nus_ data_ Handler source code interprets data types such as structure

c++ Object programming thinking is embodied in struct.
Structs are convenient for defining data types ,struct Describe data objects that require different types of data .
enumeration (Enum) type , Be able to list all possible values , And give them a name .

nodic Official website uart Introduce

## ***services_init register NUS,nus_init.data_handler = nus_data_handler;***

static void nus_data_handler(ble_nus_evt_t *p_evt)
{
    

    if (p_evt->type == BLE_NUS_EVT_RX_DATA)
    {
    
        uint32_t err_code;

        NRF_LOG_DEBUG("nus_data_handler:Received data from BLE NUS. Writing data on UART.");
        NRF_LOG_HEXDUMP_DEBUG(p_evt->params.rx_data.p_data, p_evt->params.rx_data.length);
        
        for (uint32_t i = 0; i < p_evt->params.rx_data.length; i++)
        {
    
            do
            {
    
                err_code = app_uart_put(p_evt->params.rx_data.p_data[i]);
                if ((err_code != NRF_SUCCESS) && (err_code != NRF_ERROR_BUSY))
                {
    
                    NRF_LOG_ERROR("Failed receiving NUS message. Error 0x%x. ", err_code);
                    APP_ERROR_CHECK(err_code);
                }
            } while (err_code == NRF_ERROR_BUSY);
        }
        if (p_evt->params.rx_data.p_data[p_evt->params.rx_data.length - 1] == '\r')
        {
    
            while (app_uart_put('\n') == NRF_ERROR_BUSY);
        }
    }
}

## ***ble_nus.h***
//p_evt->type(ble_nus_evt_type_t) == BLE_NUS_EVT_RX_DATA
//p_evt->params.rx_data.p_data
typedef struct
{
    
    ble_nus_evt_type_t         type;        /**< Event type. */
    ble_nus_t                * p_nus;       /**< A pointer to the instance. */
    uint16_t                   conn_handle; /**< Connection handle. */
    ble_nus_client_context_t * p_link_ctx;  /**< A pointer to the link context. */
    union
    {
    
        ble_nus_evt_rx_data_t rx_data; /**< @ref BLE_NUS_EVT_RX_DATA event data. */
        ble_nus_evt_rx_data_t led_data;// preservation led data 
    } params;
} ble_nus_evt_t;
//evt_type
typedef enum
{
    
    BLE_NUS_EVT_RX_DATA,      /**< Data received. */
    BLE_NUS_EVT_TX_RDY,       /**< Service is ready to accept new data to be transmitted. */
    BLE_NUS_EVT_LED_OPT,     // add to led Event type 
    BLE_NUS_EVT_COMM_STARTED, /**< Notification has been enabled. */
    BLE_NUS_EVT_COMM_STOPPED, /**< Notification has been disabled. */
} ble_nus_evt_type_t;
//the buffer with received data
typedef struct
{
    
    uint8_t const * p_data; /**< A pointer to the buffer with received data. */
    uint16_t        length; /**< Length of received data. */
} ble_nus_evt_rx_data_t;