edx-platform/openedx/core/djangoapps/xblock/api.py

"""
Python API for interacting with edx-platform's new XBlock Runtime.

For content in modulestore (currently all course content), you'll need to use
the older runtime.

Note that these views are only for interacting with existing blocks. Other
Studio APIs cover use cases like adding/deleting/editing blocks.
"""

import logging

from django.urls import reverse
from django.utils.translation import ugettext as _
from opaque_keys.edx.keys import UsageKeyV2
from opaque_keys.edx.locator import BundleDefinitionLocator
from rest_framework.exceptions import NotFound
import six
from xblock.core import XBlock
from xblock.exceptions import NoSuchViewError

from openedx.core.djangoapps.xblock.apps import get_xblock_app_config
from openedx.core.djangoapps.xblock.learning_context.manager import get_learning_context_impl
from openedx.core.djangoapps.xblock.runtime.blockstore_runtime import BlockstoreXBlockRuntime, xml_for_definition
from openedx.core.djangoapps.xblock.runtime.runtime import XBlockRuntimeSystem
from openedx.core.djangolib.blockstore_cache import BundleCache
from .utils import get_secure_token_for_xblock_handler, get_xblock_id_for_anonymous_user

log = logging.getLogger(__name__)


def get_runtime_system():
    """
    Get the XBlockRuntimeSystem, which is a single long-lived factory that can
    create user-specific runtimes.

    The Runtime System isn't always needed (e.g. for management commands), so to
    keep application startup faster, it's only initialized when first accessed
    via this method.
    """
    # pylint: disable=protected-access
    if not hasattr(get_runtime_system, '_system'):
        params = dict(
            handler_url=get_handler_url,
            runtime_class=BlockstoreXBlockRuntime,
        )
        params.update(get_xblock_app_config().get_runtime_system_params())
        get_runtime_system._system = XBlockRuntimeSystem(**params)
    return get_runtime_system._system


def load_block(usage_key, user):
    """
    Load the specified XBlock for the given user.

    Returns an instantiated XBlock.

    Exceptions:
        NotFound - if the XBlock doesn't exist or if the user doesn't have the
                   necessary permissions
    """
    # Is this block part of a course, a library, or what?
    # Get the Learning Context Implementation based on the usage key
    context_impl = get_learning_context_impl(usage_key)
    # Now, using the LearningContext and the Studio/LMS-specific logic, check if
    # the block exists in this context and if the user has permission to render
    # this XBlock view:
    if get_xblock_app_config().require_edit_permission:
        authorized = context_impl.can_edit_block(user, usage_key)
    else:
        authorized = context_impl.can_view_block(user, usage_key)
    if not authorized:
        # We do not know if the block was not found or if the user doesn't have
        # permission, but we want to return the same result in either case:
        raise NotFound("XBlock {} does not exist, or you don't have permission to view it.".format(usage_key))

    # TODO: load field overrides from the context
    # e.g. a course might specify that all 'problem' XBlocks have 'max_attempts'
    # set to 3.
    # field_overrides = context_impl.get_field_overrides(usage_key)

    runtime = get_runtime_system().get_runtime(user=user)

    return runtime.get_block(usage_key)


def get_block_metadata(block):
    """
    Get metadata about the specified XBlock
    """
    return {
        "block_id": six.text_type(block.scope_ids.usage_id),
        "block_type": block.scope_ids.block_type,
        "display_name": get_block_display_name(block),
    }


def resolve_definition(block_or_key):
    """
    Given an XBlock, definition key, or usage key, return the definition key.
    """
    if isinstance(block_or_key, BundleDefinitionLocator):
        return block_or_key
    elif isinstance(block_or_key, UsageKeyV2):
        context_impl = get_learning_context_impl(block_or_key)
        return context_impl.definition_for_usage(block_or_key)
    elif isinstance(block_or_key, XBlock):
        return block_or_key.scope_ids.def_id
    else:
        raise TypeError(block_or_key)


def xblock_type_display_name(block_type):
    """
    Get the display name for the specified XBlock class.
    """
    block_class = XBlock.load_class(block_type)
    if hasattr(block_class, 'display_name') and block_class.display_name.default:
        return _(block_class.display_name.default)  # pylint: disable=translation-of-non-string
    else:
        return block_type  # Just use the block type as the name


def get_block_display_name(block_or_key):
    """
    Efficiently get the display name of the specified block. This is done in a
    way that avoids having to load and parse the block's entire XML field data
    using its parse_xml() method, which may be very expensive (e.g. the video
    XBlock parse_xml leads to various slow edxval API calls in some cases).

