iOS 11 使用方法替换(Method Swizzling)，去掉导航栏返回按钮的文字

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：

• 要给整个backButtonItem赋值才可以，