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:

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.

Weather Data Providers

The example uses two unrelated weather data providers:

The provider to be used is selected automatically at runtime and can be changed if the selected provider is not available. However, it can't be specified manually.

Note: Free plans are used for both providers, which implies certain limitations on the amount of weather requests. If the limits are exceeded, the providers become temporary unavailable. When both providers are unavailable, the application would not be able to show any weather information. In this case it is required to wait until at least one of the providers becomes available again.

Application Data Models

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 use Q_PROPERTY to expose it nicely to QML later. It also uses QML_ANONYMOUS macro, which makes it recognized in QML.

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)
    QML_ANONYMOUS

public:
    explicit WeatherData(QObject *parent = 0);
    WeatherData(const WeatherData &other);
    WeatherData(const WeatherInfo &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 QGeoPositionInfoSource::createDefaultSource().

AppModel::AppModel(QObject *parent) :
        QObject(parent),
        d(new AppModelPrivate)
{
    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(errorOccurred(QGeoPositionInfoSource::Error)),
                this, SLOT(positionError(QGeoPositionInfoSource::Error)));
        d->src->startUpdates();
    } else {
        d->useGps = false;
        d->city = "Brisbane";
        emit cityChanged();
        requestWeatherByCity();
    }
}

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 weather data for the specified location.

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

    if (!d->useGps)
        return;

    requestWeatherByCoordinates();
}

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.

The model also uses QML_ELEMENT macro, which makes it available in QML.

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)
    QML_ELEMENT

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);

    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 weather forecast for the next days (the number of days is provider-specific). This makes it easy to access the forecast from QML.

Expose Custom Models to QML

To expose the models to the QML UI layer, we use the QML_ELEMENT and QML_ANONYMOUS macros. See the QQmlEngine class description for more details on these macros.

To make the types available in QML, we need to update our build accordingly.

CMake Build

For a CMake-based build, we need to add the following to the CMakeLists.txt:

qt_add_qml_module(weatherinfo
    URI WeatherInfo
    VERSION 1.0
    QML_FILES
        components/BigForecastIcon.qml
        components/ForecastIcon.qml
        components/WeatherIcon.qml
        weatherinfo.qml
    RESOURCES
        icons/weather-few-clouds.png
        icons/weather-fog.png
        icons/weather-haze.png
        icons/weather-icy.png
        icons/weather-overcast.png
        icons/weather-showers.png
        icons/weather-sleet.png
        icons/weather-snow.png
        icons/weather-storm.png
        icons/weather-sunny-very-few-clouds.png
        icons/weather-sunny.png
        icons/weather-thundershower.png
        icons/weather-showers-scattered.png
    NO_RESOURCE_TARGET_PATH
)

qmake Build

For a qmake build, we need to modify the weatherinfo.pro file in the following way:

CONFIG += qmltypes
QML_IMPORT_NAME = WeatherInfo
QML_IMPORT_MAJOR_VERSION = 1

Instantiate the Models in QML

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

import WeatherInfo 1.0

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

Once the model is 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: (appModel.hasValidWeather
                          ? appModel.weather.weatherIcon
                          : "sunny")
            }

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