make UIView in UIScrollView stick to the top when

问题:

So in a UITableView when you have sections the section view sticks to the top until the next section overlaps it and then it replaces it on top. I want to have a similar effect, where basically I have a UIView in my UIScrollView, representing the sections UIView and when it hits the top.. I want it to stay in there and not get carried up. How do I do this? I think this needs to be done in either layoutSubviews or scrollViewDidScroll and do a manipulation on the UIVIew..

回答1:

To create UIView in UIScrollView stick to the top when scrolled up do:

- (void)createHeaderView:(UIView*)headerView {
    _headerView = [headerView retain];
    _headerViewInitialY = _headerView.frame.origin.y;
    [_scrollView addSubview:_headerView];
    _scrollView.delegate = self;
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    CGRect headerFrame = _headerView.frame;
    headerFrame.origin.y = MAX(_headerViewInitialY, scrollView.contentOffset.y);
    _headerView.frame = headerFrame;
}

回答2:

Swift Solution based on EVYA's response:

var navigationBarOriginalOffset : CGFloat?

override func viewWillAppear(animated: Bool) {
    super.viewWillAppear(animated)
    navigationBarOriginalOffset = navigationBar.frame.origin.y
}

func scrollViewDidScroll(scrollView: UIScrollView) {
    navigationBar.frame.origin.y = max(navigationBarOriginalOffset!, scrollView.contentOffset.y)
}

回答3:

If I recall correctly, the 2010 WWDC ScrollView presentation discusses precisely how to keep a view in a fixed position while other elements scroll around it. Watch the video and you should have a clear-cut approach to implement.

It's essentially updating frames based on scrollViewDidScroll callbacks (although memory is a bit hazy on the finer points).

回答4:

Evya's solution works really well, however if you use Auto Layout, you should do something like this (The Auto Layout syntax is written in Masonry, but you get the idea.):

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {

    //Make the header view sticky to the top.
    [self.headerView mas_remakeConstraints:^(MASConstraintMaker *make) {
        make.top.equalTo(self.scrollView.mas_top).with.offset(scrollView.contentOffset.y);
        make.left.equalTo(self.scrollView.mas_left);
        make.right.equalTo(self.scrollView.mas_right);

        make.height.equalTo(@(headerViewHeight));
    }];

    [self.scrollView bringSubviewToFront:self.headerView];
}