Add JSON pointer support

README.md

@@ -22,7 +22,7 @@ Contributions are welcome - feel free to open issues and pull requests!
 
 - Auto-update of EPICS PVS via `I/O Intr` records;
 - Support for read/write flat MQTT topics (i.e, topics where the payload is a single value or array);
-- Support for reading arbitrarily nested fields from JSON topic payloads;
+- Support for reading JSON payloads;
 - Support for MQTT QoS levels;
 - Checks and reject invalid messages (based mostly on type-checking);
 - Auto reconnection of broker;
@@ -117,7 +117,7 @@ Where:
 - `<FORMAT>` is the format of the payload: `FLAT` or `JSON`.
 - `<TYPE>` is the general type of the expected value [`INT|FLOAT|DIGITAL|STRING|INTARRAY|FLOATARRAY`].
 - `<TOPIC>` is the MQTT topic to which the record will be subscribed/published.
-- `<FIELD>` is the dot-separated path to the field to extract from a JSON payload (e.g. `sensor.temperature`). Arbitrary nesting is supported. Required when `FORMAT` is `JSON`.
+- `<FIELD>` is JSON pointer to extract value from a JSON payload (e.g. `/sensor/temperature`). If empty, JSON root is used.
 
 > **Note on JSON write support:** Writing to JSON-formatted topics is currently **not supported**. At the moment the driver has no way of knowing the JSON structure expected by the broker ahead of time for write records. For this reason, only `FLAT` format can be used for output records.
 

mqttSup/src/drvMqtt.cpp

@@ -73,23 +73,25 @@ DeviceAddress* MqttDriver::parseDeviceAddress(std::string const& function, std::
   }
   else if (prefix == JSON_FUNC_PREFIX) {
     auto spacePos = arguments.find(' ');
+    std::string topicName;
+    std::string jsonField;
     if (spacePos == std::string::npos) {
-      fprintf(stderr, "%s::%s: JSON field not specified: %s\n", driverName, functionName, arguments.c_str());
-      delete addr;
-      return nullptr;
-    }
-    if (spacePos + 1 >= arguments.size()) {
-      fprintf(stderr, "%s::%s: JSON field is empty: %s\n", driverName, functionName, arguments.c_str());
-      delete addr;
-      return nullptr;
+      topicName = arguments;
+      jsonField = "";
+    } else {
+      if (spacePos + 1 >= arguments.size()) {
+        fprintf(stderr, "%s::%s: JSON field is empty: %s\n", driverName, functionName, arguments.c_str());
+        delete addr;
+        return nullptr;
+      }
+      topicName = arguments.substr(0, spacePos);
+      jsonField = arguments.substr(spacePos + 1, arguments.size());
     }
-    std::string topicName = arguments.substr(0, spacePos);
     if (!isValidTopicName(topicName)) {
       fprintf(stderr, "%s::%s: Invalid topic name: %s\n", driverName, functionName, topicName.c_str());
       delete addr;
       return nullptr;
     }
-    std::string jsonField = arguments.substr(spacePos + 1, arguments.size());
     addr->format = MqttTopicAddr::JSON;
     addr->topicName = topicName;
     addr->jsonField = jsonField;
@@ -256,13 +258,7 @@ void MqttDriver::onMessageCb(Autoparam::Driver* driver, const std::string& topic
     if (addr.format == MqttTopicAddr::JSON) {
       try {
         json root = json::parse(payload);
-        const json* fieldAddr = findJsonField(root, addr.jsonField);
-        if (!fieldAddr || fieldAddr->is_null())
-          throw std::invalid_argument("JSON field not found: " + addr.jsonField);
-        if (fieldAddr->is_string())
-          val = fieldAddr->get<std::string>();
-        else
-          val = fieldAddr->dump();
+        val = to_string(root.at(json::json_pointer(addr.jsonField)));
       }
       catch (const std::exception& e) {
         asynPrint(pself->pasynUserSelf, ASYN_TRACE_ERROR,
@@ -336,28 +332,6 @@ void MqttDriver::onMessageCb(Autoparam::Driver* driver, const std::string& topic
 //#############################################################################################
 // Helper methods
 
-// Recursively search for a key anywhere in the JSON structure
-const json* MqttDriver::findJsonField(const json& payload, const std::string& targetKey) {
-  if (payload.is_object()) {
-    for (auto it = payload.begin(); it != payload.end(); ++it) {
-      if (it.key() == targetKey) {
-        return &it.value();
-      }
-      else {
-        const json* found = findJsonField(it.value(), targetKey);
-        if (found) return found;
-      }
-    }
-  }
-  else if (payload.is_array()) {
-    for (const auto& el : payload) {
-      const json* found = findJsonField(el, targetKey);
-      if (found) return found;
-    }
-  }
-  return nullptr;
-}
-
 /* Checks if a string corresponds to one of the supported topic types */
 bool MqttDriver::isSupportedTopicType(const std::string& type) {
   return MqttDriver::supportedTopicTypes.find(type) != MqttDriver::supportedTopicTypes.end();

mqttSup/src/drvMqtt.h

@@ -17,6 +17,7 @@
 
 using namespace Autoparam::Convenience;
 using json = nlohmann::json;
+using std::to_string;
 
 static const char* driverName = "MqttDriver";
 
@@ -59,7 +60,6 @@ class MqttDriver : public Autoparam::Driver {
   DeviceAddress* parseDeviceAddress(std::string const& function, std::string const& arguments);
   DeviceVariable* createDeviceVariable(DeviceVariable* baseVar);
   /* helper methods */
-  static const json* findJsonField(const json& payload, const std::string& targetKey);
   static bool isInteger(const std::string& s, bool isSigned = true);
   static bool isBoolean(const std::string& s);
   static bool isFloat(const std::string& s);