[doc] Improve HOWTO_MODULE

doc/HOWTO_MODULE.md

@@ -17,10 +17,16 @@ modules, etc.
 When you feel comfortable with what you've developed, please just open
 a PR, somebody will look at it eventually :) Thanks!
 
+## Coding guidelines
+I'm pretty open to whatever style you use, but if it's all the same to you
+(and yes, I know that the current codebase is only slowly adapting to this):
+- Please favour single quotes for strings (except for docstrings, which are always """)
+- For private methods/variables, please use a leading `__` (e.g. `__output` rather than `_output`)
+
 ## Hello world
 This example will show "hello world" in the status bar:
 
-```
+```python
 """Short description"""
 
 import core.module
@@ -31,13 +37,79 @@ class Module(core.module.Module):
         super().__init__(config, core.widget.Widget(self.full_text))
 
     def full_text(self, widgets):
-        return "hello world"
+        return 'hello world'
 
 # vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4
 ```
 
-## `update` vs. `full_text`
+## Of modules and widgets
-TODO
+There are two important concepts for module writers:
+- A module is something that offers a single set of coherent functionality
+- A module has 1 to n "widgets", which translates to individual blocks in the i3bar
+
+Very often, this is a 1:1 relationship, and a single module has a single widget. If that's the
+case for you, you can stop reading now :)
+
+Otherwise, you have a number of ways to handle widgets:
+- During the `super().init__(...)` inside the module's constructor, you can specify a **list**
+  of widgets, and those will comprise the widgets (in ordered fashion)
+- During runtime, you can set a new list of widgets by using the `self.widgets(<new list>)`
+  method of the module
+
+TODO: expand on this
+
+## Periodic updates
+`bumblebee-status` modules have two different ways to update their data:
+1. Each interval, the callback registered when the widget was created is called. You can do arbitrarily complex things there
+2. Each interval, **before** the widget's callback is invoked, a generic `update(self, widgets)` method is called on the **module**
+
+Largely, where you want to put your update code is up to you. My observations:
+- If you want to change the widgets a module has, you **have** to stick with `update()`
+- For simple modules, doing the data update in the widget callback is simplest (see `kernel`, for example)
+
+## Advanced topics
+### Event handlers
+The `core.input` module can be used to execute callbacks during mouse events:
+```python
+import core.module
+import core.widget
+import core.input
+
+class Module(core.module.Module):
+    @core.decorators.every(minutes=60, seconds=20)
+    def __init__(self, config):
+        super().__init__(config=config, widgets=<widgets>)
+
+		core.input.register(widget, button=core.input.LEFT_MOUSE, cmd=<cmd>)
+```
+
+The command can be either a CLI tool that will be directly executed (e.g. `cmd='shutdown -h now'`)
+or a method that will be executed. The method's signature needs to be: `def <name>(self, event)`,
+where "event" is the event data provided by i3wm.
+
+The full list of possible bindings:
+- LEFT_MOUSE
+- RIGHT_MOUSE
+- MIDDLE_MOUSE
+- WHEEL_UP
+- WHEEL_UP
+
+### Setting a default update interval
+To change the default update interval, you can use a simple decorator:
+```python
+import core.module
+import core.widget
+import core.decorators
+
+class Module(core.module.Module):
+    @core.decorators.every(minutes=60, seconds=20)
+    def __init__(self, config):
+        super().__init__(config=config, widgets=<widgets>)
+```
+
+**NOTE**: This makes the update interval of the module independent of what the
+user configures via `-i <interval>`! It is still possible to override the module's
+interval using `-p <module>.interval=<value>`, however.
 
 ## TODOs
 - default update interval