Initialize Fact QVariant as invalid

Adjusted the default Fact _rawValue from 0 to QVariant::Invalid and
updated _variantToString to return placeholders for Facts that haven't
received a value yet. This makes it possible to distinguish them from
those that are legitimately 0 (or the equivalent for their type). It
also avoids starting every Fact's QVariant lifecycle as an integer,
which is incorrect in most cases.

Related code that set initial values for a few VehicleFactGroup Facts
(Hobbs meter, altitude above terrain, flight distance & time) has been
updated or removed for consistency.

Author: rubenp02

src/FactSystem/Fact.cc

@@ -321,6 +321,18 @@ QStringList Fact::selectedBitmaskStrings() const
 
 QString Fact::_variantToString(const QVariant &variant, int decimalPlaces) const
 {
+    if (!variant.isValid()) {
+        switch (type()) {
+        case FactMetaData::valueTypeFloat:
+        case FactMetaData::valueTypeDouble:
+            return QStringLiteral("--.--");
+        case FactMetaData::valueTypeElapsedTimeInSeconds:
+            return QStringLiteral("--:--:--");
+        default:
+            return QStringLiteral("-");
+        }
+    }
+
     QString valueString;
 
     const auto stripNegativeZero = [](QString &candidate) {

src/FactSystem/Fact.h

@@ -191,7 +191,7 @@ class Fact : public QObject
 
     QString _name;
     int _componentId = -1;
-    QVariant _rawValue = 0;
+    QVariant _rawValue; // QVariant::Invalid
     FactMetaData::ValueType_t _type = FactMetaData::valueTypeInt32;
     FactMetaData *_metaData = nullptr;
     bool _sendValueChangedSignals = true;

src/Vehicle/FactGroups/VehicleFactGroup.cc

@@ -49,7 +49,7 @@ VehicleFactGroup::VehicleFactGroup(QObject *parent)
     _addFact(&_throttlePctFact);
     _addFact(&_imuTempFact);
 
-    _hobbsFact.setRawValue(QStringLiteral("0000:00:00"));
+    _hobbsFact.setRawValue(QStringLiteral("----:--:--"));
 }
 
 void VehicleFactGroup::handleMessage(Vehicle *vehicle, const mavlink_message_t &message)

src/Vehicle/Vehicle.cc

@@ -252,9 +252,6 @@ void Vehicle::_commonInit(LinkInterface* link)
     connect(this, &Vehicle::homePositionChanged,    this, &Vehicle::_updateDistanceHeadingHome);
     connect(this, &Vehicle::hobbsMeterChanged,      this, &Vehicle::_updateHobbsMeter);
     connect(this, &Vehicle::coordinateChanged,      this, &Vehicle::_updateAltAboveTerrain);
-    // Initialize alt above terrain to Nan so frontend can display it correctly in case the terrain query had no response
-    _altitudeAboveTerrFact.setRawValue(qQNaN());
-
     connect(this, &Vehicle::vehicleTypeChanged,     this, &Vehicle::inFwdFlightChanged);
     connect(this, &Vehicle::vtolInFwdFlightChanged, this, &Vehicle::inFwdFlightChanged);
 
@@ -347,8 +344,6 @@ void Vehicle::_commonInit(LinkInterface* link)
         }
     }
 
-    _flightDistanceFact.setRawValue(0);
-    _flightTimeFact.setRawValue(0);
     _flightTimeUpdater.setInterval(1000);
     _flightTimeUpdater.setSingleShot(false);
     connect(&_flightTimeUpdater, &QTimer::timeout, this, &Vehicle::_updateFlightTime);