# Message API
主要方便用于在 Server/Client 中回复对端 ACK。
## 使用示例
```php
use Simps\MQTT\Message\SubAck;
use Simps\MQTT\Protocol\ProtocolInterface;
$codes = [0];
$message_id = 8520;
$ack = new SubAck();
$ack->setCodes($codes)
->setMessageId($message_id);
$ack_data = $ack->getContents();
$ack_data = (string) $ack;
// MQTT5
$ack->setProtocolLevel(ProtocolInterface::MQTT_PROTOCOL_LEVEL_5_0)
->setCodes($codes)
->setMessageId($message_id)
->setProperties([
'will_delay_interval' => 60,
'message_expiry_interval' => 60,
]);
$ack_data = $ack->getContents();
$ack_data = (string) $ack;
```
### Server
```php
$server->send($fd, $ack->getContents());
$server->send($fd, (string) $ack);
```
### Client
```php
// 增加参数来获取数组
$client->send($ack->getContents(true), false);
```
