给navigationbar添加背景图片，并且按比例延伸

        UIImage *bgImage = [UIImageimageNamed:@"NavBarBG"];

        bgImage = [bgImage resizableImageWithCapInsets:UIEdgeInsetsZeroresizingMode:UIImageResizingModeStretch];

        [self.navigationBarsetBackgroundImage:bgImage forBarMetrics:UIBarMetricsDefault];