创建一个 UIButton 并将其用作 UIBarButtonItem 的自定义视图.然后，在按钮的图层上设置backgroundColor:

Create a UIButton and use it as the custom view for the UIBarButtonItem. Then, set the backgroundColor on the button's layer:

UIButton* button = [UIButton buttonWithType:UIButtonTypeCustom];
[button setTitle:@"Test"];
button.layer.backgroundColor = [UIColor redColor].CGColor;
button.layer.cornerRadius = 4.0;

UIBarButtonItem* buttonItem = [[UIBarButtonItem alloc] initWithCustomView:button];
self.toolbarItems = @[buttonItem];