Xcode 12设置UINavigationBar渐变背景显示为白色问题求助

1. 核心错误：RGB颜色值未归一化 #

iOS 中 UIColor(red:green:blue:alpha:) 构造方法要求传入的RGB参数取值范围为 0.0 ~ 1.0，你直接传入85、181这类大于1的数值时，系统会默认将其截断为1.0，最终两个渐变色都被识别为纯白色，这是导航栏显示白色的主要原因。

同时gradient.colors要求传入CGColor类型数组，原代码直接传入UIColor也会导致渐变颜色识别错误。你需要修正颜色赋值逻辑：

// 修正后的颜色赋值
gradient.colors = [
    UIColor(red: 85/255.0, green: 181/255.0, blue: 180/255.0, alpha: 1).cgColor,
    UIColor(red: 76/255.0, green: 144/255.0, blue: 172/255.0, alpha: 1).cgColor
]

2. 其他可能导致不生效的问题 #

2.1 调用时机不对 #

viewDidLoad阶段导航栏的布局还未完成，此时获取的bounds尺寸不准确，建议将调用逻辑移到viewWillAppear或viewDidLayoutSubviews中。

2.2 高版本iOS系统适配缺失 #

iOS 13及以上系统引入了UINavigationBarAppearance管理导航栏样式，仅调用setBackgroundImage会被系统默认外观覆盖，需要补充适配逻辑，修正后的完整扩展代码如下：

extension UINavigationBar {
    func setGradientBackground(to navController: UINavigationController) {
        let gradient = CAGradientLayer()
        var bounds = navController.navigationBar.bounds
        // 适配新系统状态栏高度获取
        let statusBarHeight = navController.view.window?.windowScene?.statusBarManager?.statusBarFrame.height ?? 0
        bounds.size.height += statusBarHeight
        gradient.frame = bounds
        // 修正颜色值 + 转CGColor
        gradient.colors = [
            UIColor(red: 85/255.0, green: 181/255.0, blue: 180/255.0, alpha: 1).cgColor,
            UIColor(red: 76/255.0, green: 144/255.0, blue: 172/255.0, alpha: 1).cgColor
        ]
        gradient.startPoint = CGPoint(x: 0.5, y: 0)
        gradient.endPoint = CGPoint(x: 0.5, y: 1)

        if let image = getImageFrom(gradientLayer: gradient) {
            // 适配iOS13+外观规则
            if #available(iOS 13.0, *) {
                let appearance = UINavigationBarAppearance()
                appearance.backgroundImage = image
                appearance.shadowColor = .clear
                navController.navigationBar.standardAppearance = appearance
                navController.navigationBar.scrollEdgeAppearance = appearance
            } else {
                navController.navigationBar.setBackgroundImage(image, for: .default)
            }
        }
    }
    
    func getImageFrom(gradientLayer:CAGradientLayer) -> UIImage? {
        var gradientImage:UIImage?
        UIGraphicsBeginImageContext(gradientLayer.frame.size)
        if let context = UIGraphicsGetCurrentContext() {
            gradientLayer.render(in: context)
            gradientImage = UIGraphicsGetImageFromCurrentImageContext()?.resizableImage(withCapInsets: UIEdgeInsets.zero, resizingMode: .stretch)
        }
        UIGraphicsEndImageContext()
        return gradientImage
    }
}

内容的提问来源于stack exchange，提问作者Ανδρέας Λ. Βασιλείου