""" A binding between a trait on an object and an extension point. """


# Standard library imports.
import weakref

# Enthought library imports.
from traits.api import Any, HasTraits, Instance, Str, Undefined

# Local imports.
from i_extension_registry import IExtensionRegistry


class ExtensionPointBinding(HasTraits):
    """ A binding between a trait on an object and an extension point. """

    #### 'ExtensionPointBinding' *CLASS* interface ############################

    # We keep a reference to each binding alive until its associated object
    # is garbage collected.
    _bindings = weakref.WeakKeyDictionary()

    #### 'ExtensionPointBinding' interface ####################################

    # The object that we are binding the extension point to.
    obj = Any

    # The Id of the extension point.
    extension_point_id = Str

    # The extension registry used by the binding. If this trait is not set then
    # the class-scope extension registry set on the 'ExtensionPoint' class is
    # used (and if that is not set then the binding won't work ;^)
    extension_registry = Instance(IExtensionRegistry)

    # The name of the trait that we are binding the extension point to.
    trait_name = Str

    #### Private interface ####################################################

    # A flag that prevents us from setting a trait twice.
    _event_handled = False

    ###########################################################################
    # 'object' interface.
    ###########################################################################

    def __init__(self, **traits):
        """ Constructor. """

        super(ExtensionPointBinding, self).__init__(**traits)

        # Initialize the object's trait from the extension point.
        self._set_trait(notify=False)

        # Wire-up the trait change and extension point handlers.
        self._initialize()

        # We keep a reference to each binding alive until its associated
        # object is garbage collected.
        bindings = ExtensionPointBinding._bindings.setdefault(self.obj, [])
        bindings.append(self)

        return

    ###########################################################################
    # 'ExtensionPointBinding' interface.
    ###########################################################################

    #### Trait initializers ###################################################

    def _extension_registry_default(self):
        """ Trait initializer. """

        # fixme: Sneaky global!!!!!
        from extension_point import ExtensionPoint

        return ExtensionPoint.extension_registry

    ###########################################################################
    # Private interface.
    ###########################################################################

    #### Trait change handlers ################################################

    def _on_trait_changed(self, obj, trait_name, old, new):
        """ Dynamic trait change handler. """

        if not self._event_handled:
            self._set_extensions(new)

        return

    def _on_trait_items_changed(self, obj, trait_name, old, event):
        """ Dynamic trait change handler. """

        if not self._event_handled:
            self._set_extensions(getattr(obj, self.trait_name))

        return

    #### Other observer pattern listeners #####################################

    def _extension_point_listener(self, extension_registry, event):
        """ Listener called when an extension point is changed. """

        self._event_handled = True
        if event.index is not None:
            self._update_trait(event)

        else:
            self._set_trait(notify=True)
        self._event_handled = False

        return

    #### Methods ##############################################################

    def _initialize(self):
        """ Wire-up trait change handlers etc. """

        # Listen for the object's trait being changed.
        self.obj.on_trait_change(
            self._on_trait_changed, self.trait_name
        )

        self.obj.on_trait_change(
            self._on_trait_items_changed, self.trait_name + '_items'
        )

        # Listen for the extension point being changed.
        self.extension_registry.add_extension_point_listener(
            self._extension_point_listener, self.extension_point_id
        )

        return

    def _set_trait(self, notify):
        """ Set the object's trait to the value of the extension point. """

        value = self.extension_registry.get_extensions(self.extension_point_id)
        traits = {self.trait_name : value}

        self.obj.set(trait_change_notify=notify, **traits)

        return

    def _update_trait(self, event):
        """ Update the object's trait to the value of the extension point. """

        self._set_trait(notify=False)

        self.obj.trait_property_changed(
            self.trait_name + '_items', Undefined, event
        )

        return

    def _set_extensions(self, extensions):
        """ Set the extensions to an extension point. """

        self.extension_registry.set_extensions(
            self.extension_point_id, extensions
        )

        return


# Factory function for creating bindings.
def bind_extension_point(
    obj, trait_name, extension_point_id, extension_registry=None
):
    """ Create a binding to an extension point. """

    # This may seem a bit wierd, but we manually build up a dictionary of
    # the traits that need to be set at the time the 'ExtensionPointBinding'
    # instance is created.
    #
    # This is because we only want to set the 'extension_registry' trait iff
    # one is explicitly specified. If we passed it in with the default argument
    # value of 'None' then it counts as 'setting' the trait which prevents
    # the binding instance from defaulting to the appropriate registry.
    # Also, if we try to set the 'extension_registry' trait *after*
    # construction time then it is too late as the binding initialization is
    # done in the constructor (we could of course split that out, which may be
    # the 'right' way to do it ;^).
    traits = {
        'obj'                : obj,
        'trait_name'         : trait_name,
        'extension_point_id' : extension_point_id
    }

    if extension_registry is not None:
        traits['extension_registry'] = extension_registry

    return ExtensionPointBinding(**traits)

#### EOF ######################################################################
