带边框的UIImage缩放

一个带边框的UIImage如果使用常规的缩放，边框部分将被按照缩放比例拉伸或压缩，有些时候这并不是我们所期望的..比如这个边框是根据图片大小变化的外框。比如下面的类似按钮的不明物体图片：主体为渐变蓝色，边框为外圈白色，灰色底板为背景。

常见的按钮添加和背景设置如下：

UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(80, 130, 160, 44)];
[button setTitle:@”Test Button” forState:UIControlStateNormal];
// Image with without cap insets
UIImage *buttonImage = [UIImage imageNamed:@”blueButton”];
[button addTarget:self action:@selector(buttonPressed:) forControlEvents: UIControlEventTouchUpInside];
[button setBackgroundImage:buttonImage forState:UIControlStateNormal];
[[self view] addSubview:button];

所得到的按钮会相当悲剧…

边框，特别是左右边框由于按钮frame过大被惨烈拉伸… iOS5中提供了一个新的UIImage方法，resizableImageWithCapInsets:，可以将图片转换为以某一偏移值为偏移的可伸缩图像（偏移值内的图像将不被拉伸或压缩）。

用法引述如下：

resizableImageWithCapInsets:
  Creates and returns a new image object with the specified cap insets.
  - (UIImage *)resizableImageWithCapInsets:(UIEdgeInsets)capInsets
    Parameters
      capInsets
      The values to use for the cap insets.
    Return Value
      A new image object with the specified cap insets.
    Discussion
      You use this method to add cap insets to an image or to change the existing cap insets of an image. In both cases, you get back a new image and the original image remains untouched.

      During scaling or resizing of the image, areas covered by a cap are not scaled or resized. Instead, the pixel area not covered by the cap in each direction is tiled, left-to-right and top-to-bottom, to resize the image. This technique is often used to create variable-width buttons, which retain the same rounded corners but whose center region grows or shrinks as needed. For best performance, use a tiled area that is a 1x1 pixel area in size.

输入参数为一个capInsets结构体：

//Defines inset distances for views.
typedef struct {
CGFloat top, left, bottom, right;
} UIEdgeInsets;

分别表示上左下右四个方向的偏移量。于是把上面按钮的UIImage改为如下形式：

// Image with cap insets
UIImage *buttonImage = [[UIImage imageNamed:@”blueButton”]
resizableImageWithCapInsets:UIEdgeInsetsMake(0, 16, 0, 16)];

可以得到如下按钮：

问题得到解决。

但是值得注意的是该方法需要至少iOS5的运行环境，因此对于需要开发支持iOS5之前的App来说是不可行的。替代方案是stretchableImageWithLeftCapWidth:topCapHeight:，但是在iOS5中，这已经是被Deprecated的方法了，而且该方法只能以1px作为重复铺满拉伸区域，无法做到类似渐变等图片效果，是存在一定局限的。

转载自：https://onevcat.com/