    This method also defines and implements various fallback mechanisms in case
    the ID can't be loaded.

    block_or_key can be an XBlock instance, a usage key or a definition key.

    Returns the display name as a string
    """
    def_key = resolve_definition(block_or_key)
    use_draft = get_xblock_app_config().get_learning_context_params().get('use_draft')
    cache = BundleCache(def_key.bundle_uuid, draft_name=use_draft)
    cache_key = ('block_display_name', six.text_type(def_key))
    display_name = cache.get(cache_key)
    if display_name is None:
        # Instead of loading the block, just load its XML and parse it
        try:
            olx_node = xml_for_definition(def_key)
        except Exception:  # pylint: disable=broad-except
            log.exception("Error when trying to get display_name for block definition %s", def_key)
            # Return now so we don't cache the error result
            return xblock_type_display_name(def_key.block_type)
        try:
            display_name = olx_node.attrib['display_name']
        except KeyError:
            display_name = xblock_type_display_name(def_key.block_type)
        cache.set(cache_key, display_name)
    return display_name


def render_block_view(block, view_name, user):  # pylint: disable=unused-argument
    """
    Get the HTML, JS, and CSS needed to render the given XBlock view.

    The only difference between this method and calling
        load_block().render(view_name)
    is that this method can fall back from 'author_view' to 'student_view'

    Returns a Fragment.
    """
    try:
        fragment = block.render(view_name)
    except NoSuchViewError:
        fallback_view = None
        if view_name == 'author_view':
            fallback_view = 'student_view'
        if fallback_view:
            fragment = block.render(fallback_view)
        else:
            raise

    return fragment


def get_handler_url(usage_key, handler_name, user):
    """
    A method for getting the URL to any XBlock handler. The URL must be usable
    without any authentication (no cookie, no OAuth/JWT), and may expire. (So
    that we can render the XBlock in a secure IFrame without any access to
    existing cookies.)

    The returned URL will contain the provided handler_name, but is valid for
    any other handler on the same XBlock. Callers may replace any occurrences of
    the handler name in the resulting URL with the name of any other handler and
    the URL will still work. (This greatly reduces the number of calls to this
    API endpoint that are needed to interact with any given XBlock.)

    Params:
        usage_key       - Usage Key (Opaque Key object or string)
        handler_name    - Name of the handler or a dummy name like 'any_handler'
        user            - Django User (registered or anonymous)

    This view does not check/care if the XBlock actually exists.
    """
    usage_key_str = six.text_type(usage_key)
    site_root_url = get_xblock_app_config().get_site_root_url()
    if not user:
        raise TypeError("Cannot get handler URLs without specifying a specific user ID.")
    elif user.is_authenticated:
        user_id = user.id
    elif user.is_anonymous:
        user_id = get_xblock_id_for_anonymous_user(user)
    else:
        raise ValueError("Invalid user value")
    # Now generate a token-secured URL for this handler, specific to this user
    # and this XBlock:
    secure_token = get_secure_token_for_xblock_handler(user_id, usage_key_str)
    # Now generate the URL to that handler:
    path = reverse('xblock_api:xblock_handler', kwargs={
        'usage_key_str': usage_key_str,
        'user_id': user_id,
        'secure_token': secure_token,
        'handler_name': handler_name,
    })
    # We must return an absolute URL. We can't just use
    # rest_framework.reverse.reverse to get the absolute URL because this method
    # can be called by the XBlock from python as well and in that case we don't
    # have access to the request.
    return site_root_url + path