Allow payment providers to process user input

src/pretix/base/payment.py

@@ -92,3 +92,26 @@ class BasePaymentProvider:
         template = get_template('pretixpresale/event/checkout_payment_form_default.html')
         ctx = Context({'request': request, 'form': form})
         return template.render(ctx)
+
+    def checkout_prepare(self, request, total):
+        """
+        Will be called if the user selects this provider as his payment method.
+        If the payment provider provides a form to the user to enter payment data,
+        this method should at least store the user's input into his session.
+
+        It should return True or False, depending of the validity of the user's input,
+        if the frontend should continue with default behaviour, or a custom HTTP response
+        (for example, a redirect), if you need special behaviour.
+
+        On errors, it should use Django's message framework to display an error message
+        to the user (or the normal form validation error messages).
+
+        :param total: The total price of the order, including the payment method fee.
+        """
+        form = self.checkout_form(request)
+        if form.is_valid():
+            for k, v in form.cleaned_data.items():
+                request.session['payment_%s_%s' % (self.identifier, k)] = v
+            return True
+        else:
+            return False

src/pretix/plugins/banktransfer/payment.py

@@ -23,3 +23,6 @@ class BankTransfer(BasePaymentProvider):
         template = get_template('pretixplugins/banktransfer/checkout_payment_form.html')
         ctx = Context({'request': request, 'event': self.event, 'settings': self.settings})
         return template.render(ctx)
+
+    def checkout_prepare(self, request, total):
+        return True

src/pretix/presale/templates/pretixpresale/event/checkout_payment.html

@@ -21,7 +21,8 @@
                             </label>
                         </h4>
                     </div>
-                    <div id="payment_{{ p.provider.identifier }}" class="panel-collapse collapse">
+                    <div id="payment_{{ p.provider.identifier }}"
+                         class="panel-collapse collapse {% if request.POST.payment == p.provider.identifier %}in{% endif %}">
                         <div class="panel-body form-horizontal">
                             {{ p.form }}
                         </div>

src/pretix/presale/views/checkout.py

@@ -6,6 +6,7 @@ from django.shortcuts import redirect
 from django.utils.functional import cached_property
 from django.views.generic import View, TemplateView
 from django.utils.translation import ugettext_lazy as _
+from django.http import HttpResponse
 from pretix.base.models import CartPosition, Question, QuestionAnswer
 from pretix.base.signals import register_payment_providers
 
@@ -204,17 +205,20 @@ class PaymentDetails(EventViewMixin, CartDisplayMixin, EventLoginRequiredMixin,
         })
 
     @cached_property
-    def provider_forms(self):
-        total = CartPosition.objects.current.filter(
+    def _total_order_value(self):
+        return CartPosition.objects.current.filter(
             Q(user=self.request.user) & Q(event=self.request.event)
         ).aggregate(sum=Sum('price'))['sum']
+
+    @cached_property
+    def provider_forms(self):
         providers = []
         responses = register_payment_providers.send(self.request.event)
         for receiver, response in responses:
             provider = response(self.request.event)
             if not provider.is_enabled:
                 continue
-            fee = provider.calculate_fee(total)
+            fee = provider.calculate_fee(self._total_order_value)
             providers.append({
                 'provider': provider,
                 'fee': fee,
@@ -222,6 +226,18 @@ class PaymentDetails(EventViewMixin, CartDisplayMixin, EventLoginRequiredMixin,
             })
         return providers
 
+    def post(self, request, *args, **kwargs):
+        for p in self.provider_forms:
+            if p['provider'].identifier == request.POST.get('payment', ''):
+                total = self._total_order_value + p['provider'].calculate_fee(self._total_order_value)
+                resp = p['provider'].checkout_prepare(request, total)
+                if isinstance(resp, HttpResponse):
+                    return resp
+                elif resp is True:
+                    return redirect(self.get_success_url())
+                else:
+                    return self.get(request, *args, **kwargs)
+
     def get_context_data(self, **kwargs):
         ctx = super().get_context_data(**kwargs)
         ctx['providers'] = self.provider_forms