iOS7以后处理leftBarButtonItem问题

if (isBeforeIOS7)
    {
        UIButton *btn = [UIButton buttonWithType:UIButtonTypeCustom];
        btn.frame = CGRectMake(15,5,22,22);
        [btn setBackgroundImage:[UIImage imageNamed:@"icon_back_left"] forState:UIControlStateNormal];
        [btn addTarget:self action:@selector(goBackAction) forControlEvents: UIControlEventTouchUpInside];
        UIBarButtonItem*backBarBtn=[[UIBarButtonItem alloc]initWithCustomView:btn];
        self.navigationItem.leftBarButtonItem=backBarBtn;
    }
    else
    {
        UIBarButtonItem *backBarBtn = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"icon_back_left"] style:UIBarButtonItemStyleDone target:self action:@selector(goBackAction)];
        UIBarButtonItem *negativeSpacer = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace target:nil action:nil];
        negativeSpacer.width = -10;
        self.navigationItem.leftBarButtonItems = @[negativeSpacer,backBarBtn];
    }