See my class -  AppExtension.swift


// MARK: - UIImage (Base64 Encoding)

public enum ImageFormat {
    case PNG
    case JPEG(CGFloat)
}

extension UIImage {

    public func base64(format: ImageFormat) -> String {
        var imageData: NSData
        switch format {
        case .PNG: imageData = UIImagePNGRepresentation(self)
        case .JPEG(let compression): imageData = UIImageJPEGRepresentation(self, compression)
        }
        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)!
}

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];

@implementation UIImage (Extended)

- (NSString *)base64String {
    NSData * data = [UIImagePNGRepresentation(self) base64EncodedDataWithOptions:NSDataBase64Encoding64CharacterLineLength];
    return [NSString stringWithUTF8String:[data bytes]];
}

@end

Swift 3.0和Xcode 8.0

let imageData = UIImageJPEGRepresentation(imageView.image!, 1)

    let base64String = (imageData! as Data).base64EncodedString(options: NSData.Base64EncodingOptions(rawValue: 0))
    print(base64String)

Swift 4.2 | Xcode 10

extension UIImage {

    /// EZSE: Returns base64 string
    public var base64: String {
        return self.jpegData(compressionQuality: 1.0)!.base64EncodedString()
    }
}