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, we get the platform’s default position source using QGeoPositionInfo::createDefaultSource()

AppModel::AppModel(QObject *parent) :
        QObject(parent),
        d(new AppModelPrivate)
{
    d->nam = new QNetworkAccessManager(this);
    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 and Attributions

The example bundles the following images from Third-Party sources:

Tango Icons	Public Domain
Tango Weather Icon Pack by Darkobra	Public Domain

Example project @ code.qt.io