prompt: We are currently solving the following issue within our repository. Here is the issue text:
  --- BEGIN ISSUE ---
  StreamField EmbedBlock renders the url only
  Given the following setup:
  
  ```
  class EmbedOnImage(StructBlock):
      embed = EmbedBlock()
      background_image = ImageChooserBlock(required=False)
  
      class Meta:
          template = 'streamfield_blocks/embed_on_image.html'
  ```
  
  `{{ self.embed }}` outputs the embed url only in `streamfield_blocks/embed_on_image.html`.
  
  And `EmbedBlock.render_basic` is never called: https://github.com/torchbox/wagtail/blob/master/wagtail/wagtailembeds/blocks.py#L8
  
  Also, if I apply the embed filter too: `{{ self.embed|embed }}` then it outputs nothing because the embed filter won't call `embed_to_frontend_html` either, which would use the correct template to render it: https://github.com/torchbox/wagtail/blob/master/wagtail/wagtailembeds/templatetags/wagtailembeds_tags.py#L14
  
  
  --- END ISSUE ---
  
  Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
  --- BEGIN FILES ---
  Path: `wagtail/wagtailembeds/blocks.py`
  Content:
  ```
  1 from wagtail.wagtailcore import blocks
  2 
  3 from wagtail.wagtailembeds.format import embed_to_frontend_html
  4 
  5 
  6 class EmbedBlock(blocks.URLBlock):
  7     def render_basic(self, value):
  8         return embed_to_frontend_html(value)
  9
  ```
  --- END FILES ---
  
  Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.
  
  Here is an example:
  
  ```diff
  diff --git a/examples/server_async.py b/examples/server_async.py
  --- a/examples/server_async.py
  +++ b/examples/server_async.py
  @@ -313,4 +313,4 @@
   
   
   if __name__ == "__main__":
  -    asyncio.run(run_async_server("."), debug=True)
  +    asyncio.run(run_async_server(), debug=True)
  diff --git a/examples/server_sync.py b/examples/server_sync.py
  --- a/examples/server_sync.py
  +++ b/examples/server_sync.py
  @@ -313,5 +313,5 @@
   
   
   if __name__ == "__main__":
  -    server = run_sync_server(".")
  +    server = run_sync_server()
       server.shutdown()
  
  ```
  
golden_diff: diff --git a/wagtail/wagtailembeds/blocks.py b/wagtail/wagtailembeds/blocks.py
  --- a/wagtail/wagtailembeds/blocks.py
  +++ b/wagtail/wagtailembeds/blocks.py
  @@ -1,8 +1,62 @@
  +from django.utils.encoding import python_2_unicode_compatible
  +
   from wagtail.wagtailcore import blocks
   
   from wagtail.wagtailembeds.format import embed_to_frontend_html
   
   
  +@python_2_unicode_compatible
  +class EmbedValue(object):
  +    """
  +    Native value of an EmbedBlock. Should, at minimum, have a 'url' property
  +    and render as the embed HTML when rendered in a template.
  +    NB We don't use a wagtailembeds.model.Embed object for this, because
  +    we want to be able to do {{ value.url|embed:max_width=500 }} without
  +    doing a redundant fetch of the embed at the default width.
  +    """
  +    def __init__(self, url):
  +        self.url = url
  +
  +    def __str__(self):
  +        return embed_to_frontend_html(self.url)
  +
  +
   class EmbedBlock(blocks.URLBlock):
  -    def render_basic(self, value):
  -        return embed_to_frontend_html(value)
  +    def get_default(self):
  +        # Allow specifying the default for an EmbedBlock as either an EmbedValue or a string (or None).
  +        if not self.meta.default:
  +            return None
  +        elif isinstance(self.meta.default, EmbedValue):
  +            return self.meta.default
  +        else:
  +            # assume default has been passed as a string
  +            return EmbedValue(self.meta.default)
  +
  +    def to_python(self, value):
  +        # The JSON representation of an EmbedBlock's value is a URL string;
  +        # this should be converted to an EmbedValue (or None).
  +        if not value:
  +            return None
  +        else:
  +            return EmbedValue(value)
  +
  +    def get_prep_value(self, value):
  +        # serialisable value should be a URL string
  +        if value is None:
  +            return ''
  +        else:
  +            return value.url
  +
  +    def value_for_form(self, value):
  +        # the value to be handled by the URLField is a plain URL string (or the empty string)
  +        if value is None:
  +            return ''
  +        else:
  +            return value.url
  +
  +    def value_from_form(self, value):
  +        # convert the value returned from the form (a URL string) to an EmbedValue (or None)
  +        if not value:
  +            return None
  +        else:
  +            return EmbedValue(value)
  
