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。

1，效果图如下：

（在图片左上角和右下角都添加了文字。）

2，为方便使用，我们通过扩展UIImage类来实现添加水印功能

（文字大小，文字颜色，背景色，位置，边距都可以设置）

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//--- 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]          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

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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()      } }