Log File Position Source (C++)
The Logfile Position Source shows how to create and work with a custom NMEA position source, for platforms without GPS.
The data is read from a file which has positional data in NMEA format. The resulting time and position information is then displayed to the screen as simple text in date/time and latitude/longitude format.
This example class reads position data from a text file, log.txt. The file specifies position data using a simple text format: it contains one position update per line, where each line contains a date/time, a latitude and a longitude, separated by spaces. The date/time is in ISO 8601 format and the latitude and longitude are in degrees decimal format. Here is an excerpt from log.txt:
2009-08-24T22:25:01 -27.576082 153.092415 2009-08-24T22:25:02 -27.576223 153.092530 2009-08-24T22:25:03 -27.576364 153.092648
The class reads this data and distributes it via the positionUpdated() signal.
Here is the definition of the LogFilePositionSource
class:
class LogFilePositionSource : public QGeoPositionInfoSource { Q_OBJECT public: LogFilePositionSource(QObject *parent = 0); QGeoPositionInfo lastKnownPosition(bool fromSatellitePositioningMethodsOnly = false) const; PositioningMethods supportedPositioningMethods() const; int minimumUpdateInterval() const; Error error() const; public slots: virtual void startUpdates(); virtual void stopUpdates(); virtual void requestUpdate(int timeout = 5000); private slots: void readNextPosition(); private: QFile *logFile; QTimer *timer; QGeoPositionInfo lastPosition; };
The main methods overrided by the subclass are:
- startUpdates(): called by client applications to start regular position updates.
- stopUpdates(): called by client applications to stop regular position updates.
- requestUpdate(): called by client applications to request a single update, with a specified timeout.
When a position update is available, the subclass emits the positionUpdated() signal.
Here are the key methods in the class implementation:
LogFilePositionSource::LogFilePositionSource(QObject *parent) : QGeoPositionInfoSource(parent), logFile(new QFile(this)), timer(new QTimer(this)) { connect(timer, SIGNAL(timeout()), this, SLOT(readNextPosition())); logFile->setFileName(":/simplelog.txt"); if (!logFile->open(QIODevice::ReadOnly)) qWarning() << "Error: cannot open source file" << logFile->fileName(); } void LogFilePositionSource::startUpdates() { int interval = updateInterval(); if (interval < minimumUpdateInterval()) interval = minimumUpdateInterval(); timer->start(interval); } void LogFilePositionSource::stopUpdates() { timer->stop(); } void LogFilePositionSource::requestUpdate(int /*timeout*/) { // For simplicity, ignore timeout - assume that if data is not available // now, no data will be added to the file later if (logFile->canReadLine()) readNextPosition(); else emit updateTimeout(); } void LogFilePositionSource::readNextPosition() { QByteArray line = logFile->readLine().trimmed(); if (!line.isEmpty()) { QList<QByteArray> data = line.split(' '); double latitude; double longitude; bool hasLatitude = false; bool hasLongitude = false; QDateTime timestamp = QDateTime::fromString(QString(data.value(0)), Qt::ISODate); latitude = data.value(1).toDouble(&hasLatitude); longitude = data.value(2).toDouble(&hasLongitude); if (hasLatitude && hasLongitude && timestamp.isValid()) { QGeoCoordinate coordinate(latitude, longitude); QGeoPositionInfo info(coordinate, timestamp); if (info.isValid()) { lastPosition = info; emit positionUpdated(info); } } } }