成员的完整列表，这些成员属于 aiot_mqtt_download_recv_t,包括所有继承而来的类成员

data	aiot_mqtt_download_recv_t
data	aiot_mqtt_download_recv_t
data_resp	aiot_mqtt_download_recv_t
data_size	aiot_mqtt_download_recv_t
file_lenth	aiot_mqtt_download_recv_t
filename	aiot_mqtt_download_recv_t
offset	aiot_mqtt_download_recv_t
percent	aiot_mqtt_download_recv_t
type	aiot_mqtt_download_recv_t