title: generator
---
type: method
---
summary: Registers a custom generator function.
---
body:

This is very experimental API and used to register a function that can
generate source objects relative to another one.  This works in combination
with [urlresolver :ref](../urlresolver/) but handles the build-all part of
the equation.

The registered function is invoked for each source after it was build.  As
such it's important to only return items if a virtual sub resource actually
exists for a page.

## Example

```python
from lektor.sourceobj import VirtualSourceObject
from lektor.db import Record
from lektor.utils import build_url

class Source(VirtualSourceObject):

    @property
    def path(self):
        return self.record.path + '@source'

    @property
    def url_path(self):
        return build_url([self.record.url_path, 'source.txt'])

@env.generator
def generate_source_file(node):
    if isinstance(node, Record) and not node.is_attachment:
        yield Source(node)
```