1、iOS11以下设置navigationItem的边距

UIView *backView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 44, 44)];
    backView.backgroundColor = [UIColor magentaColor];
    UIBarButtonItem *backItem = [[UIBarButtonItem alloc] initWithCustomView:backView];
    // 用于控制间距的item
    UIBarButtonItem *spaceItem = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace target:nil action:nil];
    /**
     *  width为负数时相当于向左移动width数值的距离，为正数时相当于向右移动width数值的距离
     *  自己测了下，左边应该是16px,右边应该是12px
     */
    spaceItem.width = -15;
    self.navigationItem.leftBarButtonItems = @[spaceItem,backItem];

2、iOS设置navigationItem的边距

- (void)viewDidLoad {
    [super viewDidLoad];
    
    UIView *backView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 44, 44)];
    
    UIButton *backButton = [UIButton buttonWithType:UIButtonTypeCustom];
    backButton.frame = CGRectMake(0, 0, 44, 44);
    [backButton setImage:[UIImage imageNamed:@"navigation_back"] forState:UIControlStateNormal];
    [backButton addTarget:self action:@selector(backAction:) forControlEvents:UIControlEventTouchUpInside];
    [backView addSubview:backButton];
    
    if ([UIDevice currentDevice].systemVersion.floatValue >= 11.0) {
        self.navigationItem.leftBarButtonItem = [[UIBarButtonItem alloc] initWithCustomView:backView];
    } else {
        UIBarButtonItem *spaceItem = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace target:nil action:nil];
        spaceItem.width = -15;
        UIBarButtonItem *backItem = [[UIBarButtonItem alloc] initWithCustomView:backView];
        self.navigationItem.leftBarButtonItems = @[spaceItem,backItem];
    }
}

- (void)viewDidLayoutSubviews{
    if ([UIDevice currentDevice].systemVersion.floatValue >= 11.0) {
        UINavigationItem *item = self.navigationItem;
        NSArray *array = item.leftBarButtonItems;
        if (array && array.count) {
            // 注意：这里设置的第一个leftBarButtonItem的customeView不能为空，也就是不要设置UIBarButtonSystemItemFixedSpace这种风格的item
            UIBarButtonItem *btnItem = array[0];
            UIView *view = [[btnItem.customView.superview superview] superview];
            NSArray *arrayConstraint = view.constraints;
            for (NSLayoutConstraint *constant in arrayConstraint) {
                CGFloat space = fabs(constant.constant);
                // 非plus手机为16 plus手机为20
                if (space == 16 || space == 20) {
                    constant.constant = 0;
                }
            }
        }
    }
}