# Extensions

Extension plugins are implemented as **classes**. The initializer should take
only a {class}`StenoEngine<plover.engine.StenoEngine>` as a parameter.

```ini
[options.entry_points]
plover.extension =
  example_extension = plover_my_plugin.extension:Extension
```

```python
# plover_my_plugin/extension.py

class Extension:
  def __init__(self, engine):
    # Called once to initialize an instance which lives until Plover exits.
    self.engine = engine

  def start(self):
    # Called to start the extension or when the user enables the extension.
    # It can be used to start a new thread for example.
    pass

  def stop(self):
    # Called when Plover exits or the user disables the extension.
    pass
```

Extensions can interact with the engine through the
{class}`StenoEngine<plover.engine.StenoEngine>` API, and receive engine
events through its {ref}`engine hooks<engine-hooks>` mechanism.

For example, a simple extension plugin that just writes strokes to a file,
using the {js:func}`stroked<stroked>` hook:

```python
class StrokeLogger:
  def __init__(self, engine):
    self.engine = engine
    self.output_file = None

  def start(self):
    self.output_file = open("strokes.txt")

    # self.on_stroked gets called on every stroke
    self.engine.hook_connect("stroked", self.on_stroked)

  def stop(self):
    self.engine.hook_connect("stroked", self.on_stroked)
    self.output_file.close()

  def on_stroked(self, stroke):
    print(stroke, file=self.output_file)
```