UIColor子类实现状态色：深色模式下Idle颜色不更新问题

问题背景 #

我们通过UIColor子类CUIColorVariant实现状态色，包含hovered、pressed、disabled三个公开属性与私有idle属性（作为实例默认颜色）。原本运行正常，但添加深色模式支持后发现：其他状态色可随trait变化自动更新，仅idle颜色无法更新。

有趣的是，将idle设为公开属性后，它能在深色模式下正常更新，推测问题出在子类内部对idle属性的解析逻辑。

相关实现代码 #

@interface CUIColorVariant : UIColor

@property (nonatomic, readonly) UIColor *hovered;
@property (nonatomic, readonly) UIColor *pressed;
@property (nonatomic, readonly) UIColor *disabled;

- (instancetype)initWithIdle:(UIColor *)idle hovered:(UIColor *)hovered pressed:(UIColor *)pressed disabled:(UIColor *)disabled;

@end


@interface CUIColorVariant ()

@property (nonatomic, strong) UIColor *idle;

@end

@implementation CUIColorVariant

- (instancetype)initWithIdle:(UIColor *)idle hovered:(UIColor *)hovered pressed:(UIColor *)pressed disabled:(UIColor *)disabled {
    self = [super init];
    if (self) {
        _idle = idle;
        _hovered = hovered;
        _pressed = pressed;
        _disabled = disabled;
    }
    return self;
}

#pragma mark - Required UIColor methods

- (CGColorRef)CGColor {
    return self.idle.CGColor;
}

- (CIColor *)CIColor {
    return self.idle.CIColor;
}

- (void)set {
    [self.idle set];
}

- (void)setFill {
    [self.idle setFill];
}

- (void)setStroke {
    [self.idle setStroke];
}

- (UIColor *)colorWithAlphaComponent:(CGFloat)alpha {
    return [self.idle colorWithAlphaComponent:alpha];
}

- (UIColor *)resolvedColorWithTraitCollection:(UITraitCollection *)traitCollection {
    return [self.idle resolvedColorWithTraitCollection:traitCollection];
}

- (BOOL)getWhite:(nullable CGFloat *)white alpha:(nullable CGFloat *)alpha {
    return [self.idle getWhite:white alpha:alpha];
}

- (BOOL)getHue:(nullable CGFloat *)hue saturation:(nullable CGFloat *)saturation brightness:(nullable CGFloat *)brightness alpha:(nullable CGFloat *)alpha {
    return [self.idle getHue:hue saturation:saturation brightness:brightness alpha:alpha];
}

- (BOOL)getRed:(nullable CGFloat *)red green:(nullable CGFloat *)green blue:(nullable CGFloat *)blue alpha:(nullable CGFloat *)alpha {
    return [self.idle getRed:red green:green blue:blue alpha:alpha];
}

#pragma mark - Required NSObject methods

- (BOOL)isEqual:(id)object {
    if ([object isKindOfClass:CUIColorVariant.class]) {
        return [self.idle isEqual:[(CUIColorVariant*)object idle]]
        && [self.hovered isEqual:[(CUIColorVariant*)object hovered]]
        && [self.pressed isEqual:[(CUIColorVariant*)object pressed]]
        && [self.disabled isEqual:[(CUIColorVariant*)object disabled]];
    } else {
        return NO;
    }
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    SEL aSelector = [invocation selector];

    if ([self.idle respondsToSelector:aSelector]) {
        [invocation invokeWithTarget:self.idle];
    } else {
        [super forwardInvocation:invocation];
    }
}

-(NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    if ([self.idle respondsToSelector:aSelector]) {
        return [[self idle] methodSignatureForSelector:aSelector];
    } else {
        return [super methodSignatureForSelector:aSelector];
    }
}

- (NSUInteger)hash {
    return self.idle.hash ^ self.hovered.hash ^ self.pressed.hash ^ self.disabled.hash;
}

- (id)copy {
    return [[CUIColorVariant alloc] initWithIdle:[_idle copy] hovered:[_hovered copy] pressed:[_pressed copy] disabled:[_disabled copy]];
}

- (id)copyWithZone:(NSZone *)zone {
    return [[CUIColorVariant allocWithZone:zone] initWithIdle:[_idle copyWithZone:zone]
                                                      hovered:[_hovered copyWithZone:zone]
                                                      pressed:[_pressed copyWithZone:zone]
                                                     disabled:[_disabled copyWithZone:zone]];
}
@end

