Method Swizzling doesn't work for category of UIViewController

Question

I use the normal swizzle method:

void swizzleMethod(Class class, SEL originalSelector, SEL swizzledSelector)
{
    Method originalMethod = class_getInstanceMethod(class, originalSelector);
    Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);

    BOOL didAddMethod = class_addMethod(class, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod));

    if (didAddMethod) {
        class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
    }
    else {
        method_exchangeImplementations(originalMethod, swizzledMethod);
    }
}

I want to exchange viewWillAppear: with xxx_viewWillAppear:. So I create a category of UIViewController and create the method xxx_viewWillAppear:.

If I use dispatch_once in the +(void)load method to call swizzleMethod, everything goes wrong.

+ (void)load
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        swizzleMethod([self class], @selector(viewWillAppear:), @selector(xxx_viewWillAppear:));
    });
}

It would call the viewWillAppear: in the UIViewController, and when [super viewWillAppear:animated] was called, xxx_viewWillAppear was called.

But if I put the load method in the UIViewController (not in the category), it goes right.

So, Why?

I use xcode 6 and iOS 8.