How to detect if macOS changed Theme (Dark vs Light) while running?

[Hansaplast]

Not sure how others do this, but I check for Dark Theme during the Form's onPaint event - so the application can change certain things (for example controls that do not respond well to a theme change, or controls that use non-standard colors). This works for all my apps just fine. Well, except for Windows, since Lazarus doesn't change colors when you change theme in Windows.
If this is the correct way to do this; maybe we can add this as a tip to the Wiki page?

[trev]

If this is the correct way to do this; maybe we can add this as a tip to the Wiki page?

I think it's a hack

You mentioned the "correct" method in your earlier 2018 post in this thread... except you didn't manage to get it working It's beyond me too...

In which case, a hack may be better than nothing

[Wallaby]

I know it's an old thread, but I faced the same problem. I needed to adjust certain custom colours when macOS switches to Dark or Light theme.

Here is the code that will receive the event whenever the theme is changed, manually or automatically:

Code: Pascal  [Select][+][-]

1. unit Core.MacOS.DarkMode;
2.  
3. {$MODESWITCH OBJECTIVEC1}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, CocoaAll, MacOsAll;
9.  
10. implementation
11.  
12. type
13.   TThemeChangedNotification = objcclass(NSObject)
14.     procedure ThemeChangedNotification(notification: NSNotification);
15.       message 'ThemeChangedNotification:';
16.   end;
17.  
18. procedure TThemeChangedNotification.ThemeChangedNotification(
19.   notification: NSNotification);
20. begin
21.   // This will be triggered upon theme change
22. end;
23.  
24. var
25.   ThemeChangedNotification: TThemeChangedNotification;
26.   DistributedNotificationCenter: NSDistributedNotificationCenter;
27.  
28. initialization
29.   ThemeChangedNotification := TThemeChangedNotification.alloc.init;
30.   ThemeChangedNotification.retain;
31.   DistributedNotificationCenter := NSDistributedNotificationCenter.defaultCenter;
32.   if Assigned(DistributedNotificationCenter) then
33.     DistributedNotificationCenter.addObserver_selector_name_object(
34.       ThemeChangedNotification, ObjCSelector('ThemeChangedNotification:'),
35.       NSSTR('AppleInterfaceThemeChangedNotification'), nil);
36.  
37. finalization
38.   ThemeChangedNotification.release;
39.  
40. end.