Added docs for subclassing and lifecycle hooks

docs/source/extensions/example_extension/01b_basic_structure_subclass.py

@@ -0,0 +1,41 @@
+from labthings.server.extensions import BaseExtension
+from labthings.server.find import find_component
+
+
+# Create the extension class
+class MyExtension(BaseExtension):
+    def __init__(self):
+
+        # Create some instance variable
+        self.state_variable = "An example of a persistant instance variable"
+
+        # Superclass init function
+        super().__init__("com.myname.myextension", version="0.0.0")
+
+    def identify(self):
+        """
+        Demonstrate access to Microscope.camera, and Microscope.stage
+        """
+        microscope = find_component("org.openflexure.microscope")
+
+        response = (
+            f"My name is {microscope.name}. "
+            f"My parent camera is {microscope.camera}, "
+            f"and my parent stage is {microscope.stage}."
+        )
+
+        return response
+
+    def rename(self, new_name):
+        """
+        Rename the microscope
+        """
+
+        microscope = find_component("org.openflexure.microscope")
+
+        microscope.name = new_name
+        microscope.save_settings()
+
+
+# Create your extension object
+my_extension = MyExtension()

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

docs/source/extensions/structure.rst

@@ -24,3 +24,13 @@ Once this extension is loaded, any other extensions will have access to your met
         # Call a function from your extension
         if my_found_extension:
             my_found_extension.identify()
+
+
+Subclassing ``BaseExtension``
+-------------------------------
+
+The syntax used above allows novice programmers to easily start building extensions, without having to deal with subclassing. However, for more complex extensions which require persistent state, subclassing :py:class:`labthings.server.extensions.BaseExtension` is recommended.
+
+The same simple extension as seen above can be written using subclassing:
+
+.. literalinclude:: ./example_extension/01b_basic_structure_subclass.py

docs/source/plugins.rst

@@ -12,3 +12,4 @@ Developing API Extensions
    ./extensions/actions.rst
    ./extensions/tasks_locks.rst
    ./extensions/ev_gui.rst
+   ./extensions/lifecycle_hooks.rst