efficient build

- postpone building until really needed
- rebuild only if artifacts change
- no build on source update
- prune takes current resolver state instead of global var

lektor_groupby/backref.py

@@ -0,0 +1,59 @@
+from lektor.context import get_ctx
+from typing import TYPE_CHECKING, Iterator
+from weakref import WeakSet
+if TYPE_CHECKING:
+    from lektor.builder import Builder
+    from lektor.db import Record
+    from .groupby import GroupBy
+    from .vobj import GroupBySource
+
+
+class GroupByRef:
+    @staticmethod
+    def of(builder: 'Builder') -> 'GroupBy':
+        ''' Get the GroupBy object of a builder. '''
+        return builder.__groupby  # type:ignore[attr-defined,no-any-return]
+
+    @staticmethod
+    def set(builder: 'Builder', groupby: 'GroupBy') -> None:
+        ''' Set the GroupBy object of a builder. '''
+        builder.__groupby = groupby  # type: ignore[attr-defined]
+
+
+class VGroups:
+    @staticmethod
+    def of(record: 'Record') -> WeakSet:
+        '''
+        Return the (weak) set of virtual objects of a page.
+        Creates a new set if it does not exist yet.
+        '''
+        try:
+            wset = record.__vgroups  # type: ignore[attr-defined]
+        except AttributeError:
+            wset = WeakSet()
+            record.__vgroups = wset  # type: ignore[attr-defined]
+        return wset  # type: ignore[no-any-return]
+
+    @staticmethod
+    def iter(record: 'Record', *keys: str, recursive: bool = False) \
+            -> Iterator['GroupBySource']:
+        ''' Extract all referencing groupby virtual objects from a page. '''
+        ctx = get_ctx()
+        if not ctx:
+            raise NotImplementedError("Shouldn't happen, where is my context?")
+        # get GroupBy object
+        builder = ctx.build_state.builder
+        groupby = GroupByRef.of(builder)
+        groupby.make_once(builder)  # ensure did cluster before
+        # manage config dependencies
+        for dep in groupby.dependencies:
+            ctx.record_dependency(dep)
+        # find groups
+        proc_list = [record]
+        while proc_list:
+            page = proc_list.pop(0)
+            if recursive and hasattr(page, 'children'):
+                proc_list.extend(page.children)  # type: ignore[attr-defined]
+            for vobj in VGroups.of(page):
+                if not keys or vobj.config.key in keys:
+                    yield vobj