iOS8 自定义navigationbar 以及 UIBarButtonItem 边距问题

- (void)initNavigationBar{

    [self.navigationController setNavigationBarHidden:YES];

    UINavigationBar *bar = [[UINavigationBar alloc] initWithFrame:CGRectMake(0, 0, 320, 32)];

    [bar setBackgroundImage:[UIImage imageNamed:@"zhuche_bar2.png"] forBarMetrics:UIBarMetricsDefault]; 

    UINavigationItem *item = [[UINavigationItem alloc] initWithTitle:nil];

    UIButton *left = [UIButton buttonWithType:UIButtonTypeCustom];

    [left setFrame:CGRectMake(0, 2, 28, 28)];

    [left setImage:[UIImage imageNamed:@"zhuche_back.png"] forState:UIControlStateNormal];

    [left addTarget:self action:@selector(back) forControlEvents:UIControlEventTouchUpInside];

    UIBarButtonItem *leftButton = [[UIBarButtonItem alloc] initWithCustomView:left];

    [item setLeftBarButtonItem:leftButton];

    [bar pushNavigationItem:item animated:NO];

    [self.view addSubview:bar];

}


- (void)back{

    [self.navigationController popViewControllerAnimated:YES];

}

@interface UINavigationItem (margin)

@end

@implementation UINavigationItem (margin)

#if __IPHONE_OS_VERSION_MAX_ALLOWED > __IPHONE_6_1
- (void)setLeftBarButtonItem:(UIBarButtonItem *)_leftBarButtonItem
{
    if ([[[UIDevice currentDevice] systemVersion] floatValue] >= 7.0)
    {
        UIBarButtonItem *negativeSeperator = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace target:nil action:nil];
        negativeSeperator.width = -16;//此处修改到边界的距离，请自行测试
        
        if (_leftBarButtonItem)
        {
            [self setLeftBarButtonItems:@[negativeSeperator, _leftBarButtonItem]];
        }
        else
        {
            [self setLeftBarButtonItems:@[negativeSeperator]];
        }
       
        
    }
    else
    {
        [self setLeftBarButtonItem:_leftBarButtonItem animated:NO];
    }
}

- (void)setRightBarButtonItem:(UIBarButtonItem *)_rightBarButtonItem
{
    if ([[[UIDevice currentDevice] systemVersion] floatValue] >= 7.0)
    {
        UIBarButtonItem *negativeSeperator = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace target:nil action:nil];
        negativeSeperator.width = -12;//此处修改到边界的距离，请自行测试
        
        if (_rightBarButtonItem)
        {
            [self setRightBarButtonItems:@[negativeSeperator, _rightBarButtonItem]];
        }
        else
        {
            [self setRightBarButtonItems:@[negativeSeperator]];
        }
        
    }
    else
    {
        [self setRightBarButtonItem:_rightBarButtonItem animated:NO];
    }
}

#endif
@end