Add a plugin API for ticket outputs

doc/development/api/index.rst

@@ -9,4 +9,5 @@ Contents:
    plugins
    restriction
    payment
+   ticketoutput
    general

doc/development/api/payment.rst

@@ -20,11 +20,10 @@ that we'll soon create::
 
     from pretix.base.signals import register_payment_providers
 
-    from .payment import Paypal
-
 
     @receiver(register_payment_providers)
     def register_payment_provider(sender, **kwargs):
+        from .payment import Paypal
         return Paypal
 
 

doc/development/api/ticketoutput.rst

@@ -0,0 +1,70 @@
+.. highlight:: python
+   :linenothreshold: 5
+
+Writing a ticket output plugin
+==============================
+
+A ticket output is a method to offer a ticket (an order) for the user to download.
+
+In this document, we will walk through the creation of a ticket output plugin. This
+is very similar to creating a payment provider.
+
+Please read :ref:`Creating a plugin <pluginsetup>` first, if you haven't already.
+
+Output registration
+-------------------
+
+The payment provider API does not make a lot of usage from signals, however, it
+does use a signal to get a list of all available ticket outputs. Your plugin
+should listen for this signal and return the subclass of ``pretix.base.ticketoutput.BaseTicketOutput``
+that we'll soon create::
+
+    from django.dispatch import receiver
+
+    from pretix.base.signals import register_ticket_outputs
+
+
+    @receiver(register_ticket_outputs)
+    def register_ticket_output(sender, **kwargs):
+        from .ticketoutput import PdfTicketOutput
+        return PdfTicketOutput
+
+
+The output class
+----------------
+
+.. class:: pretix.base.ticketoutput.BaseTicketOutput
+
+   The central object of each ticket output is the subclass of ``BaseTicketOutput``
+   we already mentioned above. In this section, we will discuss it's interface in detail.
+
+   .. py:attribute:: BaseTicketOutput.event
+
+      The default constructor sets this property to the event we are currently
+      working for.
+
+   .. py:attribute:: BaseTicketOutput.settings
+
+      The default constructor sets this property to a ``SettingsSandbox`` object. You can
+      use this object to store settings using its ``get`` and ``set`` methods. All settings
+      you store are transparently prefixed, so you get your very own settings namespace.
+
+   .. autoattribute:: identifier
+
+      This is an abstract attribute, you **must** override this!
+
+   .. autoattribute:: verbose_name
+
+      This is an abstract attribute, you **must** override this!
+
+   .. autoattribute:: is_enabled
+
+   .. autoattribute:: settings_form_fields
+
+   .. automethod:: settings_content_render
+
+   .. automethod:: generate
+
+   .. autoattribute:: download_button_text
+
+   .. autoattribute:: download_button_icon