Document the basic process for creating custom kittens. Fixes #975

docs/index.rst

@@ -265,6 +265,8 @@ Some prominent kittens:
 :doc:`Clipboard <kittens/clipboard>`
     Copy/paste to the clipboard from shell scripts, even over SSH.
 
+You can also :doc:`Learn to create your own kittens <kittens/custom>`.
+
 
 Configuring kitty
 -------------------

docs/kittens/custom.rst

@@ -0,0 +1,80 @@
+Custom kittens
+=================
+
+You can easily create your own kittens to extend kitty. They are just
+terminal programs written in Python. When launching a kitten, kitty will
+open an overlay window over the current window and optionally pass the
+contents of the current window/scrollback to the kitten over its :file:`STDIN`.
+The kitten can then perform whatever actions it likes, just as a normal
+terminal program. After execution of the kitten is complete, it has access
+to the running kitty instance so it can perform arbitrary actions
+such as closing windows, pasting text, etc.
+
+Let's see a simple example of creating a kitten. It will ask the user for some
+input and paste it into the terminal window.
+
+Create a file in the kitty config folder, :file:`~/.config/kitty/mykitten.py`
+(you might need to adjust the path to wherever the kitty config folder is on
+your machine).
+
+
+.. code-block:: python
+
+   def main(args):
+      # this is the main entry point of the kitten, it will be executed in
+      # the overlay window when the kitten is launched
+      answer = input('Enter some text: ')
+      # whatever this function returns will be available in the
+      # handle_result() function
+      return answer
+
+   def handle_result(args, answer, target_window_id, boss):
+      # get the kitty window into which to paste answer
+      w = boss.window_id_map.get(target_window_id)
+      if w is not None:
+         w.paste(answer)
+
+
+Now in :file:`kitty.conf` add the lines::
+
+   map ctrl+k kitten mykitten.py
+
+
+Start kitty and press :kbd:`ctrl+k` and you should see the kitten running.
+The best way to develop your own kittens is to modify one of the built in
+kittens. Look in the kittens sub-directory of the kitty source code for those.
+
+
+Passing arguments to kittens
+------------------------------
+
+You can pass arguments to kittens by defining them in the map directive in
+:file:`kitty.conf`. For example::
+
+   map ctrl+k kitten mykitten.py arg1 arg2
+
+These will be available as the ``args`` parameter in the ``main()`` and
+``handle_result()`` functions. Note also that the current working directory
+of the kitten is set to the working directory of whatever program is
+running in the active kitty window.
+
+
+Passing the contents of the screen to the kitten
+---------------------------------------------------
+
+If you would like your kitten to have access to the contents of the screen
+and/or the scrollback buffer, you just need to add an annotation to the ``handle_result()``
+function, telling kitty what kind of input your kitten would like. For example:
+
+.. code-block:: py
+
+   def handle_result(...):
+      pass
+
+   handle_result.type_of_input = 'text'
+
+This will send the plain text of the active window to the kitten's
+:file:`STDIN`. For text with formatting escape codes, use ``ansi``
+instead. If you want line wrap markers as well, use ``screen-ansi``
+or just ``screen``. For the scrollback buffer as well, use
+``history``, ``ansi-history`` or ``screen-history``.