Add an :option:launch --watcher option that allows defining callbacks that are called for various events in the window's life-cycle

Fixes #2440

kitty/launch.py

@@ -6,14 +6,15 @@
 from functools import lru_cache
 from typing import Any, Dict, List, Optional, Sequence, Tuple
 
+from .boss import Boss
 from .child import Child
 from .cli import parse_args
 from .cli_stub import LaunchCLIOptions
+from .constants import resolve_custom_file
 from .fast_data_types import set_clipboard_string
-from .utils import set_primary_selection
-from .window import Window
-from .boss import Boss
 from .tabs import Tab
+from .utils import set_primary_selection
+from .window import Watchers, Window
 
 try:
     from typing import TypedDict
@@ -148,6 +149,14 @@ defaults to :code:`kitty`.
 --os-window-name
 Set the WM_NAME property on X11 for the newly created OS Window when using
 :option:`launch --type`=os-window. Defaults to :option:`launch --os-window-class`.
+
+
+--watcher -w
+type=list
+Path to a python file. Appropriately named functions in this file will be called
+for various events, such as when the window is resized or closed. See the section
+on watchers in the launch command documentation :doc:`launch`. Relative paths are
+resolved relative to the kitty config directory.
 '''
 
 
@@ -192,6 +201,23 @@ def tab_for_window(boss: Boss, opts: LaunchCLIOptions, target_tab: Optional[Tab]
     return tab
 
 
+def load_watch_modules(opts: LaunchCLIOptions) -> Optional[Watchers]:
+    if not opts.watcher:
+        return None
+    import runpy
+    ans = Watchers()
+    for path in opts.watcher:
+        path = resolve_custom_file(path)
+        m = runpy.run_path(path, run_name='__kitty_watcher__')
+        w = m.get('on_close')
+        if callable(w):
+            ans.on_close.append(w)
+        w = m.get('on_resize')
+        if callable(w):
+            ans.on_resize.append(w)
+    return ans
+
+
 class LaunchKwds(TypedDict):
 
     allow_remote_control: bool
@@ -274,7 +300,8 @@ def launch(boss: Boss, opts: LaunchCLIOptions, args: List[str], target_tab: Opti
     else:
         tab = tab_for_window(boss, opts, target_tab)
         if tab is not None:
-            new_window: Window = tab.new_window(env=env or None, **kw)
+            watchers = load_watch_modules(opts)
+            new_window: Window = tab.new_window(env=env or None, watchers=watchers or None, **kw)
             if opts.keep_focus and active:
                 boss.set_active_window(active, switch_os_window_if_needed=True)
             return new_window