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Singletons and Static Initialization

The Itemframework has a baseclass Singleton<T> which is used across the itemframework, but can also be used by the usercore developer. Also, we have a pattern for static initialization of additional components, which ensures the correct order of initialization.

In contrast to a normal singleton, our singleton system has the following additional features:

  • Singletons can depend on other Singletons, the correct order of initialization is guaranteed.
  • Dependency Cycles will be detected and reported
  • Singletons have a postInit and preDestroy method which will be called before the app starts/ends.

Note: Singleton<T> can only be used from within libraries which are loaded before the apps starts. This means the Singleton functionality can be used in the usercore, starter and itemframework project, but not in the plugins.

The program flow is as follows:

  1. main() is invoked and the dynamic linker loads the itemframework and the usercore library
  2. The subclasses of Singleton<T> and the "Components" add themself to the StartupHelper class using static initialization.
  3. The application event loop starts (QCoreApplication::exec)
  4. The dependencies marked by the Singletons and Components are resolved, and the order of initialzation is determined.
  5. The Singletons/Components are created in order.
  6. postInit() is called on all Singletons in order.
  7. The application runs untill the user closes it.
  8. preDestroy() is called on all Singletons in reverse order.
  9. The Singletons/Components are destructed/deleted in reverse order.

Components

If you only need static initialization at the right time, you can add a static void init() and/or static void deinit() function to your class. Then add a STARTUP_ADD_COMPONENT(myclass) to the source file to take care of the registration

Singletons

If your singleton class is named A you should let it inherit from Singleton<A>. This will automatically create a instance member function, which you can later use to fetch the instance. Also you will have to add the bool postInit() and bool preDestroy() members. You can return true from these members if you have nothing else to initialize/destroy or return false if initialization/destruction failed.

Finally add a STARTUP_ADD_SINGLETON(A) to the source file to take care of the registration.

Control the order of initialization

Both, Singletons and Components can control their position in the list of to be initialized singletons/components by adding the following classinfo's:

  • Q_CLASSINFO("dependsOn", "PluginManager") depend on the PluginManager Singleton/Component unconditionally.
  • Q_CLASSINFO("optionallyDependsOn", "GuiManager") depend on the GuiManager Singleton/Component only if it's available.
  • Q_CLASSINFO("guiModule","true") marks the module as Gui Module. This means the Singleton/Components will only be loaded if you are in a Gui Application and not if you are in a Testcase/Console-Application

Singletons/Components must either inherit from QObject and use the Q_OBJECT macro or use the Q_GADGET macro.