Qt 5.11 | Qt Positioning | Qt Positioning Examples | Weather Info (C++/QML)

Qt 5.11.3 Reference Documentation

# Weather Info (C++/QML)

The Weather Info example shows how to use the user's current position to retrieve local content from a web service in a C++ plugin for QML.

Key Qt Positioning classes used in this example:

- QGeoPositionInfo
- QGeoPositionInfoSource

## Running the Example

To run the example from Qt Creator, open the Welcome mode and select the example from Examples. For more information, visit Building and Running an Example.

The example uses weather data provided by http://www.openweathermap.org.

The key part of this example is the application's data model, contained in the WeatherData and AppModel classes. WeatherData represents the weather information taken from the HTTP service. It is a simple data class, but we give it Q_PROPERTies to expose it nicely to QML, later.

      class WeatherData : public QObject {
          Q_OBJECT
          Q_PROPERTY(QString dayOfWeek
                     READ dayOfWeek WRITE setDayOfWeek
                     NOTIFY dataChanged)
          Q_PROPERTY(QString weatherIcon
                     READ weatherIcon WRITE setWeatherIcon
                     NOTIFY dataChanged)
          Q_PROPERTY(QString weatherDescription
                     READ weatherDescription WRITE setWeatherDescription
                     NOTIFY dataChanged)
          Q_PROPERTY(QString temperature
                     READ temperature WRITE setTemperature
                     NOTIFY dataChanged)

      public:
          explicit WeatherData(QObject *parent = 0);
          WeatherData(const WeatherData &other);

          QString dayOfWeek() const;
          QString weatherIcon() const;
          QString weatherDescription() const;
          QString temperature() const;

          void setDayOfWeek(const QString &value);
          void setWeatherIcon(const QString &value);
          void setWeatherDescription(const QString &value);
          void setTemperature(const QString &value);

      signals:
          void dataChanged();
      };

AppModel models the state of the entire application. At startup, the application first begins by waiting for network connectivity. We do this using the QNetworkConfigurationManager and QNetworkSession family of C++ APIs.

      AppModel::AppModel(QObject *parent) :
              QObject(parent),
              d(new AppModelPrivate)
      {
          // make sure we have an active network session
          d->nam = new QNetworkAccessManager(this);

          QNetworkConfigurationManager ncm;
          d->ns = new QNetworkSession(ncm.defaultConfiguration(), this);
          connect(d->ns, SIGNAL(opened()), this, SLOT(networkSessionOpened()));
          // the session may be already open. if it is, run the slot directly
          if (d->ns->isOpen())
              this->networkSessionOpened();
          // tell the system we want network
          d->ns->open();
      }

Once the network session is open, we proceed to get the platform's default position source using QGeoPositionInfo::createDefaultSource()

      void AppModel::networkSessionOpened()
      {
          d->src = QGeoPositionInfoSource::createDefaultSource(this);

          if (d->src) {
              d->useGps = true;
              connect(d->src, SIGNAL(positionUpdated(QGeoPositionInfo)),
                      this, SLOT(positionUpdated(QGeoPositionInfo)));
              connect(d->src, SIGNAL(error(QGeoPositionInfoSource::Error)),
                      this, SLOT(positionError(QGeoPositionInfoSource::Error)));
              d->src->startUpdates();
          } else {
              d->useGps = false;
              d->city = "Brisbane";
              emit cityChanged();
              this->refreshWeather();
          }
      }

If no default source is available, we take a static position and fetch weather for that. If, however, we do have a position source, we connect its positionUpdated() signal to a slot on the AppModel and call startUpdates(), which begins regular updates of device position.

