[UIColor colorNamed:] 仅返回浅色外观颜色，即使 traitCollection.userInterfaceStyle = Dark

Question

I am working on adding iOS 13 Dark Mode support to my iOS 11+ app. The works great using named / dynamic colors all over the app.

However, when using [UIColor colorNamed:] in a custom class always the light color version ( #ffffff / white) is returned instead of the dark version ( #000000 / black):

// Some ViewController
CustomClass *custom = [customClass alloc] initWithTraitCollection:self.traitCollection]; 

- (void)traitCollectionDidChange:(UITraitCollection *)previousTraitCollection {
    [super traitCollectionDidChange:previousTraitCollection];
    custom.traitCollection = self.traitCollection;
}


// CustomClass
- (void)initWithTraitCollection:(UITraitCollection *)traitCollection {
    self = [super init];
    if (self) {
        self.traitCollection = traitCollection;
    }
    return self;
}

- (void)doSomething {    
    NSLog(@"CustomClass UIStyle: %@", (self.traitCollection.userInterfaceStyle == UIUserInterfaceStyleDark ? @"dark" : (self.traitCollection.userInterfaceStyle == UIUserInterfaceStyleLight ? @"light" : @"unspecified")));

    // Trying to get the color in three different ways (normal + specify traitCollection explicitly)
    UIColor *color1 = [UIColor colorNamed:@"whiteOrBlack"];
    UIColor *color2 = [UIColor colorNamed:@"whiteOrBlack" inBundle:nil compatibleWithTraitCollection:self.traitCollection];

    __block UIColor *color3 = color1;
    [self.traitCollection performAsCurrentTraitCollection:^{
         color3 = [UIColor colorNamed:@"whiteOrBlack"];
    }];

    // Output
    NSLog(@"   color1: %@", [self colorAsHexString:color1]);
    NSLog(@"   color2: %@", [self colorAsHexString:color2]);
    NSLog(@"   color3: %@", [self colorAsHexString:color3]);
}


// Output
CustomClass UIStyle: dark
   #ffffff
   #ffffff
   #ffffff

It does not matter if I specify the traitCollection / UIUserInterfaceStyle explictly or not. Only the light color is returned, even if UIUserInterfaceStyleDark is active.

Am I missing something?

Is there any other way so explicitly specify which color value I would like to access?

Answer 1

The UIColor that you are fetching from the asset catalog is dynamic . That means that the values of its RGB components depend on the value of [UITraitCollection currentTraitCollection] . Each time you ask for the components, the color resolves itself based on the currentTraitCollection .

You didn't show the implementation of your -colorAsHexString: method, but it must be getting the RGB components from the color, somehow.

Therefore, you want to call -colorAsHexString: at a time when you have set the currentTraitCollection , like this:

    UIColor *dynamicColor = [UIColor colorNamed:@"whiteOrBlack"];

    [self.traitCollection performAsCurrentTraitCollection:^{
         NSLog(@"Color: %@", [self colorAsHexString:dynamicColor]);
    }];

(Even better, you could put the call to performAsCurrentTraitCollection inside the implementation of -colorAsHexString: , if that makes sense in your specific case.)

Here's how to get a resolved, non-dynamic color for a specific trait collection:

    UIColor *dynamicColor = [UIColor colorNamed:@"whiteOrBlack"];
    UIColor *resolvedColor = [dynamicColor resolvedColorWithTraitCollection:self.traitCollection];