在开发中使用分类对原有的系统类进行方法扩展，是增强系统原有类功能的常见做法。

/**
 *  自由拉伸一张图片
 *
 *  @param name 图片名字
 *  @param left 左边开始位置比例  值范围0-1
 *  @param top  上边开始位置比例  值范围0-1
 *
 *  @return 拉伸后的Image
 */
 + (UIImage *)resizedImageWithName:(NSString *)name left:(CGFloat)left top:(CGFloat)top
{
    UIImage *image = [UIImage imageNamed:name];
    return [image stretchableImageWithLeftCapWidth:image.size.width * left topCapHeight:image.size.height * top];
}

/**
 *  根据颜色和大小获取Image
 *
 *  @param color 颜色
 *  @param size  大小
 *
 */
+ (UIImage *)imageWithColor:(UIColor *)color size:(CGSize)size
{
    UIGraphicsBeginImageContextWithOptions(size, 0, [UIScreen mainScreen].scale);
    [color set];
    UIRectFill(CGRectMake(0, 0, size.width, size.height));
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}
/**
 *  根据图片和颜色返回一张加深颜色以后的图片
 */
+ (UIImage *)colorizeImage:(UIImage *)baseImage withColor:(UIColor *)theColor {
    
    UIGraphicsBeginImageContext(CGSizeMake(baseImage.size.width*2, baseImage.size.height*2));
    
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    CGRect area = CGRectMake(0, 0, baseImage.size.width * 2, baseImage.size.height * 2);
    
    CGContextScaleCTM(ctx, 1, -1);
    CGContextTranslateCTM(ctx, 0, -area.size.height);
    
    CGContextSaveGState(ctx);
    CGContextClipToMask(ctx, area, baseImage.CGImage);
    
    [theColor set];
    CGContextFillRect(ctx, area);
    
    CGContextRestoreGState(ctx);
    
    CGContextSetBlendMode(ctx, kCGBlendModeMultiply);
    
    CGContextDrawImage(ctx, area, baseImage.CGImage);
    
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    
    UIGraphicsEndImageContext();
    
    return newImage;
}
/**
 *  根据图片返回一张高斯模糊的图片
 *
 *  @param blur 模糊系数
 *
 *  @return 新的图片
 */
- (UIImage *)boxblurImageWithBlur:(CGFloat)blur {
    
    NSData *imageData = UIImageJPEGRePResentation(self, 1); // convert to jpeg
    UIImage* destImage = [UIImage imageWithData:imageData];
    
    
    if (blur < 0.f || blur > 1.f) {
        blur = 0.5f;
    }
    int boxSize = (int)(blur * 40);
    boxSize = boxSize - (boxSize % 2) + 1;
    
    CGImageRef img = destImage.CGImage;
    
    vImage_Buffer inBuffer, outBuffer;
    
    vImage_Error error;
    
    void *pixelBuffer;
    
    
    //create vImage_Buffer with data from CGImageRef
    
    CGDataProviderRef inProvider = CGImageGetDataProvider(img);
    CFDataRef inBitmapData = CGDataProviderCopyData(inProvider);
    
    
    inBuffer.width = CGImageGetWidth(img);
    inBuffer.height = CGImageGetHeight(img);
    inBuffer.rowBytes = CGImageGetBytesPerRow(img);
    
    inBuffer.data = (void*)CFDataGetBytePtr(inBitmapData);
    
    //create vImage_Buffer for output
    
    pixelBuffer = malloc(CGImageGetBytesPerRow(img) * CGImageGetHeight(img));
    
    if(pixelBuffer == NULL)
        NSLog(@"No pixelbuffer");
    
    outBuffer.data = pixelBuffer;
    outBuffer.width = CGImageGetWidth(img);
    outBuffer.height = CGImageGetHeight(img);
    outBuffer.rowBytes = CGImageGetBytesPerRow(img);
    
    // Create a third buffer for intermediate processing
    void *pixelBuffer2 = malloc(CGImageGetBytesPerRow(img) * CGImageGetHeight(img));
    vImage_Buffer outBuffer2;
    outBuffer2.data = pixelBuffer2;
    outBuffer2.width = CGImageGetWidth(img);
    outBuffer2.height = CGImageGetHeight(img);
    outBuffer2.rowBytes = CGImageGetBytesPerRow(img);
    
    //perform convolution
    error = vImageBoxConvolve_ARGB8888(&inBuffer, &outBuffer2, NULL, 0, 0, boxSize, boxSize, NULL, kvImageEdgeExtend);
    if (error) {
        NSLog(@"error from convolution %ld", error);
    }
    error = vImageBoxConvolve_ARGB8888(&outBuffer2, &inBuffer, NULL, 0, 0, boxSize, boxSize, NULL, kvImageEdgeExtend);
    if (error) {
        NSLog(@"error from convolution %ld", error);
    }
    error = vImageBoxConvolve_ARGB8888(&inBuffer, &outBuffer, NULL, 0, 0, boxSize, boxSize, NULL, kvImageEdgeExtend);
    if (error) {
        NSLog(@"error from convolution %ld", error);
    }
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef ctx = CGBitmapContextCreate(outBuffer.data,
                                             outBuffer.width,
                                             outBuffer.height,
                                             8,
                                             outBuffer.rowBytes,
                                             colorSpace,
                                             (CGBitmapInfo)kCGImageAlphaNoneSkipLast);
    CGImageRef imageRef = CGBitmapContextCreateImage (ctx);
    UIImage *returnImage = [UIImage imageWithCGImage:imageRef];
    
    //clean up
    CGContextRelease(ctx);
    CGColorSpaceRelease(colorSpace);
    
    free(pixelBuffer);
    free(pixelBuffer2);
    CFRelease(inBitmapData);
    
    CGImageRelease(imageRef);
    
    return returnImage;
}
/**
 *  自由改变Image的大小
 *
 *  @param size 目的大小
 *
 *  @return 修改后的Image
 */
- (UIImage *)cropImageWithSize:(CGSize)size {
    
    float scale = self.size.width/self.size.height;
    CGRect rect = CGRectMake(0, 0, 0, 0);
    
    if (scale > size.width/size.height) {
        
        rect.origin.x = (self.size.width - self.size.height * size.width/size.height)/2;
        rect.size.width  = self.size.height * size.width/size.height;
        rect.size.height = self.size.height;
        
    }else {
        
        rect.origin.y = (self.size.height - self.size.width/size.width * size.height)/2;
        rect.size.width  = self.size.width;
        rect.size.height = self.size.width/size.width * size.height;
        
    }
    
    CGImageRef imageRef   = CGImageCreateWithImageInRect(self.CGImage, rect);
    UIImage *croppedImage = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);
    
    return croppedImage;
}