[P037] Add variable processing for JSON attributes

docs/source/Plugin/P037.rst

@@ -69,6 +69,8 @@ When JSON is enabled, regular MQTT Topics will be processed normally.
 
 The JSON Attribute column, when used, can be used to specify what data element to retrieve from a received JSON payload. If the resulting value is numeric it will be stored to the corresponding variable, and the regular event for that will be generated.
 
+With the extended JSON processing, available in most ESP32 builds, the ``attribute`` to retrieve from the JSON can be longer than the input has space for (20 characters). To allow for longer JSON attributes, the requested JSON attribute can be stored in a String variable, using the ``LetStr`` command, like ``LetStr,timeLeft,printer_data.progress.printTimeLeft``, in f.e. the ``On System#Boot Do`` event handler rule, and then use in this input field like ``[str#timeLeft]``.
+
 When receiving multiple-value attributes, like ``"svalue":"28.1;17.8;2;1010.3;0"``, an index can be appended to the attribute like ``svalue;4`` to get the 4th value, in this example ``1010.3`` (1-based index).
 
 When receiving a JSON payload, an event can (in addition to the standard event when receiving a numeric value) be generated for the received Attribute, or for all attributes if no specific attribute is configured.

src/_P037_MQTTImport.ino

@@ -11,6 +11,7 @@
 // This task reads data from the MQTT Import input stream and saves the value
 
 /**
+ * 2026-09-11 tonhuisman: Process variables to allow for longer JSON attribute names
  * 2026-08-02 tonhuisman: Add better support for JSON parsing, that handles multiple levels and arrays
  * 2025-08-20 tonhuisman: Generate events with numeric values using the configured decimals setting.
  * 2025-06-14 tonhuisman: Add support for Custom Value Type per task value
@@ -555,7 +556,12 @@ boolean Plugin_037(uint8_t function, struct EventStruct *event, String& string)
                 jsonAttribute.trim();
 
                 if (!jsonAttribute.isEmpty()) {
+                  #  if FEATURE_STRING_VARIABLES
+                  key = parseTemplate(jsonAttribute); // Process variables to allow for longer JSON attribute names
+                  key.trim();                         // The JSON parser is quite picky about extra spaces...
+                  #  else // if FEATURE_STRING_VARIABLES
                   key = jsonAttribute;
+                  #  endif // if FEATURE_STRING_VARIABLES
 
                   #  if FEATURE_JSON_PARSE
                   Payload = getJsonValue(P037_data->root, key, false);