我正在尝试在导航栏上添加自定义后退按钮,但它不起作用下面是我的代码-(void)addLeftButton{UIImage*buttonImage=[UIImageimageNamed:@"btn_back.png"];UIButton*aButton=[UIButtonbuttonWithType:UIButtonTypeCustom];[aButtonsetBackgroundImage:buttonImageforState:UIControlStateNormal];aButton.frame=CGRectMake(0.0,0.0,buttonImage.size.width,