作者:Mitchell
UIImage*image = [UIImage imageNamed:@"imageName"];
UIGraphicsBeginImageContext(CGSizeMake(self.view.frame.size.width, self.view.frame.size.height));
[image drawAtPoint:CGPointMake(0, 0)];
NSString*str = @"Mitchell";
[str drawAtPoint:CGPointMake(10, 10) withAttributes:@{NSForegroundColorAttributeName:[UIColor magentaColor],NSFontAttributeName:[UIFont systemFontOfSize:40]}];
image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
NSData *data = UIImagePNGRepresentation(image);
_img = [UIImage imageWithData:data];
