Added docs for subclassing and lifecycle hooks

docs/source/extensions/lifecycle_hooks.rst

@@ -0,0 +1,53 @@
+Lifecycle Hooks
+===============
+
+Introduction
+------------
+In some cases it is useful to have functions triggered by events in an extensions lifecycle. Currently two such lifecycle events can be used, ``on_register``, and ``on_component``.
+
+``on_register``
+---------------
+
+The ``on_register`` method can be used to have a function call as soon as the extension has been successfully registered to the microscope. For example:
+
+.. code-block:: python
+
+    class MyExtension(BaseExtension):
+        def __init__(self):
+
+            # Track if the extension has been registered
+            self.registered = False
+
+            # Add lifecycle hooks
+            self.on_register(self.on_register_handler, args=(), kwargs={})
+
+            # Superclass init function
+            super().__init__("com.myname.myextension", version="0.0.0")
+
+        def on_register_handler(self, *args, **kwargs):
+            self.registered = True
+            print("Extension has been registered!")
+
+
+``on_component``
+----------------
+
+The ``on_component`` method can be used to have a function call as soon as a particular LabThings component has been added. This can be used, for example, to get information about the microscope instance as soon as it is available. For example:
+
+.. code-block:: python
+
+    class MyExtension(BaseExtension):
+        def __init__(self):
+
+            # Hold a reference to the microscope object as soon as it is available
+            self.microscope = None
+
+            # Add lifecycle hooks
+            self.on_component("com.myname.myextension", self.on_microscope_handler)
+
+            # Superclass init function
+            super().__init__("org.openflexure.microscope", version="0.0.0")
+
+        def on_microscope_handler(self, microscope_object):
+            print("Microscope object has been found!")
+            self.microscope = microscope_object