()获取
DataWriterListener是一个抽象类，用于响应DataWriter的状态发生变化，有以下成员函数用于回调：

• on_publication_matched()
• on_offered_deadline_missed()
• on_offered_incompatible_qos()
• on_liveliness_lost()
• on_unacknowledged_sample_removed()
  创建DataWriter：使用Publisher的函数create_datawriter()函数创建。必选的参数为绑定到要发送数据的Topic和DataWriterQos，可选的参数为继承自DataWriterListener和StatusMask

DataWriter* data_writer_with_default_qos = publisher->create_datawriter(topic, DATAWRITER_QOS_DEFAULT);
if (nullptr == data_writer_with_default_qos) {
    // Error
    return;
}
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通过Profile创建：

// First load the XML with the profiles
DomainParticipantFactory::get_instance()->load_XML_profiles_file("profiles.xml");

// Create a DataWriter using a profile and no Listener
DataWriter* data_writer_with_profile = publisher->create_datawriter_with_profile(topic, "data_writer_profile");
if (nullptr == data_writer_with_profile) {
    // Error
    return;
}
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通过自定义PayloadPool创建DataWriter

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

// Create PayloadPool
std::shared_ptr<eprosima::fastrtps::rtps::IPayloadPool> payload_pool =
        std::dynamic_pointer_cast<eprosima::fastrtps::rtps::IPayloadPool>(std::make_shared<CustomPayloadPool>());

DataWriter* data_writer = publisher->create_datawriter(topic, qos, nullptr, StatusMask::all(), payload_pool);
if (nullptr == data_writer) {
    // Error
    return;
}
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删除DataWriter

// Delete the DataWriter
if (publisher->delete_datawriter(data_writer) != ReturnCode_t::RETCODE_OK) {
    // Error
    return;
}
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发送数据
write()函数有两个参数：指向要发送数值的data实例；instance的handler。如果handle是非空，那么它必须要被TypeSupport的getKey()获取到，否则write函数会失败。

// Publish the new value, deduce the instance handle
if (data_writer->write(data, eprosima::fastrtps::rtps::InstanceHandle_t()) != ReturnCode_t::RETCODE_OK) {
    // Error
    return;
}
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减少发送数据的拷贝：通过loan sample，步骤如下：

1. 通过 loan_sample()函数获取loaned sample的引用
2. 使用上一步获取的引用构建data
3. write发送
   如果loan_sample()被调用但sample未被发送，必须要使用discard_loan()释放。

void* data = nullptr;
if (ReturnCode_t::RETCODE_OK == data_writer->loan_sample(data)) {
    bool error = false;

    // Fill the data values
    // (...)

    if (error) {
        // Return the loan without publishing
        data_writer->discard_loan(data);
        return;
    }

    // Publish the new value
    if (data_writer->write(data, eprosima::fastrtps::rtps::InstanceHandle_t()) != ReturnCode_t::RETCODE_OK) {
        // Error
        return;
    }
}

// The data instance can be reused to publish new values,
// but delete it at the end to avoid leaks
custom_type_support->deleteData(data)
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