Extend ReportMixin class to support labels (#4678)

* Extend ReportMixin context to shim label templates also

* Update docs

InvenTree/label/models.py

@@ -18,6 +18,7 @@ import part.models
 import stock.models
 from InvenTree.helpers import normalize, validateFilterString
 from InvenTree.models import MetadataMixin
+from plugin.registry import registry
 
 try:
     from django_weasyprint import WeasyTemplateResponseMixin
@@ -190,6 +191,13 @@ class LabelTemplate(MetadataMixin, models.Model):
         context['width'] = self.width
         context['height'] = self.height
 
+        # Pass the context through to any registered plugins
+        plugins = registry.with_mixin('report')
+
+        for plugin in plugins:
+            # Let each plugin add its own context data
+            plugin.add_label_context(self, self.object_to_print, request, context)
+
         return context
 
     def render_as_string(self, request, **kwargs):

InvenTree/plugin/base/integration/ReportMixin.py

@@ -34,3 +34,16 @@ class ReportMixin:
             context: The context dictionary to add to
         """
         pass
+
+    def add_label_context(self, label_instance, model_instance, request, context):
+        """Add extra context to the provided label instance.
+
+        By default, this method does nothing.
+
+        Args:
+            label_instance: The label instance to add context to
+            model_instance: The model instance which initiated the label generation
+            request: The request object which initiated the label generation
+            context: The context dictionary to add to
+        """
+        pass