Implement a macOS listener

README.md

@@ -49,6 +49,13 @@ pip install darkdetect
 
 Alternatively, you are free to vendor directly a copy of Darkdetect in your app. Further information on vendoring can be found [here](https://medium.com/underdog-io-engineering/vendoring-python-dependencies-with-pip-b9eb6078b9c0).
 
+## Optional Installs
+
+To enable the macOS listener, additional components are required, these can be installed via:
+```bash
+pip install darkdetect[macos-listener]
+```
+
 ## Notes
 
 - This software is licensed under the terms of the 3-clause BSD License.

darkdetect/_mac_detect.py

@@ -6,6 +6,19 @@
 
 import ctypes
 import ctypes.util
+import subprocess
+import sys
+import os
+from pathlib import Path
+from typing import Callable
+
+try:
+    from Foundation import NSObject, NSKeyValueObservingOptionNew, NSKeyValueChangeNewKey, NSUserDefaults
+    from PyObjCTools import AppHelper
+    _can_listen = True
+except ModuleNotFoundError:
+    _can_listen = False
+
 
 try:
     # macOS Big Sur+ use "a built-in dynamic linker cache of all system-provided libraries"
@@ -69,6 +82,43 @@ def isDark():
 def isLight():
     return theme() == 'Light'
 
-#def listener(callback: typing.Callable[[str], None]) -> None:
-def listener(callback):
-    raise NotImplementedError()
+
+def _listen_child():
+    """
+    Run by a child process, install an observer and print theme on change
+    """
+    import signal
+    signal.signal(signal.SIGINT, signal.SIG_IGN)
+
+    OBSERVED_KEY = "AppleInterfaceStyle"
+
+    class Observer(NSObject):
+        def observeValueForKeyPath_ofObject_change_context_(
+            self, path, object, changeDescription, context
+        ):
+            result = changeDescription[NSKeyValueChangeNewKey]
+            try:
+                print(f"{'Light' if result is None else result}", flush=True)
+            except IOError:
+                os._exit(1)
+
+    observer = Observer.new()  # Keep a reference alive after installing
+    defaults = NSUserDefaults.standardUserDefaults()
+    defaults.addObserver_forKeyPath_options_context_(
+        observer, OBSERVED_KEY, NSKeyValueObservingOptionNew, 0
+    )
+
+    AppHelper.runConsoleEventLoop()
+
+
+def listener(callback: Callable[[str], None]) -> None:
+    if not _can_listen:
+        raise NotImplementedError()
+    with subprocess.Popen(
+        (sys.executable, "-c", "import _mac_detect as m; m._listen_child()"),
+        stdout=subprocess.PIPE,
+        universal_newlines=True,
+        cwd=Path(__file__).parent,
+    ) as p:
+        for line in p.stdout:
+            callback(line.strip())

pyproject.toml

@@ -8,15 +8,14 @@ build-backend = "setuptools.build_meta"
 name = "darkdetect"
 description = "Detect OS Dark Mode from Python"
 readme = "README.md"
-requires-python = ">=3"
+requires-python = ">=3.6"
 dynamic = [ "version" ]
 classifiers = [
     "License :: OSI Approved :: BSD License",
     "Operating System :: MacOS :: MacOS X",
     "Operating System :: Microsoft :: Windows :: Windows 10",
     "Operating System :: POSIX :: Linux",
     "Programming Language :: Python :: 3",
-    "Programming Language :: Python :: 3.5",
     "Programming Language :: Python :: 3.6",
     "Programming Language :: Python :: 3.7",
     "Programming Language :: Python :: 3.8",
@@ -35,6 +34,9 @@ email = "asottile@gmail.com"
 homepage = "http://github.com/albertosottile/darkdetect"
 download = "http://github.com/albertosottile/darkdetect/releases"
 
+[project.optional-dependencies]
+macos-listener = [ "pyobjc-framework-Cocoa; platform_system == 'Darwin'" ]
+
 # Tool Config
 
 [tool.setuptools]