Keeping the contentOffset in a UICollectionView while rotating Interface Orientation

Question

I'm trying to handle interface orientation changes in a UICollectionViewController. What I'm trying to achieve is, that I want to have the same contentOffset after an interface rotation. Meaning, that it should be changed corresponding to the ratio of the bounds change.

Starting in portrait with a content offset of {bounds.size.width * 2, 0} …

… should result to the content offset in landscape also with {bounds.size.width * 2, 0} (and vice versa).

Calculating the new offset is not the problem, but don't know, where (or when) to set it, to get a smooth animation. What I'm doing so fare is invalidating the layout in willRotateToInterfaceOrientation:duration: and resetting the content offset in didRotateFromInterfaceOrientation::

- (void)willRotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation
                                duration:(NSTimeInterval)duration;
{
    self.scrollPositionBeforeRotation = CGPointMake(self.collectionView.contentOffset.x / self.collectionView.contentSize.width,
                                                    self.collectionView.contentOffset.y / self.collectionView.contentSize.height);
    [self.collectionView.collectionViewLayout invalidateLayout];
}

- (void)didRotateFromInterfaceOrientation:(UIInterfaceOrientation)fromInterfaceOrientation;
{
    CGPoint newContentOffset = CGPointMake(self.scrollPositionBeforeRotation.x * self.collectionView.contentSize.width,
                                           self.scrollPositionBeforeRotation.y * self.collectionView.contentSize.height);
    [self.collectionView newContentOffset animated:YES];
}

This changes the content offset after the rotation.

How can I set it during the rotation? I tried to set the new content offset in willAnimateRotationToInterfaceOrientation:duration: but this results into a very strange behavior.

An example can be found in my Project on GitHub.

Tourney

Answer 1

You can either do this in the view controller:

override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
    super.viewWillTransition(to: size, with: coordinator)

    guard let collectionView = collectionView else { return }
    let offset = collectionView.contentOffset
    let width = collectionView.bounds.size.width

    let index = round(offset.x / width)
    let newOffset = CGPoint(x: index * size.width, y: offset.y)

    coordinator.animate(alongsideTransition: { (context) in
        collectionView.reloadData()
        collectionView.setContentOffset(newOffset, animated: false)
    }, completion: nil)
}

Or in the layout itself: https://mcmap.net/q/265164/-keeping-the-contentoffset-in-a-uicollectionview-while-rotating-interface-orientation

Predictor

Exact and precise answer. Thanks a lot! – Tonl

After quite a bit of research this is the most elegant solution! – Arty

It works. You just have to make sure that viewWillTransition is being called. Other point is if the next page is not a "entire" page you just need to take of the round on index variable. – Swansea

Should you be using the floor function instead of round? – Milicent

@Milicent I am using round only – Predictor

Instead of calling collectionView.reloadData() you can call collectionView.collectionViewLayout.invalidateLayout() – Mihe

Answer 2

Solution 1, "just snap"

If what you need is only to ensure that the contentOffset ends in a right position, you can create a subclass of UICollectionViewLayout and implement targetContentOffsetForProposedContentOffset: method. For example you could do something like this to calculate the page:

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset
{
    NSInteger page = ceil(proposedContentOffset.x / [self.collectionView frame].size.width);
    return CGPointMake(page * [self.collectionView frame].size.width, 0);
}

But the problem that you'll face is that the animation for that transition is extremely weird. What I'm doing on my case (which is almost the same as yours) is:

Solution 2, "smooth animation"

1) First I set the cell size, which can be managed by collectionView:layout:sizeForItemAtIndexPath: delegate method as follows:

- (CGSize)collectionView:(UICollectionView *)collectionView
                  layout:(UICollectionViewLayout  *)collectionViewLayout
  sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
    return [self.view bounds].size;
}

Note that [self.view bounds] will change according to the device rotation.