When a position update is received, we use the longitude and latitude of the returned coordinate to retrieve the current "city" name for use in the weather lookup.

      void AppModel::positionUpdated(QGeoPositionInfo gpsPos)
      {
          d->coord = gpsPos.coordinate();

          if (!(d->useGps))
              return;

          queryCity();
      }

To inform the UI about this process, the cityChanged() signal is emitted when a new city is used, and the weatherChanged() signal whenever a weather update occurs.

      class AppModel : public QObject
      {
          Q_OBJECT
          Q_PROPERTY(bool ready
                     READ ready
                     NOTIFY readyChanged)
          Q_PROPERTY(bool hasSource
                     READ hasSource
                     NOTIFY readyChanged)
          Q_PROPERTY(bool hasValidCity
                     READ hasValidCity
                     NOTIFY cityChanged)
          Q_PROPERTY(bool hasValidWeather
                     READ hasValidWeather
                     NOTIFY weatherChanged)
          Q_PROPERTY(bool useGps
                     READ useGps WRITE setUseGps
                     NOTIFY useGpsChanged)
          Q_PROPERTY(QString city
                     READ city WRITE setCity
                     NOTIFY cityChanged)
          Q_PROPERTY(WeatherData *weather
                     READ weather
                     NOTIFY weatherChanged)
          Q_PROPERTY(QQmlListProperty<WeatherData> forecast
                     READ forecast
                     NOTIFY weatherChanged)

      public:
          explicit AppModel(QObject *parent = 0);
          ~AppModel();

          bool ready() const;
          bool hasSource() const;
          bool useGps() const;
          bool hasValidCity() const;
          bool hasValidWeather() const;
          void setUseGps(bool value);
          void hadError(bool tryAgain);

          QString city() const;
          void setCity(const QString &value);

          WeatherData *weather() const;
          QQmlListProperty<WeatherData> forecast() const;

      public slots:
          Q_INVOKABLE void refreshWeather();

      signals:
          void readyChanged();
          void useGpsChanged();
          void cityChanged();
          void weatherChanged();

      };

We use a QQmlListProperty for the weather forecast information, which contains the next 4 days of forecast weather. This makes it easy to access from QML.

To expose these to the QML UI layer, we use the qmlRegisterType() function. We call this once for each type we wish to register, before loading the actual QML file.

      #include "appmodel.h"

      int main(int argc, char *argv[])
      {
          QLoggingCategory::setFilterRules("wapp.*.debug=false");
          QGuiApplication application(argc, argv);

          qmlRegisterType<WeatherData>("WeatherInfo", 1, 0, "WeatherData");
          qmlRegisterType<AppModel>("WeatherInfo", 1, 0, "AppModel");

          const QString mainQmlApp = QStringLiteral("qrc:///weatherinfo.qml");
          QQuickView view;
          view.setSource(QUrl(mainQmlApp));
          view.setResizeMode(QQuickView::SizeRootObjectToView);

          QObject::connect(view.engine(), SIGNAL(quit()), qApp, SLOT(quit()));
          view.setGeometry(QRect(100, 100, 360, 640));
          view.show();
          return application.exec();
      }

Finally, in the actual QML, we instantiate the AppModel.

      import WeatherInfo 1.0

      Item {
          id: window
          AppModel {
              id: model
              onReadyChanged: {
                  if (model.ready)
                      window.state = "ready"
                  else
                      window.state = "loading"
              }
          }
      }

Once instantiated like this, we can use its properties elsewhere in the QML document:

                  BigForecastIcon {
                      id: current

                      width: main.width -12
                      height: 2 * (main.height - 25 - 12) / 3

                      weatherIcon: (model.hasValidWeather
                                ? model.weather.weatherIcon
                                : "01d")
                  }

Files:

- weatherinfo/appmodel.cpp
- weatherinfo/appmodel.h
- weatherinfo/components/BigForecastIcon.qml
- weatherinfo/components/ForecastIcon.qml
- weatherinfo/components/WeatherIcon.qml
- weatherinfo/main.cpp
- weatherinfo/weatherinfo.pro
- weatherinfo/weatherinfo.qml
- weatherinfo/weatherinfo.qrc

Images:

- weatherinfo/icons/weather-few-clouds.png
- weatherinfo/icons/weather-fog.png
- weatherinfo/icons/weather-haze.png
- weatherinfo/icons/weather-icy.png
- weatherinfo/icons/weather-overcast.png
- weatherinfo/icons/weather-showers.png
- weatherinfo/icons/weather-sleet.png
- weatherinfo/icons/weather-snow.png
- weatherinfo/icons/weather-storm.png
- weatherinfo/icons/weather-sunny-very-few-clouds.png
- weatherinfo/icons/weather-sunny.png
- weatherinfo/icons/weather-thundershower.png