Changing navigationBar's title based on position in scrollView

Question

I'm trying to change the navigationBar's title based on a scrollView's x position. Here's the code I have. I don't know why it's not working or if it's possible though:

if(scrollView.contentOffset.x == 0) {
            self.navigationController!.navigationBar.topItem!.title = "First"
        }
        else if(scrollView.contentOffset.x == self.view.frame.size.width) {
            self.navigationController!.navigationBar.topItem!.title = "Second"
        }
        else if(scrollView.contentOffset.x == self.view.frame.size.width*2) {
            self.navigationController!.navigationBar.topItem!.title = "Third"
        }

Any ideas as to what's going wrong? I put this in the viewDidLoad of the ViewController the scrollView is in. It's only setting the navigationBar's title as "First". Is it because I put in viewDidLoad? Do I have to put it into a new method that updates?

I also tried putting the following code in, updating when the scrollView ended dragging. This does not change anything though:

func scrollViewDidEndDragging(scrollView: UIScrollView!, willDecelerate decelerate: Bool) {
        if(self.scrollView.contentOffset.x == 0) {
            self.navigationController!.navigationBar.topItem!.title = "First"
        }
        else if(self.scrollView.contentOffset.x == self.view.frame.size.width) {
            self.navigationController!.navigationBar.topItem!.title = "Second"
        }
        else if(self.scrollView.contentOffset.x == self.view.frame.size.width*2) {
            self.navigationController!.navigationBar.topItem!.title = "Third"
        }
    }

self.scrollView.contentOffset.x keeps logging null within scrollViewDidEndDragging . Is there a reason why?

Answer 1

将此代码放在scrollViewDidScroll：方法中，不要忘记将self设置为滚动视图的委托。

Answer 2

You should put your code inside scrollViewDidScroll() delegate method, and if you calculations are alright, it will work. Good Luck!