Method Swizzling

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

我们使用 Method Swizzling 的目的通常都是为了给程序增加功能，而不是完全地替换某个功能，所以我们一般都需要在自定义的实现中调用原始的实现。所以这里就会有两种情况需要我们分别进行处理：
第 1 种情况：主类本身有实现需要替换的方法，也就是 class_addMethod
方法返回 NO
。这种情况的处理比较简单，直接交换两个方法的实现就可以了：

1.png

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

将父类的实现替换到我们自定义的 mrc_viewWillAppear
方法中。这样就达到了在 mrc_viewWillAppear
方法的实现中调用父类实现的目的。