蓝牙
蓝牙GAP主要是广播设置，GATT是建立连接之后的数据交换协议。
设置好手机端蓝牙和ESP32端蓝牙之后，你将会发现设备有3个服务 ，虽然我们只是添加了1个服务。但是却显示3个服务。另外2个是GAP服务(0x1800)，它包含GAP数据，包括之前设置的一些参数；一个是GATT服务(0x1801),它包含如果初始化服务后需要更改服务所用到的特性值。所有的BLE服务必须包含这些服务，协议栈SoftDevice自动添加了它们。
static const uint8_t char_prop_read = ESP_GATT_CHAR_PROP_BIT_WRITE | ESP_GATT_CHAR_PROP_BIT_READ;
static const uint8_t char_prop_write = ESP_GATT_CHAR_PROP_BIT_WRITE | ESP_GATT_CHAR_PROP_BIT_READ;
static const uint8_t char_prop_read_write_notify = ESP_GATT_CHAR_PROP_BIT_WRITE | ESP_GATT_CHAR_PROP_BIT_READ | ESP_GATT_CHAR_PROP_BIT_NOTIFY;
上面三句变量声明代码，他们的作用是定义charistic的读写权限。就在这里改，如果不需要读的权限就把ESP_GATT_CHAR_PROP_BIT_READ去掉，不需要写的权限就把ESP_GATT_CHAR_PROP_BIT_WRITE去掉；
static const esp_gatts_attr_db_t gatt_db[HRS_IDX_NB] =
{
// Service Declaration
[IDX_SVC] =
{{ESP_GATT_AUTO_RSP}, {ESP_UUID_LEN_16, (uint8_t *)&primary_service_uuid, ESP_GATT_PERM_READ,
sizeof(uint16_t), sizeof(GATTS_SERVICE_UUID_TEST), (uint8_t *)&GATTS_SERVICE_UUID_TEST}},
/* Characteristic Declaration */
[IDX_CHAR_A] =
{{ESP_GATT_AUTO_RSP}, {ESP_UUID_LEN_16, (uint8_t *)&character_declaration_uuid, ESP_GATT_PERM_READ,
CHAR_DECLARATION_SIZE, CHAR_DECLARATION_SIZE, (uint8_t *)&char_prop_read_write_notify}},
/* Characteristic Value */
[IDX_CHAR_VAL_A] =
{{ESP_GATT_AUTO_RSP}, {ESP_UUID_LEN_16, (uint8_t *)&GATTS_CHAR_UUID_TEST_A, ESP_GATT_PERM_READ | ESP_GATT_PERM_WRITE,
GATTS_DEMO_CHAR_VAL_LEN_MAX, sizeof(char_value_a), (uint8_t *)char_value_a}},
/* Client Characteristic Configuration Descriptor */
[IDX_CHAR_CFG_A] =
{{ESP_GATT_AUTO_RSP}, {ESP_UUID_LEN_16, (uint8_t *)&character_client_config_uuid, ESP_GATT_PERM_READ | ESP_GATT_PERM_WRITE,
sizeof(uint16_t), sizeof(heart_measurement_ccc), (uint8_t *)heart_measurement_ccc}},
/* Characteristic Declaration */
[IDX_CHAR_B] =
{{ESP_GATT_AUTO_RSP}, {ESP_UUID_LEN_16, (uint8_t *)&character_declaration_uuid, ESP_GATT_PERM_READ,
CHAR_DECLARATION_SIZE, CHAR_DECLARATION_SIZE, (uint8_t *)&char_prop_read}},
/* Characteristic Value */
[IDX_CHAR_VAL_B] =
{{ESP_GATT_AUTO_RSP}, {ESP_UUID_LEN_16, (uint8_t *)&GATTS_CHAR_UUID_TEST_B, ESP_GATT_PERM_READ | ESP_GATT_PERM_WRITE,
GATTS_DEMO_CHAR_VAL_LEN_MAX, sizeof(char_value_b), (uint8_t *)char_value_b}},
/* Characteristic Declaration */
[IDX_CHAR_C] =
{{ESP_GATT_AUTO_RSP}, {ESP_UUID_LEN_16, (uint8_t *)&character_declaration_uuid, ESP_GATT_PERM_READ,
CHAR_DECLARATION_SIZE, CHAR_DECLARATION_SIZE, (uint8_t *)&char_prop_write}},
/* Characteristic Value */
[IDX_CHAR_VAL_C] =
{{ESP_GATT_AUTO_RSP}, {ESP_UUID_LEN_16, (uint8_t *)&GATTS_CHAR_UUID_TEST_C, ESP_GATT_PERM_READ | ESP_GATT_PERM_WRITE,
GATTS_DEMO_CHAR_VAL_LEN_MAX, sizeof(char_value_c), (uint8_t *)char_value_c}},
};
上面这一段实际上定义了一个服务，以及这个服务的各项参数，这是重点！！！下面实上面参数的解释：
* ``[HRS_IDX_SVC]``: Named or designated initializer in the enum table.
* ``ESP_GATT_AUTO_RSP``: Auto respond configuration, set to respond automatically by the stack.
* ``ESP_UUID_LEN_16``: UUID length set to 16 bits.
* ``(uint8_t *)&primary_service_uuid``: UUID to identify the service as a primary one (0x2800).
* ``ESP_GATT_PERM_READ``: Read Permission for the service.
* ``sizeof(uint16_t)``: Maximum length of the service UUID (16 bits).
* ``sizeof(heart_rate_svc)``: Current service length set to the size of the variable *heart_rate_svc*, which is 16 bits.
* ``(uint8_t *)&heart_rate_svc``: Service attribute value set to the variable *heart_rate_svc* which contains the Heart Rate Service UUID (0x180D).
