[IMP] l10n_it_account_stamp: Usability for stamp in invoice.

If stamp line can be added (invoice in draft): show button to add stamp line.
If stamp line can't be added (invoice not in draft): show message explaining why it can't be added.
If stamp line has already been added: show message saying it has already been added (useful if invoice has many lines).
Button and messages update live while updating the invoice.
Better messages to user for charging stamp to customer

l10n_it_account_stamp/models/account_move.py

@@ -7,7 +7,16 @@
 class AccountMove(models.Model):
     _inherit = "account.move"
 
-    tax_stamp = fields.Boolean(readonly=False, compute="_compute_tax_stamp", store=True)
+    tax_stamp = fields.Boolean(
+        help="Tax stamp is applied to this invoice.",
+        readonly=False,
+        compute="_compute_tax_stamp",
+        store=True,
+    )
+    tax_stamp_line_present = fields.Boolean(
+        string="Stamp line is present in invoice",
+        compute="_compute_tax_stamp_line_present",
+    )
     auto_compute_stamp = fields.Boolean(
         related="company_id.tax_stamp_product_id.auto_compute"
     )
@@ -86,11 +95,21 @@ def add_tax_stamp_line(self):
             inv.write({"invoice_line_ids": [(0, 0, invoice_line_vals)]})
 
     def is_tax_stamp_line_present(self):
+        self.ensure_one()
         for line in self.line_ids:
             if line.is_stamp_line:
                 return True
         return False
 
+    @api.depends(
+        "invoice_line_ids",
+        "invoice_line_ids.product_id",
+        "invoice_line_ids.product_id.is_stamp",
+    )
+    def _compute_tax_stamp_line_present(self):
+        for invoice in self:
+            invoice.tax_stamp_line_present = invoice.is_tax_stamp_line_present()
+
     def is_tax_stamp_product_present(self):
         product_stamp = self.invoice_line_ids.filtered(
             lambda line: line.product_id.is_stamp

l10n_it_account_stamp/views/account_move_view.xml

@@ -23,20 +23,37 @@
                     name="manually_apply_tax_stamp"
                     attrs="{'invisible': [('auto_compute_stamp', '=', True)]}"
                 />
+                <field name="tax_stamp_line_present" invisible="1" />
             </xpath>
             <field name="narration" position="before">
-                <img
-                    src="/l10n_it_account_stamp/static/description/icon.png"
-                    alt="Tax stamp"
+                <div
+                    name="stamp_applicability"
                     attrs="{'invisible': [('tax_stamp', '=', False)]}"
-                />
-                <button
-                    class="oe_edit_only"
-                    type="object"
-                    string="Add tax stamp line"
-                    name="add_tax_stamp_line"
-                    attrs="{'invisible': ['|',('tax_stamp', '=', False),('state', 'not in', 'draft')]}"
-                />
+                >
+                    <img
+                        src="/l10n_it_account_stamp/static/description/icon.png"
+                        alt="Tax stamp"
+                    />
+                    <span
+                        attrs="{'invisible': [('tax_stamp_line_present', '=', True)]}"
+                    >
+                        <span attrs="{'invisible': [('state', 'not in', 'draft')]}">
+                            <button
+                                type="object"
+                                string="Charge stamp to customer"
+                                name="add_tax_stamp_line"
+                            />
+                        </span>
+                        <span attrs="{'invisible': [('state', '=', 'draft')]}">
+                            Stamp can only be charged to customer when invoice is in draft state
+                        </span>
+                    </span>
+                    <span
+                        attrs="{'invisible': [('tax_stamp_line_present', '=', False)]}"
+                    >
+                        Stamp charged to customer
+                    </span>
+                </div>
                 <!--move narration down-->
                 <div />
             </field>