+load method in the category should be guarantee execute onlyonce

[robinli08]

• (void)load { Method delegateOriginalMethod = class_getInstanceMethod([self class], @selector(setDelegate:)); Method delegateSwizzledMethod = class_getInstanceMethod([self class], @selector(vi_setDelegate:));

  method_exchangeImplementations(delegateOriginalMethod, delegateSwizzledMethod); }