http://blog.leichunfeng.com/blog/2015/06/14/objective-c-method-swizzling-best-practice/

@interface UIViewController (MRCUMAnalytics)
@end
@implementation UIViewController (MRCUMAnalytics)
+ (void)load { 
   static dispatch_once_t onceToken; 
   dispatch_once(&onceToken, ^{
   Class class = [self class]; 
   SEL originalSelector = @selector(viewWillAppear:); 
   SEL swizzledSelector = @selector(mrc_viewWillAppear:); 
   Method originalMethod = class_getInstanceMethod(class, originalSelector); 
   Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector); 
   BOOL success = class_addMethod(class, originalSelector,
                 method_getImplementation(swizzledMethod),
                 method_getTypeEncoding(swizzledMethod)); 
   if (success) { 
             class_replaceMethod(class, swizzledSelector, 
             method_getImplementation(originalMethod), 
            method_getTypeEncoding(originalMethod)); 
          } else {
           method_exchangeImplementations(originalMethod, swizzledMethod);
         } 
   });
}

#pragma mark - Method Swizzling
- (void)mrc_viewWillAppear:(BOOL)animated 
{ 
   [self mrc_viewWillAppear:animated];
   [MobClick beginLogPageView:NSStringFromClass([self class])];
}
@end

我們使用 Method Swizzling 的目的通常都是為了給程序增加功能，而不是完全地替換某個功能，所以我們一般都需要在自定義的實現中調用原始的實現。所以這里就會有兩種情況需要我們分別進行處理：
第 1 種情況：主類本身有實現需要替換的方法，也就是 class_addMethod
方法返回 NO
。這種情況的處理比較簡單，直接交換兩個方法的實現就可以了：

1.png

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

將父類的實現替換到我們自定義的 mrc_viewWillAppear
方法中。這樣就達到了在 mrc_viewWillAppear
方法的實現中調用父類實現的目的。