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;\
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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)];
}
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