- Qt 5.15
- Qt Positioning
- C++ Classes
- QGeoAreaMonitorSource
- Qt 5.15.11 Reference Documentation

# QGeoAreaMonitorSource Class

The QGeoAreaMonitorSource class enables the detection of proximity changes for a specified set of coordinates. More...

Header: | #include <QGeoAreaMonitorSource>
qmake: | QT += positioning
Since: | Qt 5.2

This class was introduced in Qt 5.2.

- List of all members, including inherited members

## Public Types

enum | AreaMonitorFeature { PersistentAreaMonitorFeature, AnyAreaMonitorFeature }
flags | AreaMonitorFeatures
enum | Error { AccessError, InsufficientPositionInfo, NoError, UnknownSourceError }

## Public Functions

| QGeoAreaMonitorSource(QObject *parent)
virtual | ~QGeoAreaMonitorSource()
virtual QList<QGeoAreaMonitorInfo> | activeMonitors() const = 0
virtual QList<QGeoAreaMonitorInfo> | activeMonitors(const QGeoShape &lookupArea) const = 0
virtual Error | error() const = 0
virtual QGeoPositionInfoSource * | positionInfoSource() const
virtual bool | requestUpdate(const QGeoAreaMonitorInfo &monitor, const char *signal) = 0
virtual void | setPositionInfoSource(QGeoPositionInfoSource *newSource)
QString | sourceName() const
virtual bool | startMonitoring(const QGeoAreaMonitorInfo &monitor) = 0
virtual bool | stopMonitoring(const QGeoAreaMonitorInfo &monitor) = 0
virtual AreaMonitorFeatures | supportedAreaMonitorFeatures() const = 0

## Signals

void | areaEntered(const QGeoAreaMonitorInfo &monitor, const QGeoPositionInfo &update)
void | areaExited(const QGeoAreaMonitorInfo &monitor, const QGeoPositionInfo &update)
void | error(QGeoAreaMonitorSource::Error areaMonitoringError)
void | monitorExpired(const QGeoAreaMonitorInfo &monitor)

## Static Public Members

QStringList | availableSources()
QGeoAreaMonitorSource * | createDefaultSource(QObject *parent)
QGeoAreaMonitorSource * | createSource(const QString &sourceName, QObject *parent)

## Detailed Description

A QGeoAreaMonitorSource emits signals when the current position is in range, or has moved out of range, of a specified area. Each area is specified by a QGeoAreaMonitorInfo object. For example:

     public:
         MyClass() : QObject()
         {
             QGeoAreaMonitorSource *monitor = QGeoAreaMonitorSource::createDefaultSource(this);
             if (monitor) {
                 connect(monitor, SIGNAL(areaEntered(QGeoAreaMonitorInfo,QGeoPositionInfo)),
                         this, SLOT(areaEntered(QGeoAreaMonitorInfo,QGeoPositionInfo)));
                 connect(monitor, SIGNAL(areaExited(QGeoAreaMonitorInfo,QGeoPositionInfo)),
                         this, SLOT(areaExited(QGeoAreaMonitorInfo,QGeoPositionInfo)));

                 QGeoAreaMonitorInfo bigBen("Big Ben");
                 QGeoCoordinate position(51.50104, -0.124632);
                 bigBen.setArea(QGeoCircle(position, 100));

                 monitor->startMonitoring(bigBen);

             } else {
                 qDebug() << "Could not create default area monitor";
             }
         }

     public Q_SLOTS:
         void areaEntered(const QGeoAreaMonitorInfo &mon, const QGeoPositionInfo &update)
         {
             Q_UNUSED(mon);

             qDebug() << "Now within 100 meters, current position is" << update.coordinate();
         }

         void areaExited(const QGeoAreaMonitorInfo &mon, const QGeoPositionInfo &update)
         {
             Q_UNUSED(mon);

             qDebug() << "No longer within 100 meters, current position is" << update.coordinate();
         }

`QGeoAreaMonitorSource` follows a singleton pattern. Each instance of the class with the same sourceName() shares the same area monitoring backend. If a new QGeoAreaMonitorInfo object is added via startMonitoring() or requestUpdate() it can be retrieved by another instance of this class (provided that they are sourced from the same area monitor provider plug-in). The same singleton pattern applies to the QGeoPositionInfoSource instance used by this class. The following code snippet emphasizes the behavior:

     QGeoAreaMonitorSource *s1 = QGeoAreaMonitorSource::createSource("blah", this);
     QGeoAreaMonitorSource *s2 = QGeoAreaMonitorSource::createSource("blah", this);
     QVERIFY(s1->positionInfoSource() == s2->positionInfoSource);

## Member Type Documentation

### enum QGeoAreaMonitorSource::AreaMonitorFeature
flags QGeoAreaMonitorSource::AreaMonitorFeatures

Defines the types of area monitoring capabilities.

