Autolayout is changing UIScrollView's contentOffset on rotation

Question

To experiment with autolayout and uiscrollview's I have been using this example

which I have edited to include 2 views in the scroll view, I have setup the autolayout constraints to position the views horizontally adjacent with their size set to fill the scroll view frame.

UIView *beeView = [[[NSBundle mainBundle] loadNibNamed:@"BeeView" owner:nil options:nil] firstObject];
beeView.translatesAutoresizingMaskIntoConstraints = NO;
[self.scrollView addSubview:beeView];
UIView *beeView2 = [[[NSBundle mainBundle] loadNibNamed:@"BeeView" owner:nil options:nil] firstObject];
beeView2.backgroundColor= [UIColor orangeColor];
beeView2.translatesAutoresizingMaskIntoConstraints = NO;
[self.scrollView addSubview:beeView2];

NSDictionary *views = @{@"beeView":beeView,@"beeView2":beeView2, @"scrollView":self.scrollView};
NSDictionary *metrics = @{@"height" : @200};
[self.scrollView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[beeView(==scrollView)][beeView2(==beeView)]|" options:NSLayoutFormatAlignAllTop | NSLayoutFormatAlignAllBottom metrics:metrics views:views]];
[self.scrollView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[beeView(==scrollView)]|" options:kNilOptions metrics:metrics views:views]];

which nicely produces what I intended.

However, if the scroll view's contentOffset is nonzero and the device is rotated from portrait to landscape, the content offset of the scroll view is automatically set to 32px. (see screenshot)

I have tried saving contentOffset and setting it to this saved value when scrollViewDidEndDecelerating: is called which works but is ugly as the scroll view scrolls to a 32px offset and then back to where I want it to be.

How do I control the scroll view's contentOffset? Are the autolayout constraints wrong? Are there extra constraints I can add to control the contentOffset when resizing the view?

Answer 1

Where the 32px comes from? Is it related to your left and right scrollView margin?

Does it keep the wrong offset every time you change page ? If that the case, you should look at your scrollView's contentInsets values.

Otherwise, what I do to manage rotation on scrollView with paging is observing the scrollView's contentSize:

First, when you load the view, add the observer:

[self.scrollView addObserver:self forKeyPath:NSStringFromSelector(@selector(contentSize)) options:0 context:nil];

Then, when the contentSize value change, adjust the contentOffset:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {

    if (object == self.scrollView && [keyPath isEqualToString:NSStringFromSelector(@selector(contentSize))]) {

        //Note that you should track your page index
        self.scrollView.contentOffset = CGPointMake(self.pageIndex * self.scrollView.bounds.size.width, self.scrollView.contentOffset.y);

    } else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

Finally, remove the observer when you unload the scrollView:

[self.scrollView removeObserver:self forKeyPath:NSStringFromSelector(@selector(contentSize)) context:nil];