Add reload() command for interactive scene reloading (#2240)

* Init reload command (lots of things not working yet)

* Add back in class line (accidentally deleted)

* Add back in key modifiers (accidentally deleted)

* Unpack tuple from changed `get_module`

* Init MainRunManager & respawn IPython shell

* Init cleanup of scenes from manager

* Restore string quotes

* Still take `self.preview` into account

* Remove left-over code from module experimentation

* Remove double window activation

* Reset scenes array in RunManager

* Move self.args None check up

* Use first available window

* Don't use constructor for RunManager

* Use self. syntax

* Init moderngl context manually

* Add some comments for failed attempts to reset scene

* Reuse existing shell (this fixed the bug 🎉)

* Remove unused code

* Remove unnecessary intermediate ReloadSceneException

* Allow users to finally exit

* Rename main_run_manager to reload_manager

* Add docstrings to `ReloadManager`

* Improve reset management in window

* Clarify why we use magic exit_raise command

* Add comment about window reuse

* Improve docstrings in ReloadManager & handle case of 0 scenes

* Set scene and title earlier

* Run linter suggestions

manimlib/reload_manager.py

@@ -0,0 +1,84 @@
+from typing import Any
+from IPython.terminal.embed import KillEmbedded
+
+
+class ReloadManager:
+    """
+    Manages the loading and running of scenes and is called directly from the
+    main entry point of ManimGL.
+
+    The name "reload" comes from the fact that this class handles the
+    reinitialization of scenes when requested by the user via the `reload()`
+    command in the IPython shell.
+    """
+
+    args: Any = None
+    scenes: list[Any] = []
+    window = None
+
+    # The line number to load the scene from when reloading
+    start_at_line = None
+
+    def set_new_start_at_line(self, start_at_line):
+        """
+        Sets/Updates the line number to load the scene from when reloading.
+        """
+        self.start_at_line = start_at_line
+
+    def run(self):
+        """
+        Runs the scenes in a loop and detects when a scene reload is requested.
+        """
+        while True:
+            try:
+                # blocking call since a scene will init an IPython shell()
+                self.retrieve_scenes_and_run(self.start_at_line)
+                return
+            except KillEmbedded:
+                # Requested via the `exit_raise` IPython runline magic
+                # by means of our scene.reload() command
+                print("Reloading...")
+
+                for scene in self.scenes:
+                    scene.tear_down()
+
+                self.scenes = []
+
+            except KeyboardInterrupt:
+                break
+
+    def retrieve_scenes_and_run(self, overwrite_start_at_line: int | None = None):
+        """
+        Creates a new configuration based on the CLI args and runs the scenes.
+        """
+        import manimlib.config
+        import manimlib.extract_scene
+
+        # Args
+        if self.args is None:
+            raise RuntimeError("Fatal error: No args were passed to the ReloadManager")
+        if overwrite_start_at_line is not None:
+            self.args.embed = str(overwrite_start_at_line)
+
+        # Args to Config
+        config = manimlib.config.get_configuration(self.args)
+        if self.window:
+            config["existing_window"] = self.window  # see scene initialization
+
+        # Scenes
+        self.scenes = manimlib.extract_scene.main(config)
+        if len(self.scenes) == 0:
+            print("No scenes found to run")
+            return
+
+        # Find first available window
+        for scene in self.scenes:
+            if scene.window is not None:
+                self.window = scene.window
+                break
+
+        for scene in self.scenes:
+            scene.run()
+
+
+reload_manager = ReloadManager()