\brief HbIndicatorPluginInterface is an abstract interface class for indicator plugins.

    Indicators are activated, deactivated and updated using HbIndicator class. Indicator

    framework takes care of displaying indicators in satus bar and/or indicator menu.

    Indicator plugins contain implentation of indicators. Single pluging can implement

    one or several indicators. HbIndicatorInterface defines functionality required from an

    indicator implementation.

    Indicators are identified by a string. By convention the string should follow

    inverted domain name format. For example com.nokia.hb.indicator.xxx/1.0.

    Function indicatorTypes() returns a list of indicators the plugin implements.

    Appending version number into indicator type string enables versioning.

    A plugin should handle versioning by returning indicator type string for all versions it

    implements in indicatorTypes() and then in createIndicator() create an indicator

    instance compatible with the version requested. This could always be the latest version if it

    is backwards compatible with older versions. Indicator framework is unaware of version

    numbers in type strings. It performs string comparison of the whole string when searching

    for a plugin.

    Plugins are responsible for maintaining system security for their own part. If plugin

    performs operations that may compromise security or want's to limit access to specific

    clients, it should check client security credentials in accessAllowed() function.

    Indicator framework calls this function before activating/deactivating indicators.

    \section _exceptions Exception handling

    Indicator service uses two strategies in exception handling: Avoidance and trapping.

    Memory allocation exceptions while an indicator is running are avoided by ensuring there is

    sufficient heap space available before allowing new indicators to be activated.

    Trapping is used while an indicator is created. A call to createIndicator() is enclosed

    in try/catch block. Memory allocation exception causes indicator activation to fail

    and an error is returned to a client. Plugin should take care there are no memory leaks

    if exception is thrown inside createIndicator(). Calls to HbIndicatorInterface

    functions are trapped and thrown allocation exceptions are ignored. Plugins can

    provide more fine grained exception handling by trapping exceptions themselves.

    \section _platform_hbindicatorplugin Platform-specific implementation notes for HbIndicatorPluginInterface

    \subsection _nonsymbian Non-Symbian

    Indicator plugins are loaded into client process. Plugin executables are searched from

    application's current directory and HB_PLUGINS_DIR/indicators directory.

    \subsection _symbian Symbian

    Plugins are run by a server with platform security capabilities ProtServ, SwEvent,

    TrustedUI and ReadDeviceData. If a plugin doesn't have required platform security

    capabilities it will not load.

    Device dialog plugin stubs (.qtplugin) are searched from /resource/plugins/indicators directory

    and executables in /sys/bin directory in each drive.

    \section _example_code Example code

    Below is an example of how to create a simple indicator plugin.

    If plugin implements only one indicator, plugin can also inherit from

    \snippet{tsrc\unit\unittest_hbindicator\codesnippetplugin\hbcodesnippetplugin.h,1}

    \snippet{tsrc\unit\unittest_hbindicator\codesnippetplugin\hbcodesnippetplugin.cpp,1}

    If more than one indicators are implemented inside a plugin, plugin inherits from HbIndicatorPluginInterface.

    \snippet{tsrc\unit\unittest_hbindicator\codesnippetplugin\hbcodesnippetplugin.h,2}

    And createIndicator should create new object based on indicator type.

    \snippet{tsrc\unit\unittest_hbindicator\codesnippetplugin\hbcodesnippetplugin.cpp,2}

    \sa HbIndicator, HbIndicatorInterface

    Should return the indicator types the plugin implements.

    Creates an indicator of type \a indicatorType. Ownership is passed to the caller.