常见的几种会引发离屏渲染的设置

1、使用了mask的layer（layer.mask）
2、需要进行裁剪的layer（layer.masksToBounds / view.clipsToBounds）（模拟器会离屏渲染，真机测试暂无离屏渲染的情况）
3、设置了组透明度true,并且透明度不为1的layer (layer.allowsGroupOpacity/layer.opacity)
4、添加了投影的layer（layer.shadow）
5、采用了光栅化的layer（layer.shouldRasterize）
6、绘制了文字的layer （UILabel，CATextLayer，CoreText等）

已知的解决方法：

Shadow 可以通过指定路径来取消离屏渲染。 Mask 无法取消离屏渲染，可以利用混合layer图层或者另添加新的图片图层来进行优化。

示例代码

/** 切角必须明确View的大小 可重写View的layoutSubviews方法使用，剪切超出部分View本身自行控制*/

    override func layoutSubviews() {
        super.layoutSubviews()
        // 有圆角或边框的都统一创建，不需要考虑渐变，阴影
        if cornerRadius > 0 || borderWidth > 0 {
            let originalImg = createOriginalImage()
            createCornerImage(originalImg)
        }// 渐变无切角设置原图片
        else if isGradient {
            let originalImg = createOriginalImage()
            addSubviewImage(originalImg)
        }// 阴影有填充背景色
        else if isShadow {
            layer.backgroundColor = fillColor?.cgColor
            // 阴影设置消除离屏渲染
            layer.shadowPath = UIBezierPath(rect: bounds).cgPath
        }
    }
    /// fillColor色或者渐变色生成图片
    private func createOriginalImage() -> UIImage? {
        if isGradient, let gradient = gradientView {
            gradient.frame = bounds
            return gradient.asImage()
        }
        UIGraphicsBeginImageContextWithOptions(bounds.size, false, UIScreen.main.scale)
        let context = UIGraphicsGetCurrentContext()
        context?.setFillColor((fillColor ?? UIColor.white).cgColor)
        context?.fill(bounds)
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return image
    }
    /// 对图片进行切角+边框
    private func createCornerImage(_ image: UIImage?) {
        let borderW = borderWidth; let borderC = borderColor ?? .clear
        let cornerRadii = CGSize(width: cornerRadius, height: cornerRadius)
        let rect = CGRect(origin: .zero, size: bounds.size)
        let bezierPath = UIBezierPath(roundedRect: rect, byRoundingCorners: corners ?? .allCorners, cornerRadii: cornerRadii)
        UIGraphicsBeginImageContextWithOptions(rect.size, false, UIScreen.main.scale)
        let context = UIGraphicsGetCurrentContext()
        context?.addPath(bezierPath.cgPath)
        context?.clip()
        image?.draw(in: rect)
        borderC.setStroke()
        bezierPath.lineWidth = borderW
        bezierPath.stroke()
        let cornerImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        addSubviewImage(cornerImage)
    }
    /// 给当前LCornerView视图对象添加显示图片
    private func addSubviewImage(_ img: UIImage?) {
        subviews.forEach { subView in subView.removeFromSuperview() }
        let imageView = UIImageView(image: img)
        imageView.frame = bounds
        addSubview(imageView)
    }

使用方法：

/// 圆角视图的固定tag值

fileprivate let cornerTag: Int = 99999999

/// 解决UIView离屏渲染的切圆角问题。 纯切角，默认颜色为白色， 默认切角为all。 全切角以及对UIImageView切角，不需要添加图片设置圆角推荐使用系统方法layer.cornerRadius

    /// - Parameters:
    ///   - value: 圆角半径值
    ///   - corners: 指定切角类型
    ///   - fillColor: 圆角视图背景填充色 默认白色
    func cornerRadius(value: CGFloat = 0, corners: UIRectCorner? = nil, fillColor: UIColor? = nil) {
        let imageView = viewWithTag(cornerTag) as? LCornerView ?? LCornerView()
        imageView.setupCorner(radius: value, corners, fillColor)
        remakeConstraints(view: imageView)
    }
    /// 设置边框。配合func cornerRadius(_ radius: CGFloat, corners: UIRectCorner? = nil, color: UIColor? = nil)方法使用
    /// - Parameters:
    ///   - width: 边框宽度
    ///   - borderColor: 边框颜色
    func borderWidth(_ value: CGFloat, borderColor: UIColor? = nil) {
        let imageView = viewWithTag(cornerTag) as? LCornerView ?? LCornerView()
        imageView.setupBorder(width: value, borderColor: borderColor)
        remakeConstraints(view: imageView)
    }
    /// 渐变+纯切角。默认颜色为白色， 默认切角为all。可同时使用func borderWidth(_ width: CGFloat, borderColor: UIColor? = nil)
    /// - Parameters:
    ///   - colors: 渐变色数组
    ///   - start: 开始点位置
    ///   - end: 结束点位置
    ///   - locations: 渐变色分布位置
    func gradientColors(_ colors: [UIColor], _ start: CGPoint? = nil, _ end: CGPoint? = nil, locations: [NSNumber]? = nil) {
        let imageView = viewWithTag(cornerTag) as? LCornerView ?? LCornerView()
        let gradientView = imageView.gradientView ?? LGradientView()
        gradientView.setGradientColors(colors, start, end, locations)
        imageView.setupGradient(gradientView: gradientView)
        remakeConstraints(view: imageView)
    }
    /// 阴影 当前以及其frame相等的父视图不能做切除操作，否则会把阴影切掉。可同时使用func borderWidth(_ width: CGFloat, borderColor: UIColor? = nil)
    /// - Parameters:
    ///   - color: 阴影颜色
    ///   - opacity: 阴影透明度 0-1 默认1不透明
    ///   - shadowRadius: 阴影的圆角半径
    ///   - offset: 阴影偏移量
    ///   - fillColor: 阴影视图背景填充色 默认白色
    func shadowColor(_ color: UIColor, opacity: CGFloat = 1, shadowRadius: CGFloat, offset: CGSize? = nil, fillColor: UIColor? = nil) {
        let shadowView = viewWithTag(cornerTag) as? LCornerView ?? LCornerView()
        shadowView.setupShadow(shadowColor: color, opacity, shadowRadius, offset, fillColor: fillColor)
        remakeConstraints(view: shadowView)
    }
    private func remakeConstraints(view: UIView) {
        view.tag = cornerTag
        if !subviews.contains(view) { insertSubview(view, at: 0) }
        view.snp.remakeConstraints { make in
            make.edges.equalToSuperview()
        }
    }