Getting the current page

问题:

In my scroll view, I want to get the current page that's being displayed (maybe page isn't the correct term). I can't find any variable that holds this. But I think it must be held somewhere, since the indicator is able to show which sub-view of the scroll view is currently being displayed.

Is this hidden from us completely or is there a way for me to access it?

回答1:

There is no UIScrollView property for the current page. You can calculate it with:

int page = scrollView.contentOffset.x / scrollView.frame.size.width;

If you want to round up or down to the nearest page, use:

CGFloat width = scrollView.frame.size.width;
NSInteger page = (scrollView.contentOffset.x + (0.5f * width)) / width;

回答2:

I pretty recommend you to use this code

int indexOfPage = scrollView.contentOffset.x / scrollView.frame.size.width;

but if you use this code your view doesn't need to be exactly on the page that indexOfPage gives you. It because I also recommend you to use this code only in this method

-(void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView{}

which is called when your scrollView finishes scrolling and to have number of your page really sharp

I recommend you to set your scrollView to paged enabled with this code

[scrollView setPagingEnabled:YES];

So finally it should look like that way

-(void) methodWhereYouSetYourScrollView
{
   //set scrollView
   [scrollView setPagingEnabled:YES];
   scrollView.delegate = self;
}

-(void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
   int indexOfPage = scrollView.contentOffset.x / scrollView.frame.size.width;
   //your stuff with index
}

回答3:

In swift I would do it in extension:

extension UIScrollView {
    var currentPage:Int{
        return Int((self.contentOffset.x+(0.5*self.frame.size.width))/self.frame.width)+1
    }
}

Then just call:

scrollView.currentPage

回答4:

As above but as a category

@interface UIScrollView (CurrentPage)
-(int) currentPage;
@end
@implementation UIScrollView (CurrentPage)
-(int) currentPage{
    CGFloat pageWidth = self.frame.size.width;
    return floor((self.contentOffset.x - pageWidth / 2) / pageWidth) + 1;
}
@end

回答5:

Just divide the current offset by the page size:

CGFloat pageNum = (int)(scrollView.contentOffset.x / scrollView.frame.size.width);

回答6:

There's some good answers here already. However, for scenarios where content doesn't fit exactly into a page - and if like me you want to use the result for a UIPageControl, then it's essential to use the ceil function.

Let's take an example where I have "four and a bit" pages of content.

I'll base this on my current real life example. The content size width is 3140 points. Based upon my collection view frame size, page width is 728.

So number of pages equals:

 3140 / 728 = 4.313 

My numberOfPages is going to be five. Four of which will show in entirety, and the last of which - page five - will show that remaining 0.313 of content.

Now, numberOfPages being five means that the page indices will be 0, 1, 2, 3, and 4.

When I swipe rightwards, paging towards the final offset, the scrollViewDidEndDecelerating method gives a final X offset of 2412.

Applying the rounding calculation:

2412 / 728 = 3.313 then rounded = 3

That's incorrect. Page user is viewing by offset should be:

Offset / Page User Is Viewing
0        0
728      1
1456     2
2184     3
2412     4

The correct calculation using ceil:

private func pageForOffset(offset:CGFloat, pageWidth width:CGFloat) -> Int
{
    let page = Int(ceil(offset / width))
    NSLog("\(__FUNCTION__) offset \(offset) width \(width) page \(page) ")
    return page
}

回答7:

In xCode 7.x swift 2.x you can do :

//MARK: - ScrollView Extensions
// Get the current page number
extension UIScrollView {
    var currentPage: Int {
        return Int(round(self.contentOffset.x / self.bounds.size.width))
    }

// If you have reversed offset (start from contentSize.width to 0)
    var reverseCurrentPage: Int {
        return Int(round((contentSize.width - self.contentOffset.x) / self.bounds.size.width))-1
    }
}

回答8:

The solution of Aoakenfo is amazing, this is another solution combining your code with the function scrollViewDidScroll and PageController

- (void)scrollViewDidScroll:(UIScrollView *)sender {
    if (sender == self.scroll) {
        int pageNum = (int)(self.scrPage.contentOffset.x / self.scrPage.frame.size.width);
        self.pagecontroller.currentPage =pageNum;
    }
}

回答9:

I extracted this out of a couple of our apps...

- (NSInteger)currentPage
{
    if (0 == self.frame.size.width) {
        NSLog(@"frame width == 0!");
        return 0;
    }
    if (! self.currentPageValid) {
        _currentPage = round(self.contentOffset.x / self.frame.size.width);
        self.currentPageValid = YES;
    }
    return _currentPage;
}

Full source here

回答10:

For swift I would just use this

    let width = scrollView.frame.width
    let page = round(scrollView.contentOffset.x/width)

Pretty straightforward, it basically takes the scrollview x position, divides it by the scrollview width, then rounds it.

回答11:

Are you using a UIPageControl? If so, this has a currentPage property. If not, I think you'll need to calculate the page index from the scrollView offset.

回答12:

Another way:

extension MyViewController: UIScrollViewDelegate {
    func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
        let width = scrollView.frame.width
        let page = Int(round(scrollView.contentOffset.x/width))
        print("CurrentPage:\(page)")
    }
}

回答13:

If Any one Looking for way of doing in C# for Xamarin

    public int CurrentIndex
    {
        get => (int)Math.Round(this.scrollView.ContentOffset.X / this.scrollView.Frame.Width);
        set => this.scrollView.ScrollRectToVisible(new CGRect(new CGPoint(this.scrollView.Frame.Size.Width * value, 0), this.scrollView.Frame.Size), true);
    }

This getter and Setter should provide you current page as well let you scroll to the specified page