Handling trait changes (for dark mode)

Tried the following in Objective C:

UITraitUserInterfaceStyle* style = [[UITraitUserInterfaceStyle alloc] init];

NSArray* traits = [[NSArray alloc] initWithObjects: style, nil];

[self registerForTraitChanges:traits withTarget:self action:@selector(handleStyleChange)];

[traits release];
[style release];

Unfortunately, this crashes in registerForTraitChanges:

UIKitCore`-[UIViewController registerForTraitChanges:withTarget:action:]:

  0x1acce3f90 <+60>:  bl     0x1acf55568               ; +[UITraitCollection _traitTokensIncludingPlaceholdersForTraits:]

-> 0x1acce3f94 <+64>: bl 0x1afc953c0

The beta documentation isn’t super helpful on this (yet) but notice that the UITrait type is defined like this:

typedef Class<UITraitDefinition> UITrait API_AVAILABLE(ios(17.0), tvos(17.0), watchos(10.0));

So -registerForTraitChanges:withTarget:action: takes an array of Classes, not instances of those classes. You can make it like this:

NSArray<UITrait> *traits = @[UITraitUserInterfaceStyle.self];

Note that if you had declared your array type as NSArray<UITrait> * then the compiler would warn about the type error.

[traits release];
[style release];

Also, if you’re working on cleaning up deprecated code, I’d vote to start by converting your app to use ARC. Manual -release has been obsolete for many years.

Hi Scott,

Thank you very much for your input, it now works.

Did not know this notation: UITraitUserInterfaceStyle.self

It seems to be the same as UITraitUserInterfaceStyle.class which is perhaps a more intuitive notation? Anyway, both notations work, super!

I'm mainly programming in C and always a bit afraid that memory is freed behind the scenes. But will give ARC a serious try. Thanks for the suggestion.