Added support for requesting HVX.

ble/DiscoveredCharacteristic.h

@@ -261,6 +261,20 @@ class DiscoveredCharacteristic {
      */
     ble_error_t write(uint16_t length, const uint8_t *value, const GattClient::WriteCallback_t& onWrite) const;
 
+	/**
+     * Request Server to send notify/inform messages for the characteristic
+     *
+     * @param  type
+     *           BLE_HVX_NOTIFICATION or BLE_HVX_INDICATION.
+     *
+     * @retval BLE_ERROR_NONE Successfully requested notification/informa messages
+     *         BLE_ERROR_INVALID_STATE if some internal state about the connection is invalid, or
+     *         BLE_STACK_BUSY if some client procedure already in progress, or
+     *         BLE_ERROR_NO_MEM if there are no available buffers left to process the request, or
+     *         BLE_ERROR_OPERATION_NOT_PERMITTED due to the characteristic's properties.
+     */
+    ble_error_t requestHVX(HVXType_t type) const;
+
     void setupLongUUID(UUID::LongUUIDBytes_t longUUID, UUID::ByteOrder_t order = UUID::MSB) {
         uuid.setupLong(longUUID, order);
     }

source/DiscoveredCharacteristic.cpp

@@ -105,6 +105,28 @@ DiscoveredCharacteristic::writeWoResponse(uint16_t length, const uint8_t *value)
     return gattc->write(GattClient::GATT_OP_WRITE_CMD, connHandle, valueHandle, length, value);
 }
 
+ble_error_t
+DiscoveredCharacteristic::requestHVX(HVXType_t type) const
+{
+    if (type == BLE_HVX_NOTIFICATION && !props.notify()) {
+        return BLE_ERROR_OPERATION_NOT_PERMITTED;
+    }
+
+    if (type == BLE_HVX_INDICATION && !props.indicate()) {
+        return BLE_ERROR_OPERATION_NOT_PERMITTED;
+    }
+
+    if (!gattc) {
+        return BLE_ERROR_INVALID_STATE;
+    }
+
+    /* Cargo Culted from: https://developer.mbed.org/teams/Bluetooth-Low-Energy/code/BLE_ButtonSense/file/2dec89c76f68/main.cpp */
+    /* HACK. We're assuming that CCCD descriptor immediately follows the value attribute. */
+    /* TODO actually discover the handle for the CCCD descriptor */
+    uint16_t value = (uint16_t)type;
+    return gattc->write(GattClient::GATT_OP_WRITE_REQ, connHandle, valueHandle + 1, sizeof(value), (const uint8_t*)(&value));
+}
+
 struct OneShotWriteCallback {
     static void launch(GattClient* client, Gap::Handle_t connHandle,
                        GattAttribute::Handle_t handle, const GattClient::WriteCallback_t& cb) {