[FR] Refactor plugin registry (#4340)

* add mixin order ref

* move import

* fix import order

* reorder import

* move activation/deactivation to mixins

* move loaded/unloaded mixins out into seperate modules

* fix deactivation sequence

* switch to classmethods for loading

* only run (de)activation if defined for mixin
Fixes #4184

* fix deactivating

* move reloading back to registry

* fix merge error

* move app mixin deactivation

* fix migration reloading

* reverse deactivation sequence

* Revert "reverse deactivation sequence"

This reverts commit aff17dd07d.

InvenTree/plugin/base/integration/SettingsMixin.py

@@ -0,0 +1,75 @@
+"""Plugin mixin class for SettingsMixin."""
+import logging
+
+from django.db.utils import OperationalError, ProgrammingError
+
+logger = logging.getLogger('inventree')
+
+
+class SettingsMixin:
+    """Mixin that enables global settings for the plugin."""
+
+    class MixinMeta:
+        """Meta for mixin."""
+        MIXIN_NAME = 'Settings'
+
+    def __init__(self):
+        """Register mixin."""
+        super().__init__()
+        self.add_mixin('settings', 'has_settings', __class__)
+        self.settings = getattr(self, 'SETTINGS', {})
+
+    @classmethod
+    def _activate_mixin(cls, registry, plugins, *args, **kwargs):
+        """Activate plugin settings.
+
+        Add all defined settings form the plugins to a unified dict in the registry.
+        This dict is referenced by the PluginSettings for settings definitions.
+        """
+        logger.info('Activating plugin settings')
+
+        registry.mixins_settings = {}
+
+        for slug, plugin in plugins:
+            if plugin.mixin_enabled('settings'):
+                plugin_setting = plugin.settings
+                registry.mixins_settings[slug] = plugin_setting
+
+    @classmethod
+    def _deactivate_mixin(cls, registry, **kwargs):
+        """Deactivate all plugin settings."""
+        logger.info('Deactivating plugin settings')
+        # clear settings cache
+        registry.mixins_settings = {}
+
+    @property
+    def has_settings(self):
+        """Does this plugin use custom global settings."""
+        return bool(self.settings)
+
+    def get_setting(self, key, cache=False):
+        """Return the 'value' of the setting associated with this plugin.
+
+        Arguments:
+            key: The 'name' of the setting value to be retrieved
+            cache: Whether to use RAM cached value (default = False)
+        """
+        from plugin.models import PluginSetting
+
+        return PluginSetting.get_setting(key, plugin=self, cache=cache)
+
+    def set_setting(self, key, value, user=None):
+        """Set plugin setting value by key."""
+        from plugin.models import PluginConfig, PluginSetting
+
+        try:
+            plugin, _ = PluginConfig.objects.get_or_create(key=self.plugin_slug(), name=self.plugin_name())
+        except (OperationalError, ProgrammingError):  # pragma: no cover
+            plugin = None
+
+        if not plugin:  # pragma: no cover
+            # Cannot find associated plugin model, return
+            logger.error(f"Plugin configuration not found for plugin '{self.slug}'")
+            return
+
+        PluginSetting.set_setting(key, value, user, plugin=plugin)