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本文為英文版的機器翻譯版本，如內容有任何歧義或不一致之處，概以英文版為準。

# 何時使用簡訊
<a name="working-with_messaging_when-to-use"></a>

中的傳訊 SimSpace Weaver 提供另一種模式，用於在模擬應用程式之間交換資訊。訂閱提供從特定應用程式或模擬區域讀取資料的提取機制；訊息提供推送機制，可將資料傳送至特定應用程式或模擬區域。

以下是兩個使用案例，其中使用傳訊推送資料比透過訂閱提取或讀取資料更有幫助。

**Example 1：將命令傳送至另一個應用程式以變更實體位置**  

```
// Message struct definition
struct MessageMoveEntity
{
     uint64_t entityId;
    std::array<float, 3> destinationPos;
};

// Create the message 
MessageMoveEntity message {45, {236.67, 826.22, 0.0} };

// Create the payload out of the struct
const Api::MessagePayload& payload = Api::Utils::CreateMessagePayload(
    reinterpret_cast<const std::uint8_t*>(&message), 
    sizeof(MessageTickAndId)
);

// Grab the MessageEndpoint of the recipient app.
Api::MessageEndpoint destination = ...

// One way is to resolve it from the domain name and position
WEAVERRUNTIME_TRY(
    Api::MessageEndpoint destination,
    Api::Utils::MessageEndpointResolver::ResolveFromPosition(
    txn,
        "MySpatialSimulation",
        Api::Vector2F32 {200.0, 100.0}
    )
);

// Then send the message 
Api::SendMessage(txn, payload, destination);
```
在接收端，應用程式會更新實體的位置，並將其寫入 State Fabric。  

```
Result<void> ReceiveMessages(Txn& txn) noexcept
{
    WEAVERRUNTIME_TRY(auto messages, Api::ReceiveMessages(txn));
    for (Api::Message& message : messages.messages)
    {
        std::cout << "Received message: " << message << std::endl;
         // Deserialize payload to the message struct
        const MessageMoveEntity& receivedMessage 
            = Api::Utils::ExtractMessage<MessageMoveEntity>(message);
            
        ProcessMessage(txn, receivedMessage);
    }

    return Aws::WeaverRuntime::Success();
}

void ProcessMessage(Txn& txn, const MessageMoveEntity& receivedMessage)
{
     // Get the entity corresponding to the entityId
    Entity entity = EntityFromEntityId (receivedMessage.entityId);
    
    // Update the position and write to StateFabric
    WEAVERRUNTIME_TRY(Api::StoreEntityIndexKey(
            txn,
            entity,
            k_vector3f32TypeId, // type id of the entity
            reinterpret_cast<std::int8_t*>(&receivedMessage.destinationPos),
            sizeof(receivedMessage.destinationPos)));
    
}
```

**Example 2：將建立實體訊息傳送至空間應用程式**  

```
struct WeaverMessage
{
    const Aws::WeaverRuntime::Api::TypeId messageTypeId;
};

const Aws::WeaverRuntime::Api::TypeId k_createEntityMessageTypeId = { 1 };

struct CreateEntityMessage : WeaverMessage
{
    const Vector3 position;
   const Aws::WeaverRuntime::Api::TypeId typeId;
}; 


CreateEntityMessage messageData { 
    k_createEntityMessageTypeId,                           
    Vector3{ position.GetX(), position.GetY(), position.GetZ() },
    Api::TypeId { 0 }
}

WEAVERRUNTIME_TRY(Api::MessageEndpoint destination, Api::Utils::MessageEndpointResolver::ResolveFromPosition(
    transaction, "MySpatialDomain", DemoFramework::ToVector2F32(position)
));

Api::MessagePayload payload = Api::Utils::CreateMessagePayload(
    reinterpret_cast<const uint8_t*>(&messageData),
    sizeof(CreateEntityMessage));
        
Api::SendMessage(transaction, payload, destination);
```
在接收端，應用程式會在 State Fabric 中建立新的實體，並更新其位置。  

```
Result<void> ReceiveMessages(Txn& txn) noexcept
{
    WEAVERRUNTIME_TRY(auto messageList, Api::ReceiveMessages(transaction));
    WEAVERRUNTIME_TRY(auto tick, Api::CurrentTick(transaction));
    for (auto& message : messageList.messages)
    {
        // cast to base WeaverMessage type to determine MessageTypeId
        WeaverMessage weaverMessageBase = Api::Utils::ExtractMessage<WeaverMessage>(message);
        if (weaverMessageBase.messageTypeId == k_createEntityMessageTypeId)
        {
            CreateEntityMessage createEntityMessageData =
                Api::Utils::ExtractMessage<CreateEntityMessage>(message);
        CreateActorFromMessage(transaction, createEntityMessageData));
        }
        else if (weaverMessageBase.messageTypeId == k_tickAndIdMessageTypeId)
        {
            ...
        }
    }
}

void ProcessMessage(Txn& txn, const CreateEntityMessage& receivedMessage)
{
    // Create entity
    WEAVERRUNTIME_TRY(
        Api::Entity entity,
        Api::CreateEntity(transaction, receivedMessage.typeId)
    );
    
    // Update the position and write to StateFabric
    WEAVERRUNTIME_TRY(Api::StoreEntityIndexKey(
        transaction,
        entity,
        receivedMessage.typeId,
        reinterpret_cast<std::int8_t*>(&receivedMessage.position),
        sizeof(receivedMessage.position)));
}
```