void wifi_init_sta(void)
{
    s_wifi_event_group = xEventGroupCreate();

    ESP_ERROR_CHECK(esp_netif_init());
    ESP_ERROR_CHECK(esp_event_loop_create_default());
    esp_netif_create_default_wifi_sta();
    wifi_init_config_t cfg = WIFI_INIT_CONFIG_DEFAULT();
    ESP_ERROR_CHECK(esp_wifi_init(&cfg));

    esp_event_handler_instance_t instance_any_id;
    esp_event_handler_instance_t instance_got_ip;
    ESP_ERROR_CHECK(esp_event_handler_instance_register(WIFI_EVENT,
                                                        ESP_EVENT_ANY_ID,
                                                        &event_handler,
                                                        NULL,
                                                        &instance_any_id));
    ESP_ERROR_CHECK(esp_event_handler_instance_register(IP_EVENT,
                                                        IP_EVENT_STA_GOT_IP,
                                                        &event_handler,
                                                        NULL,
                                                        &instance_got_ip));
    wifi_config_t wifi_config = {
        .sta = {
            .ssid = IOT_CORE_ESP_WIFI_SSID,
            .password = IOT_CORE_ESP_WIFI_PASS,
	        .threshold.authmode = WIFI_AUTH_WPA2_PSK,
            .pmf_cfg = {
                .capable = true,
                .required = false
            },
        },
    };
    ESP_ERROR_CHECK(esp_wifi_set_mode(WIFI_MODE_STA) );
    ESP_ERROR_CHECK(esp_wifi_set_config(WIFI_IF_STA, &wifi_config) );
    ESP_ERROR_CHECK(esp_wifi_start() );
    ESP_LOGI(TAG, "wifi_init_sta finished.");
}

其中xEventGroupCreate()用于创建一个事件标志组，返回值是事件标志组句柄，属于frerertos里面的东西；
esp_netif_init()用于初始化tcpip协议栈；
esp_event_loop_create_default()创建一个默认系统事件调度循环，之后可以注册回调函数来处理系统的一些事件；
esp_netif_create_default_wifi_sta()创建wifi sta；
wifi_init_config_t cfg=WIFI_INIT_CONFIG_DEFAULT()用于获取wifi的默认配置；
esp_wifi_init(&cfg)用于初始化wifi；
esp_event_handler_instance_register 用于向上面的esp_event_loop_create_default()注册回调函数，在回调函数里面可以处理各种系统事件，比如wfi连接，断开等；
wifi_config是一个结构体变量，用于设置wifi的参数，如ssid，password等；
esp_wifi_set_mode用于设置wifi的模式，在这里使用sta模式；
esp_wifi_set_config设置wifi参数；
esp_wifi_start启动wifi。

2.2、回调函数

static void event_handler(void* arg, esp_event_base_t event_base,
                                int32_t event_id, void* event_data)
{
    if (event_base == WIFI_EVENT && event_id == WIFI_EVENT_STA_START) 
    {
        esp_wifi_connect();
    } 
    else if (event_base == WIFI_EVENT && event_id == WIFI_EVENT_STA_DISCONNECTED) 
    {
        if (s_retry_num < IOT_CORE_ESP_MAXIMUM_RETRY) 
        {
            esp_wifi_connect();
            s_retry_num++;
            ESP_LOGI(TAG, "retry to connect to the AP");
        } 
        else 
        {
            xEventGroupSetBits(s_wifi_event_group, WIFI_FAIL_BIT);
        }
        ESP_LOGI(TAG,"connect to the AP fail");
    } 
    else if (event_base == IP_EVENT && event_id == IP_EVENT_STA_GOT_IP) 
    {
        ip_event_got_ip_t* event = (ip_event_got_ip_t*) event_data;
        ESP_LOGI(TAG, "got ip:" IPSTR, IP2STR(&event->ip_info.ip));
        s_retry_num = 0;
        xEventGroupSetBits(s_wifi_event_group, WIFI_CONNECTED_BIT);
    }
}

void esp_wifi_sta_check_state(void)
{
    /* Waiting until either the connection is established (WIFI_CONNECTED_BIT) or connection failed for the maximum
     * number of re-tries (WIFI_FAIL_BIT). The bits are set by event_handler() (see above) */
    EventBits_t bits = xEventGroupWaitBits(s_wifi_event_group,
            WIFI_CONNECTED_BIT | WIFI_FAIL_BIT,
            pdFALSE,
            pdFALSE,
            portMAX_DELAY);

    /* xEventGroupWaitBits() returns the bits before the call returned, hence we can test which event actually
     * happened. */
    if (bits & WIFI_CONNECTED_BIT) 
    {
        xEventGroupClearBits(s_wifi_event_group,WIFI_CONNECTED_BIT);  
        ESP_LOGI(TAG, "connected to ap SSID:%s password:%s",
                 IOT_CORE_ESP_WIFI_SSID, IOT_CORE_ESP_WIFI_PASS);
    } 
    else if (bits & WIFI_FAIL_BIT) 
    {
        xEventGroupClearBits(s_wifi_event_group,WIFI_FAIL_BIT);       
        ESP_LOGI(TAG, "Failed to connect to SSID:%s, password:%s",
                 IOT_CORE_ESP_WIFI_SSID, IOT_CORE_ESP_WIFI_PASS);
    } 
    else 
    {
        ESP_LOGE(TAG, "UNEXPECTED EVENT");
    }
}