带有3个UIViewController子视图的UIScrollView代理不工作

Question

我在UIViewControllers中有3个UIScrollView，使用以下代码进行实例化：

self.oneVC = [self.storyboard instantiateViewControllerWithIdentifier:@"OneVCSBI"];
CGRect frame = self.oneVC.view.frame;
frame.origin.x = 640;
self.oneVC.view.frame = frame;

[self addChildViewController:self.oneVC];
[self.scrollView addSubview:self.oneVC.view];
[self.oneVC didMoveToParentViewController:self];


self.twoVC = [self.storyboard instantiateViewControllerWithIdentifier:@"twoVCSBI"];
CGRect cframe = self.twoVC.view.frame;
cframe.origin.x = 320;
self.twoVC.view.frame = cframe;

[self addChildViewController:self.twoVC];
[self.scrollView addSubview:self.twoVC.view];
[self.twoVC didMoveToParentViewController:self];

self.threeVC = [self.storyboard instantiateViewControllerWithIdentifier:@"threeVCSBI"];
CGRect pframe = self.threeVC.view.frame;
pframe.origin.x = 0;
self.threeVC.view.frame = pframe;

[self addChildViewController:self.threeVC];
[self.scrollView addSubview:self.threeVC.view];
[self.threeVC didMoveToParentViewController:self];

self.scrollView.contentSize = CGSizeMake(960, self.view.frame.size.height);
self.scrollView.pagingEnabled = YES;
[self.scrollView setShowsHorizontalScrollIndicator:NO];
[self.scrollView scrollRectToVisible:CGRectMake(320*2, 0, 320 , 240) animated:NO];

问题是当我从页面滚动时，UIViewController委托方法（如-(void)viewWillAppear）永远不会被触发。我该如何解决这个问题？