s_packaging_calculator: add handy mixin

This mixin can be used to provide qty by packaging features
to any model.

stock_packaging_calculator/models/__init__.py

@@ -1 +1,2 @@
 from . import product
+from . import product_qty_by_packaging_mixin

stock_packaging_calculator/models/product_qty_by_packaging_mixin.py

@@ -0,0 +1,48 @@
+# Copyright 2021 Camptocamp SA
+# @author: Simone Orsi <simone.orsi@camptocamp.com>
+# @author: Sébastien Alix <sebastien.alix@camptocamp.com>
+# License LGPL-3.0 or later (https://www.gnu.org/licenses/lgpl)
+from odoo import api, fields, models
+
+
+class ProductQtyByPackagingMixin(models.AbstractModel):
+    """Allow displaying product qty by packaging.
+    """
+
+    _name = "product.qty_by_packaging.mixin"
+    _description = "Product Qty By Packaging (Mixin)"
+
+    _qty_by_pkg__product_field_name = "product_id"
+    _qty_by_pkg__qty_field_name = None
+
+    product_qty_by_packaging_display = fields.Char(
+        compute="_compute_product_qty_by_packaging_display", string="Qty by packaging"
+    )
+
+    def _product_qty_by_packaging_display_depends(self):
+        depends = []
+        if self._qty_by_pkg__product_field_name:
+            depends.append(self._qty_by_pkg__product_field_name)
+        if self._qty_by_pkg__qty_field_name:
+            depends.append(self._qty_by_pkg__qty_field_name)
+        return depends
+
+    @api.depends_context("lang", "qty_by_pkg_total_units")
+    @api.depends(lambda self: self._product_qty_by_packaging_display_depends())
+    def _compute_product_qty_by_packaging_display(self):
+        include_total_units = self.env.context.get("qty_by_pkg_total_units", False)
+        for record in self:
+            value = ""
+            product = record._qty_by_packaging_get_product()
+            if product:
+                value = product.product_qty_by_packaging_as_str(
+                    record._qty_by_packaging_get_qty(),
+                    include_total_units=include_total_units,
+                )
+            record.product_qty_by_packaging_display = value
+
+    def _qty_by_packaging_get_product(self):
+        return self[self._qty_by_pkg__product_field_name]
+
+    def _qty_by_packaging_get_qty(self):
+        return self[self._qty_by_pkg__qty_field_name]