当需要保留图片中间、拉伸两端时

原理：

1、计算图片大小与目标大小差距，获得需拉伸的宽度；\
2、计算拉伸左边区域的 `UIEdgeInsets`，第一次拉伸后的图片宽度 ` tempStrecthWith`;\
3、生成拉伸左侧部分的图片;\
4、计算拉伸右侧区域的 `UIEdgeInsets` 及拉伸后的图片宽度;\
5、生成拉伸右侧部分的图片，结束;\

注意：desSize目标大小，最好传入整数

代码

/**
 拉伸两端，保留中间
 
 @param image 需要拉伸的图片
 @param desSize 目标大小
 @param stretchLeftBorder 拉伸图片距离左边的距离
 @param top inset.top
 @param bottom inset.bottom
 @return 拉伸收缩后的图片
 */
static UIImage *tt_stretch_both_sides_image(UIImage *image, CGSize desSize, CGFloat stretchLeftBorder, CGFloat top, CGFloat bottom) {
    if (!image) {
        return nil;
    }
    if (desSize.width == 0) {
        return nil;
    }
    CGSize imageSize = image.size;
    
    if (fabs(desSize.width - imageSize.width) <= 4) {
        return image;
    }
    
    imageSize.width = floor(imageSize.width);
    desSize.width   = floor(desSize.width);
    
    BOOL desSizeThan = desSize.width > imageSize.width;
    
    //各需要拉伸的宽度
    CGFloat needWidth = 0;
    needWidth = (desSize.width - imageSize.width) /2.0;
    
    //先拉取左边
    CGFloat left = stretchLeftBorder;
    CGFloat right = desSizeThan? (imageSize.width - left -1): (imageSize.width - fabs(needWidth) -left);
    
    
    //画图， 生成拉伸的左边后的图片
    CGFloat tempStrecthWith = 0;
    tempStrecthWith = imageSize.width + needWidth;
    
    //生成拉伸后的图片-》左
    CGFloat height = imageSize.height;
    UIImage *strectedImage = [image resizableImageWithCapInsets:UIEdgeInsetsMake(0, left, 0, right)];
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(tempStrecthWith, height), NO, image.scale);
    [strectedImage drawInRect:CGRectMake(0, 0, tempStrecthWith, height)];
    strectedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    //拉伸右边
    right = stretchLeftBorder;
    left  = desSizeThan? (strectedImage.size.width - right - 1): (strectedImage.size.width - right - fabs(needWidth));
    
    //生成拉伸后的图片-》右
    tempStrecthWith = desSize.width;
    strectedImage = [strectedImage resizableImageWithCapInsets:UIEdgeInsetsMake(0, left, 0, right)];
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(tempStrecthWith, height), NO, image.scale);
    [strectedImage drawInRect:CGRectMake(0, 0, tempStrecthWith, height)];
    strectedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return [strectedImage resizableImageWithCapInsets:UIEdgeInsetsMake(top, 0, bottom, 0)];
}