我是这样做的：
我在Swift中做了这个，只是为了在playgrounds中检查它，我认为您可以轻松地将它翻译为Objective-C：

import UIKit

func drawOutlie(#image:UIImage, color:UIColor) -> UIImage
{
  var newImageKoef:CGFloat = 1.08
  
  var outlinedImageRect = CGRect(x: 0.0, y: 0.0, width: image.size.width * newImageKoef, height: image.size.height * newImageKoef)
  
  var imageRect = CGRect(x: image.size.width * (newImageKoef - 1) * 0.5, y: image.size.height * (newImageKoef - 1) * 0.5, width: image.size.width, height: image.size.height)
  
  UIGraphicsBeginImageContextWithOptions(outlinedImageRect.size, false, newImageKoef)
  
  image.drawInRect(outlinedImageRect)
  
  var context = UIGraphicsGetCurrentContext()
  CGContextSetBlendMode(context, kCGBlendModeSourceIn)
  
  CGContextSetFillColorWithColor(context, color.CGColor)
  CGContextFillRect(context, outlinedImageRect)
  image.drawInRect(imageRect)
  
  var newImage = UIGraphicsGetImageFromCurrentImageContext()
  UIGraphicsEndImageContext()
  
  return newImage
  
}

var imageIn = UIImage(named: "158jM")

var imageOut = drawOutlie(image: imageIn, UIColor.redColor())

那么它是如何工作的呢？
1.我们创建干净的背景（又名画布），尺寸比原始图像（用于轮廓）大一点
1.我们在整个画布上画出自己形象
1.我们用颜色填充图像
1.我们在上面画一个小图
您可以通过更改此属性来更改轮廓大小：var newImageKoef:CGFloat = 1.08
这是我在Playground里得到的结果

“雨燕5号"

extension UIImage {
    
    func drawOutline(imageKeof: CGFloat = 0.2, color: UIColor = .white)-> UIImage? {
        let outlinedImageRect = CGRect(x: 0.0, y: 0.0, width: size.width * imageKeof, height: size.height * imageKeof)
        let imageRect = CGRect(x: self.size.width * (imageKeof - 1) * 0.5, y: self.size.height * (imageKeof - 1) * 0.5, width: size.width, height: size.height)
        UIGraphicsBeginImageContextWithOptions(outlinedImageRect.size, false, imageKeof)
        draw(in: outlinedImageRect)
        let context = UIGraphicsGetCurrentContext()
        context!.setBlendMode(.sourceIn)
        context!.setFillColor(color.cgColor)
        context!.fill(outlinedImageRect)
        draw(in: imageRect)
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return newImage
    }
}

斯威夫特5

extension UIImage {
    func drawOutlie(imageKeof: CGFloat = 1.01, color: UIColor) -> UIImage? {

        let outlinedImageRect = CGRect(x: 0.0, y: 0.0,
                                   width: size.width * imageKeof,
                                   height: size.height * imageKeof)

        let imageRect = CGRect(x: size.width * (imageKeof - 1) * 0.5,
                           y: size.height * (imageKeof - 1) * 0.5,
                           width: size.width,
                           height: size.height)

        UIGraphicsBeginImageContextWithOptions(outlinedImageRect.size, false, imageKeof)

        draw(in: outlinedImageRect)

        guard let context = UIGraphicsGetCurrentContext() else {return nil}
        context.setBlendMode(.sourceIn)
        context.setFillColor(color.cgColor)
        context.fill(outlinedImageRect)
        draw(in: imageRect)

        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        return newImage
    }
}

这种方法有点不同，但更简单。

imageView.layer.shadowColor = UIColor.red.cgColor
imageView.layer.shadowOffset = CGSize(width: 0, height: 0)
imageView.layer.shadowOpacity = 1
imageView.layer.shadowRadius = 10.0
imageView.clipsToBounds = false