Label plugin refactor (#5251)

* Add skeleton for builtin label printing plugin

* Force selection of plugin when printing labels

* Enhance LabelPrintingMixin class

- Add render_to_pdf method
- Add render_to_html method

* Enhance plugin mixin

- Add class attribute to select blocking or non-blocking printing
- Add render_to_png method
- Add default method for printing multiple labels
- Add method for offloding print job

* Simplify print_label background function

- All arguments now handled by specific plugin

* Simplify label printing API

- Simply pass data to the particular plugin
- Check result type
- Return result

* Updated sample plugin

* Working on client side code

* Cleanup

* Update sample plugin

* Add new model type

- LabelOutput model
- Stores generated label file to the database
- Makes available for download

* Update label printing plugin mixin

* Add background task to remove any old label outputs

* Open file if response contains filename

* Remove "default printer" option which does not specify a plugin

* Delete old labels after 5 days

* Remove debug statements

* Update API version

* Changed default behaviour to background printing

* Update label plugin mixin docs

* Provide default printer if none provided (legacy)

* Update unit test

* unit test updates

* Further fixes for unit tests

* unit test updates

InvenTree/plugin/builtin/labels/inventree_label.py

@@ -0,0 +1,96 @@
+"""Default label printing plugin (supports PDF generation)"""
+
+from django.core.files.base import ContentFile
+from django.http import JsonResponse
+from django.utils.translation import gettext_lazy as _
+
+from label.models import LabelOutput, LabelTemplate
+from plugin import InvenTreePlugin
+from plugin.mixins import LabelPrintingMixin, SettingsMixin
+
+
+class InvenTreeLabelPlugin(LabelPrintingMixin, SettingsMixin, InvenTreePlugin):
+    """Builtin plugin for label printing.
+
+    This plugin merges the selected labels into a single PDF file,
+    which is made available for download.
+    """
+
+    NAME = "InvenTreeLabel"
+    TITLE = _("InvenTree PDF label printer")
+    DESCRIPTION = _("Provides native support for printing PDF labels")
+    VERSION = "1.0.0"
+    AUTHOR = _("InvenTree contributors")
+
+    BLOCKING_PRINT = True
+
+    SETTINGS = {
+        'DEBUG': {
+            'name': _('Debug mode'),
+            'description': _('Enable debug mode - returns raw HTML instead of PDF'),
+            'validator': bool,
+            'default': False,
+        },
+    }
+
+    def print_labels(self, label: LabelTemplate, items: list, request, **kwargs):
+        """Handle printing of multiple labels
+
+        - Label outputs are concatenated together, and we return a single PDF file.
+        - If DEBUG mode is enabled, we return a single HTML file.
+        """
+
+        debug = self.get_setting('DEBUG')
+
+        outputs = []
+        output_file = None
+
+        for item in items:
+
+            label.object_to_print = item
+
+            outputs.append(self.print_label(label, request, debug=debug, **kwargs))
+
+        if self.get_setting('DEBUG'):
+            html = '\n'.join(outputs)
+
+            output_file = ContentFile(html, 'labels.html')
+        else:
+            pages = []
+
+            # Following process is required to stitch labels together into a single PDF
+            for output in outputs:
+                doc = output.get_document()
+
+                for page in doc.pages:
+                    pages.append(page)
+
+            pdf = outputs[0].get_document().copy(pages).write_pdf()
+
+            # Create label output file
+            output_file = ContentFile(pdf, 'labels.pdf')
+
+        # Save the generated file to the database
+        output = LabelOutput.objects.create(
+            label=output_file,
+            user=request.user
+        )
+
+        return JsonResponse({
+            'file': output.label.url,
+            'success': True,
+            'message': f'{len(items)} labels generated'
+        })
+
+    def print_label(self, label: LabelTemplate, request, **kwargs):
+        """Handle printing of a single label.
+
+        Returns either a PDF or HTML output, depending on the DEBUG setting.
+        """
+
+        debug = kwargs.get('debug', self.get_setting('DEBUG'))
+
+        if debug:
+            return self.render_to_html(label, request, **kwargs)
+        else:
+            return self.render_to_pdf(label, request, **kwargs)