verification_info: {"golden_diff": "diff --git a/wagtail/wagtailembeds/blocks.py b/wagtail/wagtailembeds/blocks.py\n--- a/wagtail/wagtailembeds/blocks.py\n+++ b/wagtail/wagtailembeds/blocks.py\n@@ -1,8 +1,62 @@\n+from django.utils.encoding import python_2_unicode_compatible\n+\n from wagtail.wagtailcore import blocks\n \n from wagtail.wagtailembeds.format import embed_to_frontend_html\n \n \n+@python_2_unicode_compatible\n+class EmbedValue(object):\n+    \"\"\"\n+    Native value of an EmbedBlock. Should, at minimum, have a 'url' property\n+    and render as the embed HTML when rendered in a template.\n+    NB We don't use a wagtailembeds.model.Embed object for this, because\n+    we want to be able to do {{ value.url|embed:max_width=500 }} without\n+    doing a redundant fetch of the embed at the default width.\n+    \"\"\"\n+    def __init__(self, url):\n+        self.url = url\n+\n+    def __str__(self):\n+        return embed_to_frontend_html(self.url)\n+\n+\n class EmbedBlock(blocks.URLBlock):\n-    def render_basic(self, value):\n-        return embed_to_frontend_html(value)\n+    def get_default(self):\n+        # Allow specifying the default for an EmbedBlock as either an EmbedValue or a string (or None).\n+        if not self.meta.default:\n+            return None\n+        elif isinstance(self.meta.default, EmbedValue):\n+            return self.meta.default\n+        else:\n+            # assume default has been passed as a string\n+            return EmbedValue(self.meta.default)\n+\n+    def to_python(self, value):\n+        # The JSON representation of an EmbedBlock's value is a URL string;\n+        # this should be converted to an EmbedValue (or None).\n+        if not value:\n+            return None\n+        else:\n+            return EmbedValue(value)\n+\n+    def get_prep_value(self, value):\n+        # serialisable value should be a URL string\n+        if value is None:\n+            return ''\n+        else:\n+            return value.url\n+\n+    def value_for_form(self, value):\n+        # the value to be handled by the URLField is a plain URL string (or the empty string)\n+        if value is None:\n+            return ''\n+        else:\n+            return value.url\n+\n+    def value_from_form(self, value):\n+        # convert the value returned from the form (a URL string) to an EmbedValue (or None)\n+        if not value:\n+            return None\n+        else:\n+            return EmbedValue(value)\n", "issue": "StreamField EmbedBlock renders the url only\nGiven the following setup:\n\n```\nclass EmbedOnImage(StructBlock):\n    embed = EmbedBlock()\n    background_image = ImageChooserBlock(required=False)\n\n    class Meta:\n        template = 'streamfield_blocks/embed_on_image.html'\n```\n\n`{{ self.embed }}` outputs the embed url only in `streamfield_blocks/embed_on_image.html`.\n\nAnd `EmbedBlock.render_basic` is never called: https://github.com/torchbox/wagtail/blob/master/wagtail/wagtailembeds/blocks.py#L8\n\nAlso, if I apply the embed filter too: `{{ self.embed|embed }}` then it outputs nothing because the embed filter won't call `embed_to_frontend_html` either, which would use the correct template to render it: https://github.com/torchbox/wagtail/blob/master/wagtail/wagtailembeds/templatetags/wagtailembeds_tags.py#L14\n\n", "before_files": [{"content": "from wagtail.wagtailcore import blocks\n\nfrom wagtail.wagtailembeds.format import embed_to_frontend_html\n\n\nclass EmbedBlock(blocks.URLBlock):\n    def render_basic(self, value):\n        return embed_to_frontend_html(value)\n", "path": "wagtail/wagtailembeds/blocks.py"}], "after_files": [{"content": "from django.utils.encoding import python_2_unicode_compatible\n\nfrom wagtail.wagtailcore import blocks\n\nfrom wagtail.wagtailembeds.format import embed_to_frontend_html\n\n\n@python_2_unicode_compatible\nclass EmbedValue(object):\n    \"\"\"\n    Native value of an EmbedBlock. Should, at minimum, have a 'url' property\n    and render as the embed HTML when rendered in a template.\n    NB We don't use a wagtailembeds.model.Embed object for this, because\n    we want to be able to do {{ value.url|embed:max_width=500 }} without\n    doing a redundant fetch of the embed at the default width.\n    \"\"\"\n    def __init__(self, url):\n        self.url = url\n\n    def __str__(self):\n        return embed_to_frontend_html(self.url)\n\n\nclass EmbedBlock(blocks.URLBlock):\n    def get_default(self):\n        # Allow specifying the default for an EmbedBlock as either an EmbedValue or a string (or None).\n        if not self.meta.default:\n            return None\n        elif isinstance(self.meta.default, EmbedValue):\n            return self.meta.default\n        else:\n            # assume default has been passed as a string\n            return EmbedValue(self.meta.default)\n\n    def to_python(self, value):\n        # The JSON representation of an EmbedBlock's value is a URL string;\n        # this should be converted to an EmbedValue (or None).\n        if not value:\n            return None\n        else:\n            return EmbedValue(value)\n\n    def get_prep_value(self, value):\n        # serialisable value should be a URL string\n        if value is None:\n            return ''\n        else:\n            return value.url\n\n    def value_for_form(self, value):\n        # the value to be handled by the URLField is a plain URL string (or the empty string)\n        if value is None:\n            return ''\n        else:\n            return value.url\n\n    def value_from_form(self, value):\n        # convert the value returned from the form (a URL string) to an EmbedValue (or None)\n        if not value:\n            return None\n        else:\n            return EmbedValue(value)\n", "path": "wagtail/wagtailembeds/blocks.py"}]}