关键观察点 #

• 将idle设为公开属性时，可在深色模式下正常更新；
• hovered、pressed、disabled等状态色更新正常；
• idle属性虽动态初始化，但作为子类默认颜色时似乎静态解析；
• hovered、pressed、disabled为UIDynamicCatalogColor实例，idle则不是（除非直接通过属性访问）。

问题根源 #

确实和UIColor的动态解析机制直接相关：

1. UIColor动态解析依赖属性暴露与KVO：系统对动态颜色（如UIDynamicCatalogColor）的trait变化更新，依赖于对颜色属性的KVO监听。当idle是私有属性时，系统无法通过常规路径监听其变化，导致动态颜色无法被正确解析更新。
2. 子类解析方法的错误实现：当前resolvedColorWithTraitCollection:直接返回self.idle的解析结果，相当于丢弃了子类的状态包装，系统无法将子类实例与动态颜色的trait更新逻辑关联。当idle变为公开属性后，系统能识别到这是一个动态颜色属性，从而自动处理trait变化时的更新。

解决方案 #

无需将idle设为公开属性，通过以下调整即可修复：

1. 修正resolvedColorWithTraitCollection:方法 #

当前实现直接返回idle的解析颜色，正确做法是返回一个新的CUIColorVariant实例，确保所有状态色都用传入的traitCollection解析：

- (UIColor *)resolvedColorWithTraitCollection:(UITraitCollection *)traitCollection {
    UIColor *resolvedIdle = [self.idle resolvedColorWithTraitCollection:traitCollection];
    UIColor *resolvedHovered = [self.hovered resolvedColorWithTraitCollection:traitCollection];
    UIColor *resolvedPressed = [self.pressed resolvedColorWithTraitCollection:traitCollection];
    UIColor *resolvedDisabled = [self.disabled resolvedColorWithTraitCollection:traitCollection];
    return [[CUIColorVariant alloc] initWithIdle:resolvedIdle hovered:resolvedHovered pressed:resolvedPressed disabled:resolvedDisabled];
}

2. 调整方法转发逻辑 #

当前的方法转发直接绑定私有idle，导致系统误将子类实例当作idle实例处理。调整为使用当前trait解析后的idle进行转发：

- (void)forwardInvocation:(NSInvocation *)invocation {
    SEL aSelector = [invocation selector];
    UIColor *resolvedIdle = [self.idle resolvedColorWithTraitCollection:[UITraitCollection currentTraitCollection]];
    if ([resolvedIdle respondsToSelector:aSelector]) {
        [invocation invokeWithTarget:resolvedIdle];
    } else {
        [super forwardInvocation:invocation];
    }
}

-(NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    UIColor *resolvedIdle = [self.idle resolvedColorWithTraitCollection:[UITraitCollection currentTraitCollection]];
    if ([resolvedIdle respondsToSelector:aSelector]) {
        return [resolvedIdle methodSignatureForSelector:aSelector];
    } else {
        return [super methodSignatureForSelector:aSelector];
    }
}

3. 添加trait变化回调 #

实现traitCollectionDidChange:方法，在trait变化时手动更新所有状态色：

- (void)traitCollectionDidChange:(UITraitCollection *)previousTraitCollection {
    [super traitCollectionDidChange:previousTraitCollection];
    if (![self.traitCollection hasDifferentColorAppearanceComparedToTraitCollection:previousTraitCollection]) {
        return;
    }
    _idle = [self.idle resolvedColorWithTraitCollection:self.traitCollection];
    _hovered = [self.hovered resolvedColorWithTraitCollection:self.traitCollection];
    _pressed = [self.pressed resolvedColorWithTraitCollection:self.traitCollection];
    _disabled = [self.disabled resolvedColorWithTraitCollection:self.traitCollection];
}

核心原理总结 #

• UIColor的动态更新依赖系统对颜色属性的KVO监听和traitCollectionDidChange:回调；
• 子类必须正确实现resolvedColorWithTraitCollection:，返回自身的解析实例，而非直接返回内部颜色的结果；
• 私有属性无法被系统自动监听，需在子类内部手动处理trait变化时的颜色解析更新。

内容的提问来源于stack exchange，提问作者Marcel Voss