Callback

The embarker.callback module is a small event system used to hook plugin code into application lifecycle events.

from embarker import callback

Registering callbacks

A plugin module usually declares a module-level CALLBACKS dict mapping an event constant to a list of functions:

def on_session_opened():
    print('Session opened')

CALLBACKS = {
    callback.AFTER_OPEN_SESSION: [on_session_opened],
}

When the plugin is loaded, PluginRegistry registers each function under the plugin's own id automatically, calling register_callback internally.

register_callback(event, plugin_id, function)

Registers function to be called when event is performed. Raises ValueError if event is not one of the constants below.

unregister_callbacks(plugin_id)

Removes every callback registered by plugin_id, for every event. Called automatically when a plugin is unloaded.

perform(event, plugin_id=None, *args, **kwargs)

Calls every function registered for event. If plugin_id is provided, only the callback(s) registered under that exact plugin_id are called. Any extra args/kwargs are forwarded to the callback function(s).

Available callback events

Constant	Value	Triggered by	Arguments passed to the callback
BEFORE_NEW_SESSION	'before_new_session'	commands.new_session(), before the session state is cleared	none
AFTER_NEW_SESSION	'after_new_session'	commands.new_session(), after the session state is cleared	none
BEFORE_OPEN_SESSION	'before_open_session'	commands.open_session(), before the session file is loaded	session_data (dict), as keyword argument
AFTER_OPEN_SESSION	'after_open_session'	commands.open_session() / commands.open_session_data(), once the session has finished loading	none
AFTER_PLAYLIST_CHANGED	'after_playlist_changed'	commands.load_videos(), commands.replace_video(), commands.remove_current_video(), after the playlist content changed	none
ON_APPLICATION_EXIT	'on_application_exit'	MainWindow.closeEvent(), when the application is closing	none
ON_PLUGIN_STARTUP	'on_plugin_startup'	PluginRegistry, once a given plugin has finished loading	none — only the callback(s) registered by that specific plugin are called

Notes: - commands.open_session_data() also performs BEFORE_OPEN_SESSION, but passes its data argument positionally as plugin_id instead of as session_data, so on that code path no callback is currently invoked. - MainWindow.closeEvent() performs ON_APPLICATION_EXIT with the current Session instance as plugin_id, which never matches a plugin's own id, so in practice no callback is currently invoked for this event either. - ON_PLUGIN_STARTUP is always performed with a specific plugin_id, so it behaves as a per-plugin "I just finished loading" notification rather than a broadcast to every registered plugin.