简介：有时候我们只是想把图片部分拉伸，而不是全部拉伸，那么就要用到下面这个函数，并附上实例验证
- (UIImage *)stretchableImageWithLeftCapWidth:(NSInteger)leftCapWidth topCapHeight:(NSInteger)topCapHeight;
//leftCapWidth:左边不拉伸区域
//topCapHeight:上面不拉伸区域
对距离leftCapWidth的1竖排像素，和具体topCapHeight的1横排像素进行拉伸，其它像素不拉伸!
练习一：理解拉伸点，左边和上面分开设置，从效果图来理解。
纵向拉伸
self.view.backgroundColor = [UIColor darkGrayColor];
//不设置拉伸点，直接设置
UIImage *image1 = [UIImage imageNamed:@QQ];
UIImageView *imageView1 = [[UIImageView alloc] initWithFrame:CGRectMake(10, 100, 170, 170)];
imageView1.image = image1;
[self.view addSubview:imageView1];
//设置拉伸点，对左边和上面分开设置，理解其拉伸效果
UIImage *image2 = [UIImage imageNamed:@QQ];
image2 = [image2 stretchableImageWithLeftCapWidth:0 topCapHeight:image2.size.height*0.5];
UIImageView *imageView2 = [[UIImageView alloc] initWithFrame:CGRectMake(190, 100, 170, 170)];
imageView2.image = image2;
[self.view addSubview:imageView2];
效果图
横向拉伸：
image2 = [image2 stretchableImageWithLeftCapWidth:image2.size.width*0.5 topCapHeight:0];
效果图：
练习二：最近做到一个聊天框的联系，对文字背景图片的拉伸应用，左边的小三角我不希望拉伸，拉伸影响美观，只拉伸右边方框部分，那么调用此函数就可以实现，这个较为常用。
//不设置拉伸点，直接设置
UIImage *image1 = [UIImage imageNamed:@chat];
UIImageView *imageView1 = [[UIImageView alloc] initWithFrame:CGRectMake(100, 100, 200, 80)];
imageView1.image = image1;
[self.view addSubview:imageView1];
//设置拉伸点
UIImage *image2 = [UIImage imageNamed:@chat];
image2 = [image2 stretchableImageWithLeftCapWidth:image2.size.width*0.5 topCapHeight:image2.size.width*0.8];
UIImageView *imageView2 = [[UIImageView alloc] initWithFrame:CGRectMake(100, 200, 200, 80)];
imageView2.image = image2;
[self.view addSubview:imageView2];
效果图：
