图片拉伸

UIImage *normalImage = [UIImage imageNamed:@"xx.png"];

//ios5以前实现拉伸图片指定位置的方法
UIImage *newImage =  [normalImage stretchableImageWithLeftCapWidth:
normalImage.size.width / 2 topCapHeight:normalImage.size.height / 2];
// stretchableImageWithLeftCapWidth: topCapHeight:该方法可以返回一张拉伸指定位置的图片.
    //LeftCapWidth:左边多大的距离不可以拉伸(水平方向)
    //topCapHeight:上边有多大的距离不可以拉伸(垂直方法)
    //1 = width - leftCapWidth   - right
    //1 = height - topCapWidth  - bottom

//ios5
//指定图片的上边,下边,左边,右边 多少距离是不可以拉伸的,默认的拉伸方案是平铺
CGFloat w = norImage.size.width;
CGFloat h = norImage.size.height;
UIImage *newImage = [norImage resizableImageWithCapInsets:UIEdgeInsetsMake(h * 0.5, w * 0.5, h * 0.5 , w * 0.5)];

//ios6
//相比IOS5的方法,多了一个指定拉伸模式的参数:
   //参数有两种:平铺, 拉伸
CGFloat w = norImage.size.width * 0.5;
CGFloat h = norImage.size.height * 0.5;
UIImage *newImage = [norImage resizableImageWithCapInsets:UIEdgeInsetsMake(h, w, h, w) resizingMode:UIImageResizingModeStretch];