gaphor/docs/service_oriented.md

Service Oriented Architecture

Gaphor has a service oriented architecture. What does this mean? Well, Gaphor is built as a set of small islands (services). Each island provides a specific piece of functionality. For example, we use separate services to load/save models, provide the menu structure, and to handle the undo system.

We define services as entry points in the pyproject.toml. With entry points, applications can register functionality for specific purposes. We also group entry points in to entry point groups. For example, we use the console_scripts entry point group to start an application from the command line.

Services

Gaphor is modeled around the concept of services. Each service can be registered with the application and then it can be used by other services or other objects living within the application.

Each service should implement the Service interface. This interface defines one method:

shutdown(self)

Which is called when a service needs to be cleaned up.

We allow each service to define its own methods, as long as the service is implemented too.

Services should be defined as entry points in the pyproject.toml file.

Typically, a service does some work in the background. Services can also expose actions that can be invoked by users. For example, the Ctrl-z key combo (undo) is implemented by the UndoManager service.

A service can also depend on another services. Service initialization resolves these dependencies. To define a service dependency, just add it to the constructor by its name defined in the entry point:

class MyService(Service):

    def __init__(self, event_manager, element_factory):
        self.event_manager = event_manager
        self.element_factory = element_factory
        event_manager.subscribe(self._element_changed)

    def shutdown(self):
        self.event_manager.unsubscribe(self._element_changed)

    @event_handler(ElementChanged)
    def _element_changed(self, event):

Services that expose actions should also inherit from the ActionProvider interface. This interface does not require any additional methods to be implemented. Action methods should be annotated with an @action annotation.

Example: ElementFactory

A nice example of a service in use is the ElementFactory. It is one of the core services.

The UndoManager depends on the events emitted by the ElementFactory. When an important events occurs, like an element is created or destroyed, that event is emitted. We then use an event handler for ElementFactory that stores the add/remove signals in the undo system. Another example of events that are emitted are with UML.Elements. Those classes, or more specifically, the properties, send notifications every time their state changes.

Entry Points

Gaphor uses a main entry point group called gaphor.services.

Services are used to perform the core functionality of the application while breaking the functions in to individual components. For example, the element factory and undo manager are both services.

Plugins can also be created to extend Gaphor beyond the core functionality as an add-on. For example, a plugin could be created to connect model data to other applications. Plugins are also defined as services. For example a new XMI export plugin would be defined as follows in the pyproject.toml:

[tool.poetry.plugins."gaphor.services"]
"xmi_export" = "gaphor.plugins.xmiexport:XMIExport"

Interfaces

Each service (and plugin) should implement the gaphor.abc.Service interface:

.. autoclass:: gaphor.abc.Service
   :members:

Another more specialized service that also inherits from gaphor.abc.Service, is the UI Component service. Services that use this interface are used to define windows and user interface functionality. A UI component should implement the gaphor.ui.abc.UIComponent interface:

.. autoclass:: gaphor.ui.abc.UIComponent
   :members:

Typically, a service and UI component would like to present some actions to the user, by means of menu entries. Every service and UI component can advertise actions by implementing the gaphor.abc.ActionProvider interface:

.. autoclass:: gaphor.abc.ActionProvider
   :members: