from lxml import etree
import pkg_resources
import logging
from keystore import Location

log = logging.getLogger('mitx.' + __name__)

def dummy_track(event_type, event):
    pass


class ModuleMissingError(Exception):
    pass


class Plugin(object):
    @classmethod
    def load_class(cls, identifier):
        classes = list(pkg_resources.iter_entry_points(cls.entry_point, name=identifier))
        if len(classes) > 1:
            log.warning("Found multiple classes for {entry_point} with identifier {id}: {classes}. Returning the first one.".format(
                entry_point=cls.entry_point,
                id=identifier,
                classes=", ".join(class_.module_name for class_ in classes)))

        if len(classes) == 0:
            raise ModuleMissingError(identifier)

        return classes[0].load()


class XModule(object):
    ''' Implements a generic learning module. 
        Initialized on access with __init__, first time with state=None, and
        then with state

        See the HTML module for a simple example
    '''
    id_attribute='id' # An attribute guaranteed to be unique

    @classmethod
    def get_xml_tags(c):
        ''' Tags in the courseware file guaranteed to correspond to the module '''
        return []
        
    @classmethod
    def get_usage_tags(c):
        ''' We should convert to a real module system
            For now, this tells us whether we use this as an xmodule, a CAPA response type
            or a CAPA input type '''
        return ['xmodule']

    def get_name(self):
        name = self.__xmltree.get('name')
        if name: 
            return name
        else: 
            raise "We should iterate through children and find a default name"

    def rendered_children(self):
        '''
        Render all children. 
        This really ought to return a list of xmodules, instead of dictionaries
        '''
        children = [self.render_function(e) for e in self.__xmltree]
        return children            

    def __init__(self, system = None, xml = None, item_id = None, 
                 json = None, track_url=None, state=None):
        ''' In most cases, you must pass state or xml'''
        if not item_id: 
            raise ValueError("Missing Index")
        if not xml and not json:
            raise ValueError("xml or json required")
        if not system:
            raise ValueError("System context required")

        self.xml = xml
        self.json = json
        self.item_id = item_id
        self.state = state
        self.DEBUG = False
        
        self.__xmltree = etree.fromstring(xml) # PRIVATE

        if system: 
            ## These are temporary; we really should go 
            ## through self.system. 
            self.ajax_url = system.ajax_url
            self.tracker = system.track_function
            self.filestore = system.filestore
            self.render_function = system.render_function
            self.DEBUG = system.DEBUG
        self.system = system

    ### Functions used in the LMS

    def get_state(self):
        ''' State of the object, as stored in the database 
        '''
        return ""

    def get_score(self):
        ''' Score the student received on the problem. 
        '''
        return None

    def max_score(self):
        ''' Maximum score. Two notes: 
            * This is generic; in abstract, a problem could be 3/5 points on one randomization, and 5/7 on another
            * In practice, this is a Very Bad Idea, and (a) will break some code in place (although that code
              should get fixed), and (b) break some analytics we plan to put in place. 
        ''' 
        return None

    def get_html(self):
        ''' HTML, as shown in the browser. This is the only method that must be implemented
        '''
        return "Unimplemented"

    def handle_ajax(self, dispatch, get):
        ''' dispatch is last part of the URL. 
            get is a dictionary-like object ''' 
        return ""


class XModuleDescriptor(Plugin):
    """
    An XModuleDescriptor is a specification for an element of a course. This could
    be a problem, an organizational element (a group of content), or a segment of video,
    for example.

    XModuleDescriptors are independent and agnostic to the current student state on a
    problem. They handle the editing interface used by instructors to create a problem,
    and can generate XModules (which do know about student state).
    """
    entry_point = "xmodule.v1"

    @staticmethod
    def load_from_json(json_data, system):
        """
        This method instantiates the correct subclass of XModuleDescriptor based
        on the contents of json_data.

        json_data must contain a 'location' element, and must be suitable to be
        passed into the subclasses `from_json` method.
        """
        class_ = XModuleDescriptor.load_class(json_data['location']['category'])
        return class_.from_json(json_data, system)

    @classmethod
    def from_json(cls, json_data, system):
        """
        Creates an instance of this descriptor from the supplied json_data.
        This may be overridden by subclasses

        json_data: Json data specifying the data, children, and metadata for the descriptor
        system: An XModuleSystem for interacting with external resources
        """
        return cls(system=system, **json_data)

    def __init__(self,
                 system,
                 definition=None,
                 **kwargs):
        """
        Construct a new XModuleDescriptor. The only required arguments are the
        system, used for interaction with external resources, and the definition,
        which specifies all the data needed to edit and display the problem (but none
        of the associated metadata that handles recordkeeping around the problem).

        This allows for maximal flexibility to add to the interface while preserving
        backwards compatibility.

        system: An XModuleSystem for interacting with external resources
        definition: A dict containing `data` and `children` representing the problem definition

        Current arguments passed in kwargs:
            location: A keystore.Location object indicating the name and ownership of this problem
            goals: A list of strings of learning goals associated with this module
        """
        self.system = system
        self.definition = definition if definition is not None else {}
        self.name = Location(kwargs.get('location')).name
        self.type = Location(kwargs.get('location')).category

        # For now, we represent goals as a list of strings, but this
        # is one of the things that we are going to be iterating on heavily
        # to find the best teaching method
        self.goals = kwargs.get('goals', [])

        self._child_instances = None

    def get_children(self, categories=None):
        """Returns a list of XModuleDescriptor instances for the children of this module"""
        if self._child_instances is None:
            self._child_instances = [self.system.load_item(child) for child in self.definition['children']]

        if categories is None:
            return self._child_instances
        else:
            return [child for child in self._child_instances if child.type in categories]

    def get_xml(self):
        ''' For conversions between JSON and legacy XML representations.
        '''
        if self.xml: 
            return self.xml
        else: 
            raise NotImplementedError("JSON->XML Translation not implemented")

    def get_json(self):
        ''' For conversions between JSON and legacy XML representations.
        '''
        if self.json: 
            raise NotImplementedError
            return self.json # TODO: Return context as well -- files, etc. 
        else: 
            raise NotImplementedError("XML->JSON Translation not implemented")

    #def handle_cms_json(self):
    #    raise NotImplementedError

    #def render(self, size):
    #    ''' Size: [thumbnail, small, full] 
    #    Small ==> what we drag around
    #    Full ==> what we edit
    #    '''
    #    raise NotImplementedError


class DescriptorSystem(object):
    def __init__(self, load_item):
        """
        load_item: Takes a Location and returns and XModuleDescriptor
        """

        self.load_item = load_item