Constant | Value | Description
--- | --- | ---
`QGeoAreaMonitorSource::PersistentAreaMonitorFeature` | `0x00000001` | QGeoAreaMonitorInfo instances can be made persistent. A persistent monitor continues to be active even when the application managing the monitor is not running.
`QGeoAreaMonitorSource::AnyAreaMonitorFeature` | `0xffffffff` | Matches all possible area monitoring features.

The AreaMonitorFeatures type is a typedef for QFlags<AreaMonitorFeature>. It stores an OR combination of AreaMonitorFeature values.

### enum QGeoAreaMonitorSource::Error

Defines the types of positioning methods.

The Error enumeration represents the errors which can occur.

Constant | Value | Description
--- | --- | ---
`QGeoAreaMonitorSource::AccessError` | `0` | The connection setup to the remote area monitoring backend failed because the application lacked the required privileges.
`QGeoAreaMonitorSource::InsufficientPositionInfo` | `1` | The area monitoring source could not retrieve a location fix or the accuracy of the fix is not high enough to provide an effective area monitoring.
`QGeoAreaMonitorSource::NoError` | `3` | No error has occurred.
`QGeoAreaMonitorSource::UnknownSourceError` | `2` | An unidentified error occurred.

## Member Function Documentation

### QGeoAreaMonitorSource::QGeoAreaMonitorSource(QObject *parent)

Creates a monitor with the given parent.

### `[signal]` void QGeoAreaMonitorSource::areaEntered(const QGeoAreaMonitorInfo &monitor, const QGeoPositionInfo &update)

Emitted when the current position has moved from a position outside of the active monitor to a position within the monitored area.

The update holds the new position.

### `[signal]` void QGeoAreaMonitorSource::areaExited(const QGeoAreaMonitorInfo &monitor, const QGeoPositionInfo &update)

Emitted when the current position has moved from a position within the active monitor to a position outside the monitored area.

The update holds the new position.

### `[signal]` void QGeoAreaMonitorSource::error(QGeoAreaMonitorSource::Error areaMonitoringError)

This signal is emitted after an error occurred. The areaMonitoringError parameter describes the type of error that occurred.

Note: Signal error is overloaded in this class. To connect to this signal by using the function pointer syntax, Qt provides a convenient helper for obtaining the function pointer as shown in this example:

     connect(geoAreaMonitorSource, QOverload<QGeoAreaMonitorSource::Error>::of(&QGeoAreaMonitorSource::error),
         [=](QGeoAreaMonitorSource::Error areaMonitoringError){ /* ... */ });

### `[signal]` void QGeoAreaMonitorSource::monitorExpired(const QGeoAreaMonitorInfo &monitor)

Emitted when monitor has expired. An expired area monitor is automatically removed from the list of activeMonitors().

See also activeMonitors().

### `[virtual]` QGeoAreaMonitorSource::~QGeoAreaMonitorSource()

Destroys the monitor.

### `[pure virtual]` QList<QGeoAreaMonitorInfo> QGeoAreaMonitorSource::activeMonitors() const

Returns the list of all active monitors known to the QGeoAreaMonitorSource object.

An active monitor was started via startMonitoring() the source object will emit the required signals such as areaEntered() or areaExited(). Multiple QGeoAreaMonitorSource instances within the same application share the same active monitor objects.

Unless an active QGeoAreaMonitorInfo isPersistent() an active QGeoAreaMonitorInfo will be stopped once the current application terminates.

### `[pure virtual]` QList<QGeoAreaMonitorInfo> QGeoAreaMonitorSource::activeMonitors(const QGeoShape &lookupArea) const

Returns the list of all active monitors known to the QGeoAreaMonitorSource object whose center lies within lookupArea. If lookupArea is empty the returned list will be empty.

An active monitor was started via startMonitoring() and the source object will emit the required signals such as areaEntered() or areaExited(). Multiple QGeoAreaMonitorSource instances within the same application share the same monitor objects.

Unless an active QGeoAreaMonitorInfo isPersistent() an active QGeoAreaMonitorInfo will be stopped once the current application terminates.

See also QGeoShape.

### `[static]` QStringList QGeoAreaMonitorSource::availableSources()

Returns a list of available monitor plugins, including the default system backend if one is available.

### `[static]` QGeoAreaMonitorSource *QGeoAreaMonitorSource::createDefaultSource(QObject *parent)

Creates and returns a monitor with the given parent that monitors areas using resources on the underlying system.

Returns 0 if the system has no support for position monitoring.

