方法一：设置BarButtonItem的文本样式为透明颜色，代码如下：

[[UIBarButtonItem appearance] setTitleTextAttributes:@{NSForegroundColorAttributeName: [UIColor clearColor]} forState:UIControlStateNormal];

[[UIBarButtonItem appearance] setTitleTextAttributes:@{NSForegroundColorAttributeName: [UIColor clearColor]} forState:UIControlStateHighlighted];

此外这种方法会导致title不能居中，被偏移很多，如下所示（虽然不被显示，也占了导航栏左边很大一部分位置）

方法二：给UIViewController添加类别，然后在load方法里面用Method Swzilling方法替换 交换ViewDidAppear，部分代码如下

+(void)load {

    swizzleMethod([self class], @selector(viewDidAppear:), @selector(ac_viewDidAppear));

}

- (void)ac_viewDidAppear{

    self.navigationItem.backBarButtonItem = [[UIBarButtonItem alloc]

                                              initWithTitle:@""

                                              style:UIBarButtonItemStylePlain

                                              target:self

                                              action:nil];

    [self ac_viewDidAppear];

}

void swizzleMethod(Class class, SEL originalSelector, SEL swizzledSelector)

{

    // the method might not exist in the class, but in its superclass

    Method originalMethod = class_getInstanceMethod(class, originalSelector);

    Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);

    // class_addMethod will fail if original method already exists

    BOOL didAddMethod = class_addMethod(class, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod));

    // the method doesn’t exist and we just added one

    if (didAddMethod) {

        class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));

    }

    else {

        method_exchangeImplementations(originalMethod, swizzledMethod);

    }

}

注意事项：