获取指定类型的子控制器

- (UIViewController *)childViewControllerWithType:(Class)type {
	for (UIViewController *controller in self.childViewControllers) {
		if ([controller isKindOfClass:type]) {
			return controller;
		}
	}
	NSLog(@"Requested controller of type %@ was not found.", NSStringFromClass(type));
	return nil;
}