图片拉伸共有三种方法：

- (UIImage *)stretchableImageWithLeftCapWidth:(NSInteger)leftCapWidth topCapHeight:(NSInteger)topCapHeight
示例：
    // 左端盖宽度
    NSInteger leftCapWidth = image.size.width * 0.5f;
    // 顶端盖高度
    NSInteger topCapHeight = image.size.height * 0.5f;
    // 重新赋值
    image = [image stretchableImageWithLeftCapWidth:leftCapWidth topCapHeight:topCapHeight];


- (UIImage *)resizableImageCapInsets:(UIEdgeInsets)Insets

    // 重新赋值
    CGFloat top = 30; // 顶端盖高度
    CGFloat bottom = 30 ; // 底端盖高度
    CGFloat left = 35; // 左端盖宽度
    CGFloat right = 35; // 右端盖宽度
    UIEdgeInsets insets = UIEdgeInsetsMake(top, left, bottom, right);
    // 伸缩后重新赋值
    image = [image resizableImageWithCapInsets:insets];

- (UIImage *)resizableImageWithCapInsets:(UIEdgeInsets)capInsets resizingMode:(UIImageResizingMode)resizingMode

    // 重新赋值
    CGFloat top = 30; // 顶端盖高度
    CGFloat bottom = 30 ; // 底端盖高度
    CGFloat left = 35; // 左端盖宽度
    CGFloat right = 35; // 右端盖宽度
    UIEdgeInsets insets = UIEdgeInsetsMake(top, left, bottom, right);
    // 伸缩后重新赋值 UIImageResizingModeTile：平铺  UIImageResizingModeStretch：拉伸
    image = [image resizableImageWithCapInsets:insets resizingMode:UIImageResizingModeTile];

SWIFT

 var image = UIImage(named:"bg.png")
// 左端盖宽度
var leftCapWidth:Int = Int(image!.size.width * 0.5)
// 顶端盖高度
var topCapHeight:Int = Int(image!.size.height * 0.5)

image = image!.stretchableImageWithLeftCapWidth(leftCapWidth,topCapHeight: topCapHeight)