iPhone - knowing if a UIScrollView reached the top or bottom

问题

Is there a way to know if a UIScrollView has reached the top or bottom inside

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView 
                  willDecelerate:(BOOL)decelerate

?

thanks.

回答1:

Implement the UIScrollViewDelegate in your class, and then add this:

-(void)scrollViewDidScroll: (UIScrollView*)scrollView
{
    float scrollViewHeight = scrollView.frame.size.height;
    float scrollContentSizeHeight = scrollView.contentSize.height;
    float scrollOffset = scrollView.contentOffset.y;

    if (scrollOffset == 0)
    {
        // then we are at the top
    }
    else if (scrollOffset + scrollViewHeight == scrollContentSizeHeight)
    {
        // then we are at the end
    }
}

Hope this is what you are after! Else have a tinker by adding more conditions to the above code and NSLog the value of scrollOffset.

回答2:

Well, contentInsets are also involved, when you try to determine whether scrollView is at the top or at the bottom. You might also be interested in cases when your scrollView is above the top and below the bottom. Here is the code I use to find top and bottom positions:

Swift:

extension UIScrollView {

    var isAtTop: Bool {
        return contentOffset.y <= verticalOffsetForTop
    }

    var isAtBottom: Bool {
        return contentOffset.y >= verticalOffsetForBottom
    }

    var verticalOffsetForTop: CGFloat {
        let topInset = contentInset.top
        return -topInset
    }

    var verticalOffsetForBottom: CGFloat {
        let scrollViewHeight = bounds.height
        let scrollContentSizeHeight = contentSize.height
        let bottomInset = contentInset.bottom
        let scrollViewBottomOffset = scrollContentSizeHeight + bottomInset - scrollViewHeight
        return scrollViewBottomOffset
    }

}

Objective-C:

@implementation UIScrollView (Additions)

- (BOOL)isAtTop {
    return (self.contentOffset.y <= [self verticalOffsetForTop]);
}

- (BOOL)isAtBottom {
    return (self.contentOffset.y >= [self verticalOffsetForBottom]);
}

- (CGFloat)verticalOffsetForTop {
    CGFloat topInset = self.contentInset.top;
    return -topInset;
}

- (CGFloat)verticalOffsetForBottom {
    CGFloat scrollViewHeight = self.bounds.size.height;
    CGFloat scrollContentSizeHeight = self.contentSize.height;
    CGFloat bottomInset = self.contentInset.bottom;
    CGFloat scrollViewBottomOffset = scrollContentSizeHeight + bottomInset - scrollViewHeight;
    return scrollViewBottomOffset;
}


@end

回答3:

If you want the code in swift:

override func scrollViewDidScroll(scrollView: UIScrollView) {

    if (scrollView.contentOffset.y >= (scrollView.contentSize.height - scrollView.frame.size.height)) {
        //reach bottom
    }

    if (scrollView.contentOffset.y <= 0){
        //reach top
    }

    if (scrollView.contentOffset.y > 0 && scrollView.contentOffset.y < (scrollView.contentSize.height - scrollView.frame.size.height)){
        //not top and not bottom
    }
}

回答4:

Simple and clean extension:

import UIKit

extension UIScrollView {

    var scrolledToTop: Bool {
        let topEdge = 0 - contentInset.top
        return contentOffset.y <= topEdge
    }

    var scrolledToBottom: Bool {
        let bottomEdge = contentSize.height + contentInset.bottom - bounds.height
        return contentOffset.y >= bottomEdge
    }

}

On GitHub:

https://github.com/salutis/swift-scroll-view-edges

回答5:

I figured out exactly how to do it:

CGFloat maxPosition = scrollView.contentInset.top + scrollView.contentSize.height + scrollView.contentInset.bottom - scrollView.bounds.size.height;
CGFloat currentPosition = scrollView.contentOffset.y + self.topLayoutGuide.length;

if (currentPosition == maxPosition) {
  // you're at the bottom!
}

回答6:

Do it like this(for Swift 3):

class MyClass: UIScrollViewDelegate {

   func scrollViewDidScroll(_ scrollView: UIScrollView) {

        if (scrollView.contentOffset.y >= (scrollView.contentSize.height - scrollView.frame.size.height)) {
           //scrolled to top, do smth
        }


    }
}

来源：https://stackoverflow.com/questions/7706152/iphone-knowing-if-a-uiscrollview-reached-the-top-or-bottom