### `[static]` QGeoAreaMonitorSource *QGeoAreaMonitorSource::createSource(const QString &sourceName, QObject *parent)

Creates and returns a monitor with the given parent, by loading the plugin named sourceName.

Returns 0 if the plugin cannot be found.

### `[pure virtual]` Error QGeoAreaMonitorSource::error() const

Returns the type of error that last occurred.

### `[virtual]` QGeoPositionInfoSource *QGeoAreaMonitorSource::positionInfoSource() const

Returns the current QGeoPositionInfoSource used by this QGeoAreaMonitorSource object. The function will return QGeoPositionInfoSource::createDefaultSource() if no other object has been set.

The function returns 0 if not even a default QGeoPositionInfoSource exists.

Any usage of the returned QGeoPositionInfoSource instance should account for the fact that it may reside in a different thread.

See also QGeoPositionInfoSource and setPositionInfoSource().

### `[pure virtual]` bool QGeoAreaMonitorSource::requestUpdate(const QGeoAreaMonitorInfo &monitor, const char *signal)

Enables single shot area monitoring. Area monitoring for monitor will be performed until this QGeoAreaMonitorSource instance emits signal for the first time. Once the signal was emitted, monitor is automatically removed from the list of activeMonitors(). If monitor is invalid or has an expiry date that has been passed this function returns false.

     QGeoAreaMonitor singleShotMonitor;
     QGeoAreaMonitorSource * source = QGeoAreaMonitorSource::createDefaultSource(this);
     //...
     bool ret = source->requestUpdate(singleShotMonitor,
                           SIGNAL(areaExited(QGeoAreaMonitor,QGeoPositionInfo)));

The above `singleShotMonitor` object will cease to send updates once the areaExited() signal was emitted for the first time. Until this point in time any other signal may be emitted zero or more times depending on the area context.

It is not possible to simultanously request updates for more than one signal of the same monitor object. The last call to this function determines the signal upon which the updates cease to continue. At this stage only the areaEntered() and areaExited() signals can be used to terminate the monitoring process.

Requesting persistent monitoring on a QGeoAreaMonitorSource instance fails if the area monitoring backend doesn't support QGeoAreaMonitorSource::PersistentAreaMonitorFeature.

If monitor was already registered via startMonitoring() it is converted to a single shot behavior.

See also startMonitoring() and stopMonitoring().

### `[virtual]` void QGeoAreaMonitorSource::setPositionInfoSource(QGeoPositionInfoSource *newSource)

Sets the new QGeoPositionInfoSource to be used by this QGeoAreaMonitorSource object. The area monitoring backend becomes the new QObject parent for newSource. The previous QGeoPositionInfoSource object will be deleted. All QGeoAreaMonitorSource instances based on the same sourceName() share the same QGeoPositionInfoSource instance.

This may be useful when it is desirable to manipulate the positioning system used by the area monitoring engine.

Note that ownership must be taken care of by subclasses of QGeoAreaMonitorSource. Due to the singleton pattern behind this class newSource may be moved to a new thread.

See also positionInfoSource().

### QString QGeoAreaMonitorSource::sourceName() const

Returns the unique name of the area monitor source implementation in use.

This is the same name that can be passed to createSource() in order to create a new instance of a particular area monitor source implementation.

### `[pure virtual]` bool QGeoAreaMonitorSource::startMonitoring(const QGeoAreaMonitorInfo &monitor)

Returns `true` if the monitoring of monitor could be successfully started; otherwise returns false. A reason for not being able to start monitoring could be the unavailability of an appropriate default position info source while no alternative QGeoPositionInfoSource has been set via setPositionInfoSource().

If monitor is already active the existing monitor object will be replaced by the new monitor reference. The identification of QGeoAreaMonitorInfo instances happens via QGeoAreaMonitorInfo::identifier(). Therefore this function can also be used to update active monitors.

If monitor has an expiry date that has been passed this function returns false. Calling this function for an already via requestUpdate() registered single shot monitor switches the monitor to a permanent monitoring mode.

Requesting persistent monitoring on a QGeoAreaMonitorSource instance fails if the area monitoring backend doesn't support QGeoAreaMonitorSource::PersistentAreaMonitorFeature.

See also stopMonitoring().

### `[pure virtual]` bool QGeoAreaMonitorSource::stopMonitoring(const QGeoAreaMonitorInfo &monitor)

Returns true if monitor was successfully removed from the list of activeMonitors(); otherwise returns false. This behavior is independent on whether monitor was registered via startMonitoring() or requestUpdate().

### `[pure virtual]` AreaMonitorFeatures QGeoAreaMonitorSource::supportedAreaMonitorFeatures() const

Returns the area monitoring features available to this source.