iOS 11 使用两种方法替换(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，👇这种方法不行，因为backBarButtonItem默认为空，给nil方法消息，默认声明都不执行（参考官网）

self.navigationItem.backBarButtonItem.title = @" ";

leftBarButtonItem 与backBarButtonItem 的显示关系：

有leftBarButtonItem则优先显示当前VC的leftBarButtonItem，无则显示上个VC的backBarButtonItem，再无则显示上个VC的title（参考官网还是官网解释的清楚）

总结

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