Merge pull request #758 from Carreau/document-extensions

Start documenting how to write frontend extensions

docs/source/extending/frontend_extensions.rst

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+Custom front-end extensions
+===========================
+
+This describes the basic steps to write a JavaScript extension for the Jupyter
+notebook front-end. This allows you to customize the behaviour of the various
+pages like the dashboard, the notebook, or the text editor.
+
+The structure of an extension
+-----------------------------
+
+.. note::
+
+    The notebook frontend and Javascript API are not stable, and are subject to
+    a lot of changes. Any extension written for the current notebook is almost
+    guaranteed to break in the next release.
+
+A front-end extension is a JavaScript file that defines an AMD module
+exposing at least a function called `load_ipython_extension`, which takes no
+arguments. We will not get into the details of what each of these terms are,
+but here is the minimal code needed for a working extension:
+
+.. code:: javascript
+
+    //file myext/main.js
+
+    define(function(){
+
+        function load_ipython_extension(){
+            console.info('this is my first extension')
+        }
+
+        return {load_ipython_extension: load_ipython_extension };
+    })
+
+.. note::
+
+    For historical reasons, the function is called `load_ipython_extension`,
+    but it does apply to the Jupyter notebook as well, and will work
+    regardless of the kernel you use. 
+
+If you are familiar with JavaScript, you can use this template to require any
+Jupyter module and modify its configuration. Your extension will be loaded
+at the right time during the notebook page initialisation for you to set up a
+listener for the various events that the page can trigger.
+
+You might want to access to the current instances of the various component on
+the page, these are exposed by a module named `base/js/namespace`. If you plan
+on accessing instances on the page, you should require this module rather than
+accessing the global variable `Jupyter`. Here is how to do that:
+
+.. code:: javascript
+
+    //file myext/main.js
+
+    define(['base/js/namespace'],function(Jupyter){
+
+        function load_ipython_extension(){
+            console.log('This is the current notebook application instance', Jupyter.notebook);
+        }
+
+        return {load_ipython_extension: load_ipython_extension };
+    })
+
+
+
+Modifying key bindings
+----------------------
+
+Extensions have the ability to modify key bindings. Once again, this API
+is not guaranteed to be stable, but changing key bindings is a common request.
+This is how to do it in the current version of the notebook.
+
+Here is an example of extension that will unbind `0,0` in command mode, which
+normally restarts the kernel, and bind `0,0,0` in it's place.
+
+.. code:: javascript
+
+    //file myext/main.js
+
+    define(['base/js/namespace'],function(Jupyter){
+
+        function load_ipython_extension(){
+            Jupyter.keyboard_manager.command_shortcuts.remove_shortcut('0,0')
+            Jupyter.keyboard_manager.command_shortcuts.add_shortcut('0,0,0', 'jupyter-notebook:restart-kernel')
+        }
+
+        return {load_ipython_extension: load_ipython_extension };
+    })
+
+.. note::
+
+    Keybindings might not work correctly on non-US keyboards.
+    Unfortunately, this is a limitation of browser implementations and the status of
+    keyboard event handling on the web. We appreciate your feedback if you have
+    issues binding keys, or ideas to help improve the situation.
+
+You can see that I have used the **command name**
+`jupyter-notebook:restart-kernel` to bind the new shortcut. There is no API yet
+to access the list of all available *commands*, though the following in the
+JavaScript console of your browser on a notebook page should give you an idea
+of what of available:
+
+.. code:: javascript
+
+    Object.keys(IPython.actions._actions)
+
+In this example, we changed keyboard shortcuts in **command mode**; you can also
+customise keyboard shortcuts in edit mode.
+However, most of the keyboard shortcuts in edit mode are handled by CodeMirror,
+which supports custom key bindings via a completely different API.
+
+You can also define and register your own action to be used, but the
+documentation for this has not been written yet. If you need to do it,
+please ask us, we can give you the necessary information, and we would appreciate
+if you could format them in a detailed way in place of this paragraph.
+
+Installing and enabling extensions
+----------------------------------
+
+You can install your nbextension with the command:
+
+    jupyter nbextension install path/to/myext/
+
+Where myext is the directory containing the Javascript files. This will copy
+it to a Jupyter data directory (the exact location is platform dependent - see
+:ref:`jupyter_path`).
+
+For development, you can use the ``--symlink`` flag to symlink your extension
+rather than copying it, so there's no need to reinstall after changes.
+
+To use your extension, you'll also need to **enable** it, which tells the
+notebook interface to load it. You can do that with another command:
+
+    jupyter nbextension enable myext/main
+
+The argument refers to the Javascript module containing your ``load_ipython_extension``
+function, which is ``myext/main.js`` in this example. There is a corresponding
+``disable`` command to stop using an extension without installing it.