feat(Library): add app lifecycle hooks

core/systems/launcher/app_lifecycle_hook.gd

@@ -0,0 +1,39 @@
+extends RefCounted
+class_name AppLifecycleHook
+
+## Base class for executing callbacks at certain points of an app's lifecycle.
+##
+## This class provides an interface for executing arbitrary callbacks at certain
+## points of an application's lifecycle. This can allow [Library] implementations
+## the ability to execute actions when apps are about to start, have started,
+## or have exited.
+
+## The type of hook determines where in the application's lifecycle this hook
+## should be executed.
+enum TYPE {
+	## Executes right before an app is launched
+	PRE_LAUNCH,
+	## Executes right after an app is launched
+	LAUNCH,
+	## Executed after app exit
+	EXIT,
+}
+
+var _hook_type: TYPE
+
+
+func _init(hook_type: TYPE) -> void:
+	_hook_type = hook_type
+
+
+## Executes whenever an app from this library reaches the stage in its lifecycle
+## designated by the hook type. E.g. a `PRE_LAUNCH` hook will have this method
+## called whenever an app is about to launch.
+func execute(item: LibraryLaunchItem) -> void:
+	pass
+
+
+## Returns the hook type, which designates where in the application's lifecycle
+## the hook should be executed.
+func get_type() -> TYPE:
+	return _hook_type

core/systems/library/library.gd

@@ -94,5 +94,13 @@ func has_update(item: LibraryLaunchItem) -> bool:
 	return false
 
 
+## This method should be overridden if the library requires executing callbacks
+## at certain points in an app's lifecycle, such as when an app is starting or
+## stopping.
+func get_app_lifecycle_hooks() -> Array[AppLifecycleHook]:
+	var hooks: Array[AppLifecycleHook]
+	return hooks
+
+
 func _exit_tree() -> void:
 	LibraryManager.unregister_library(self)