swift - 如何让同一个类的对象成为初始化器的返回值？

Question

我有这个功能:

extension UIImage {

    static func from(layer: CALayer) -> UIImage? {
        UIGraphicsBeginImageContext(layer.frame.size)
        layer.render(in: UIGraphicsGetCurrentContext()!)
        let outputImage: UIImage? = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return outputImage
    }

}

我怎样才能把它改成这样呢？

extension UIImage {

    convenience init(layer: CALayer) {
        self.init();
        UIGraphicsBeginImageContext(layer.frame.size)
        layer.render(in: UIGraphicsGetCurrentContext()!)
        let outputImage: UIImage? = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        self = outputImage; // Error: cannot assign to self: self is immutable
    }

}

我希望 UIGraphicsGetImageFromCurrentImageContext() 的结果成为启动的“ self ”。但结果已经是一个 UIImage。我不认为我应该在此函数中使用“变异”？

Answer 1

由于错误状态 UIImage 是不可变的。您可以做的是从 outputImage 获取 CGImage 并使用 UIImage init(cgImage: CGImage) initializer从中初始化一个新图像:

extension UIImage {
    convenience init?(layer: CALayer) {
        UIGraphicsBeginImageContextWithOptions(layer.frame.size, false, 0)
        defer { UIGraphicsEndImageContext() }
        guard let context = UIGraphicsGetCurrentContext() else {
            return nil
        }
        layer.render(in: context)
        guard let outputImage = UIGraphicsGetImageFromCurrentImageContext()?.cgImage else {
            return nil
        }
        self.init(cgImage: outputImage)
    }
}