Swift-Extension:

extension UIImage{
     func toBase64() -> String{
          var imageData = UIImagePNGRepresentation(self)
          return imageData.base64EncodedStringWithOptions(.allZeros)
     }
}

4.2扩展方法

extension UIImage {
    func toBase64() -> String? {
        guard let imageData = self.pngData() else { return nil }
        return imageData.base64EncodedString(options: .lineLength64Characters)
    }
}

XCode 9.1和Swift 4.0

//
// Convert UIImage to a base64 representation
//
class func convertImageToBase64(image: UIImage) -> String {
    let imageData = UIImagePNGRepresentation(image)!
    return imageData.base64EncodedString(options: Data.Base64EncodingOptions.lineLength64Characters)
}

//
// Convert a base64 representation to a UIImage
//
class func convertBase64ToImage(imageString: String) -> UIImage {
    let imageData = Data(base64Encoded: imageString, options: Data.Base64DecodingOptions.ignoreUnknownCharacters)!
    return UIImage(data: imageData)!
}

解码使用方便的初始化器- Swift 5

extension UIImage {
    convenience init?(base64String: String) {
        guard let data = Data(base64Encoded: base64String) else { return nil }
        self.init(data: data)
    }
} 

对于iOS 7+， Objective-C，下面是如何从一个图像URL开始转换:

NSURL *url = [NSURL URLWithString:self.groove.thumbnailURL];

UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:url]];

NSString *base64String = [UIImagePNGRepresentation(image)
base64EncodedStringWithOptions:NSDataBase64Encoding64CharacterLineLength];

在swift 2.0中使用此扩展(credit to Jonas Franz)

extension UIImage{
  func toBase64() -> String{
    let imageData = UIImagePNGRepresentation(self)!
    return imageData.base64EncodedStringWithOptions(.Encoding64CharacterLineLength)
  }
}

我尝试了所有的解决方案，没有一个对我有效(使用Swift 4)，这是对我有效的解决方案，如果将来有人面临同样的问题。

let temp = base64String.components(separatedBy: ",")
let dataDecoded : Data = Data(base64Encoded: temp[1], options: 
 .ignoreUnknownCharacters)!
let decodedimage = UIImage(data: dataDecoded)

yourImage.image = decodedimage