图片拉伸

   UIImageView *imageView = [[UIImageView alloc]init];
    UIImage *image = [UIImage imageNamed:@"popover_background"];

    imageView.image = [image stretchableImageWithLeftCapWidth:<#(NSInteger)#> topCapHeight:<#(NSInteger)#>]

   imageView.image = [image stretchableImageWithLeftCapWidth:image.size.width* 0.5 topCapHeight:image.size.height* 0.5];

   imageView.size = CGSizeMake(100, 100);

    
    UIWindow *window = [UIApplication sharedApplication].keyWindow;
    [window addSubview:imageView];

第三种
 UIImageView *imageView = [[UIImageView alloc]init];
    UIImage *image = [UIImage imageNamed:@"popover_background"];
    
    imageView.image = [image resizableImageWithCapInsets:UIEdgeInsetsMake(20, 0, 20, 0) resizingMode:UIImageResizingModeStretch];
    imageView.size = CGSizeMake(100, 100);
    
    UIWindow *window = [UIApplication sharedApplication].keyWindow;
    [window addSubview:imageView];