backport CellularSignal::isValid()

wiring/inc/spark_wiring_cellular_printable.h

@@ -58,6 +58,9 @@ class CellularSignal : public particle::Signal, public Printable {
 
     virtual size_t printTo(Print& p) const;
 
+    virtual bool isValid() const;
+    virtual operator bool() const;
+
 private:
     cellular_signal_t sig_ = {0};
 };

wiring/src/spark_wiring_cellular_printable.cpp

@@ -93,6 +93,26 @@ size_t CellularSignal::printTo(Print& p) const
 }
 #pragma GCC diagnostic pop
 
+bool CellularSignal::isValid() const
+{
+    return (sig_.rat != NET_ACCESS_TECHNOLOGY_NONE &&
+            sig_.rssi != std::numeric_limits<int32_t>::min() &&
+#if (PLATFORM_ID == PLATFORM_ELECTRON_PRODUCTION)
+            // U-blox GSM radios may not always support quality as it depends on the packet switching mode
+            // at the time of network connection, which is not possible to query. For now, we will return "true"
+            // for GSM Electrons and will not check if quality is actually supported / valid.
+            // Hence, `isValid()` can return "true" for invalid signal value. To add a note in docs.
+            (sig_.rat == NET_ACCESS_TECHNOLOGY_GSM || sig_.qual != std::numeric_limits<int32_t>::min()));
+#else
+            sig_.qual != std::numeric_limits<int32_t>::min());
+#endif
+}
+
+CellularSignal::operator bool() const
+{
+    return isValid();
+}
+
 size_t CellularData::printTo(Print& p) const
 {
     size_t n = 0;