Tc3JsonXmlSampleJsonDomReader

This sample illustrates how a JSON message can be run through programmatically on the basis of DOM. The function block FB_JsonDomParser is used as the basis.

Declaration range

PROGRAM MAIN
VAR
  fbJson     : FB_JsonDomParser;
  jsonDoc    : SJsonValue;
  jsonProp   : SJsonValue;
  jsonValue  : SJsonValue;
  bHasMember : BOOL;
  sMessage   : STRING(255) := '{"serialNumber":"G030PT028191AC4R","batteryVoltage":"1547mV","clickType":"SINGLE"}';
  stReceivedData : ST_ReceivedData;
END_VAR

Implementation range

The JSON message is loaded into the DOM tree using the ParseDocument() method. You can subsequently check whether it contains a certain property using the HasMember() method. The FindMember() method selects the property. The GetString() method extracts its value.

jsonDoc := fbJson.ParseDocument(sMessage);
bHasMember := fbJson.HasMember(jsonDoc, 'serialNumber');
IF (bHasMember) THEN
  bHasMember := FALSE;
  jsonProp := fbJson.FindMember(jsonDoc, 'serialNumber');
  stReceivedData.serialNumber := fbJson.GetString(jsonProp);
END_IF

bHasMember := fbJson.HasMember(jsonDoc, 'batteryVoltage');
IF (bHasMember) THEN
  bHasMember := FALSE;
  jsonProp := fbJson.FindMember(jsonDoc, 'batteryVoltage');
  stReceivedData.batteryVoltage := fbJson.GetString(jsonProp);
END_IF

bHasMember := fbJson.HasMember(jsonDoc, 'clickType');
IF (bHasMember) THEN
  bHasMember := FALSE;
  jsonProp := fbJson.FindMember(jsonDoc, 'clickType');
  stReceivedData.clickType := fbJson.GetString(jsonProp);
END_IF

The use of the method HasMember() is not absolutely necessary, since the method FindMember() already returns 0 if a property was not found. The code shown above can also be implemented as follows:

jsonDoc := fbJson.ParseDocument(sMessage);

jsonProp := fbJson.FindMember(jsonDoc, 'serialNumber');
IF (jsonProp <> 0) THEN
  stReceivedData.serialNumber := fbJson.GetString(jsonProp);
END_IF

jsonProp := fbJson.FindMember(jsonDoc, 'batteryVoltage');
IF (jsonProp <> 0) THEN
  stReceivedData.batteryVoltage := fbJson.GetString(jsonProp);
END_IF

jsonProp := fbJson.FindMember(jsonDoc, 'clickType');
IF (jsonProp <> 0) THEN
  stReceivedData.clickType := fbJson.GetString(jsonProp);
END_IF

Nested JSON objects

The approach is similar with nested JSON objects. Since the entire document is located in the DOM, it is simple to navigate. Let's take a JSON object that looks like this:

sMessage : STRING(255) := '{"Values":{"serial":"G030PT028191AC4R"}}';

The property we are looking for is located in the sub-object "Values". The following code shows how to extract the property. 

jsonDoc := fbJson.ParseDocument(sMessage);
bHasMember := fbJson.HasMember(jsonDoc, 'Values');
IF (bHasMember) THEN
  bHasMember := FALSE;
  jsonProp := fbJson.FindMember(jsonDoc, 'Values');
  IF jsonProp <> 0 THEN
    jsonSerial := fbJson.FindMember(jsonProp, 'serial');
    stReceivedData.serialNumber := fbJson.GetString(jsonSerial);
  END_IF
END_IF