Ios 圆角巴布托石

出于某种原因，当我将背景图像应用到uibarbuttonite时，它会使角变成锐利的矩形。我如何使用UIS外观修复该问题

 // bar button
    id barButtonAppearance = [UIBarButtonItem appearance];
    [barButtonAppearance setBackgroundImage:[theme imageForBarButtonNormal] forState:UIControlStateNormal barMetrics:UIBarMetricsDefault];

-(UIImage *) imageForBarButtonNormal
{
    return [[UIImage imageNamed:@"bar_button_item_blue_background"] resizableImageWithCapInsets:UIEdgeInsetsMake(0, 0, 0, 0)];
}

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您的图像需要有圆角。这与在iOS中在幕后完成界面元素的方式相同