stretchableImageWithLeftCapWidth通常用来做图片的局部拉伸，常用的例子是聊天的背景图的部分拉伸
stretchableImageWithLeftCapWidth这个方法 有2个参数
leftCapWidth: 左边不拉伸的像素
topCapHeight:上边不拉伸的像素
例如如下的点：
则取横坐标轴26的像素点横向拉伸到一定宽度
取纵坐标轴11的像素点纵向拉伸到一定长度 （具体长度又imageview的frame决定）
image = [image stretchableImageWithLeftCapWidth:25 topCapHeight:10];
UIImage *image = [UIImage imageNamed:@"ic_nav_back_dark"];
UIImageView *view = [[UIImageView alloc] init];
view.backgroundColor = [UIColor whiteColor];
view.image = image;
view.frame = CGRectMake(60, 60, 100, 100);
[self.view addSubview:view];
UIImageView *view1 = [[UIImageView alloc] init];
view1.backgroundColor = [UIColor whiteColor];
image = [image stretchableImageWithLeftCapWidth:25 topCapHeight:10];
view1.image = image;
view1.frame = CGRectMake(60, 300, 100, 100);
[self.view addSubview:view1];
UIImageView *view2 = [[UIImageView alloc] init];
view2.backgroundColor = [UIColor whiteColor];
UIImage *image1 = [UIImage imageNamed:@"ic_nav_back_dark"];
image1 = [image1 stretchableImageWithLeftCapWidth:1 topCapHeight:1];
view2.image = image1;
view2.frame = CGRectMake(200, 300, 100, 100);
[self.view addSubview:view2];
效果如下：
855CAC19-ED4F-494C-BD32-21D00213D10A.png
