想要给图片添加文字水印或者注释，我们需要实现在UIImage上写字的功能。

//--- UIImageExtension.swift ---
import UIKit

extension UIImage{
    
    //水印位置枚举
    enum WaterMarkCorner{
        case TopLeft
        case TopRight
        case BottomLeft
        case BottomRight
    }
    
    //添加水印方法
    func waterMarkedImage(waterMarkText:String, corner:WaterMarkCorner = .BottomRight,
                          margin:CGPoint = CGPoint(x: 20, y: 20),
                          waterMarkTextColor:UIColor = UIColor.whiteColor(),
                          waterMarkTextFont:UIFont = UIFont.systemFontOfSize(20),
                          backgroundColor:UIColor = UIColor.clearColor()) -> UIImage{
        
        let textAttributes = [NSForegroundColorAttributeName:waterMarkTextColor,
                              NSFontAttributeName:waterMarkTextFont,
                              NSBackgroundColorAttributeName:backgroundColor]
        let textSize = NSString(string: waterMarkText).sizeWithAttributes(textAttributes)
        var textFrame = CGRectMake(0, 0, textSize.width, textSize.height)
        
        let imageSize = self.size
        switch corner{
        case .TopLeft:
            textFrame.origin = margin
        case .TopRight:
            textFrame.origin = CGPoint(x: imageSize.width - textSize.width - margin.x, y: margin.y)
        case .BottomLeft:
            textFrame.origin = CGPoint(x: margin.x, y: imageSize.height - textSize.height - margin.y)
        case .BottomRight:
            textFrame.origin = CGPoint(x: imageSize.width - textSize.width - margin.x,
                                       y: imageSize.height - textSize.height - margin.y)
        }
        
        // 开始给图片添加文字水印
        UIGraphicsBeginImageContext(imageSize)
        self.drawInRect(CGRectMake(0, 0, imageSize.width, imageSize.height))
        NSString(string: waterMarkText).drawInRect(textFrame, withAttributes: textAttributes)
        
        let waterMarkedImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        return waterMarkedImage
    }
}

3，使用样例

import UIKit

class ViewController: UIViewController {
    
    @IBOutlet weak var imageView: UIImageView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        //使用链式调用方法，给图片添加两条水印
        imageView.image = UIImage(named:"bg")?
            .waterMarkedImage("做最好的开发者知识平台")
            .waterMarkedImage("hangge.com", corner: .TopLeft,
                              margin: CGPoint(x: 20, y: 20),
                              waterMarkTextColor: UIColor.blackColor(),
                              waterMarkTextFont: UIFont.systemFontOfSize(45),
                              backgroundColor: UIColor.clearColor())
    }
    
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
}