2) When the device is about to rotate, I'm adding an imageView on top of the collection view with all resizing masks. This view will actually hide the collectionView weirdness (because it is on top of it) and since the willRotatoToInterfaceOrientation: method is called inside an animation block it will rotate accordingly. I'm also keeping the next contentOffset according to the shown indexPath so I can fix the contentOffset once the rotation is done:

- (void)willRotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation
                                duration:(NSTimeInterval)duration
{
    // Gets the first (and only) visible cell.
    NSIndexPath *indexPath = [[self.collectionView indexPathsForVisibleItems] firstObject];
    KSPhotoViewCell *cell = (id)[self.collectionView cellForItemAtIndexPath:indexPath];

    // Creates a temporary imageView that will occupy the full screen and rotate.
    UIImageView *imageView = [[UIImageView alloc] initWithImage:[[cell imageView] image]];
    [imageView setFrame:[self.view bounds]];
    [imageView setTag:kTemporaryImageTag];
    [imageView setBackgroundColor:[UIColor blackColor]];
    [imageView setContentMode:[[cell imageView] contentMode]];
    [imageView setAutoresizingMask:0xff];
    [self.view insertSubview:imageView aboveSubview:self.collectionView];

    // Invalidate layout and calculate (next) contentOffset.
    contentOffsetAfterRotation = CGPointMake(indexPath.item * [self.view bounds].size.height, 0);
    [[self.collectionView collectionViewLayout] invalidateLayout];
}

Note that my subclass of UICollectionViewCell has a public imageView property.

3) Finally, the last step is to "snap" the content offset to a valid page and remove the temporary imageview.

- (void)didRotateFromInterfaceOrientation:(UIInterfaceOrientation)fromInterfaceOrientation
{
    [self.collectionView setContentOffset:contentOffsetAfterRotation];
    [[self.view viewWithTag:kTemporaryImageTag] removeFromSuperview];
}

Lombardi

Your answer is brilliant! But you missed calls to super in willRotateToInterfaceOrientation and didRotateFromInterfaceOrientation. – Elene

This didn't work for me as it was, I instead changed it to not calculate an offset after rotation, instead I store the NSIndexPath pulled when getting cell information and instead of setting the offset I scroll to that with no animation. If someone has interest I can put together an actual answer with code (swift). – Suppletion

Answer 3

The "just snap" answer above didn't work for me as it frequently didn't end on the item that was in view before the rotate. So I derived a flow layout that uses a focus item (if set) for calculating the content offset. I set the item in willAnimateRotationToInterfaceOrientation and clear it in didRotateFromInterfaceOrientation. The inset adjustment seems to be need on IOS7 because the Collection view can layout under the top bar.

@interface HintedFlowLayout : UICollectionViewFlowLayout
@property (strong)NSIndexPath* pathForFocusItem;
@end

@implementation HintedFlowLayout

-(CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset
{
    if (self.pathForFocusItem) {
        UICollectionViewLayoutAttributes* layoutAttrs = [self layoutAttributesForItemAtIndexPath:self.pathForFocusItem];
        return CGPointMake(layoutAttrs.frame.origin.x - self.collectionView.contentInset.left, layoutAttrs.frame.origin.y-self.collectionView.contentInset.top);
    }else{
        return [super targetContentOffsetForProposedContentOffset:proposedContentOffset];
    }
}
@end

Ptah

by far the simplest solution – Greaves

Answer 4

Swift 4.2 subclass:

class RotatableCollectionViewFlowLayout: UICollectionViewFlowLayout {

    private var focusedIndexPath: IndexPath?

    override func prepare(forAnimatedBoundsChange oldBounds: CGRect) {
        super.prepare(forAnimatedBoundsChange: oldBounds)
        focusedIndexPath = collectionView?.indexPathsForVisibleItems.first
    }

    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint) -> CGPoint {
        guard let indexPath = focusedIndexPath
            , let attributes = layoutAttributesForItem(at: indexPath)
            , let collectionView = collectionView else {
                return super.targetContentOffset(forProposedContentOffset: proposedContentOffset)
        }
        return CGPoint(x: attributes.frame.origin.x - collectionView.contentInset.left,
                       y: attributes.frame.origin.y - collectionView.contentInset.top)
    }

    override func finalizeAnimatedBoundsChange() {
        super.finalizeAnimatedBoundsChange()
        focusedIndexPath = nil
    }
}

Laetitia

This is more what I was looking for. I wanted rotation code to be contained within my custom UICollectionView subclass, so that rotation code wouldn't need to be duplicated throughout the app. This does cause scrolling to occur though, which means cell re-use kicks in. That may or may not have unintentional side-effects. – Rida

Unfortunately, targetContentOffset isn't called for me – Breland

now with UICollectionViewCompositionalLayout is this answer still valid, because I am unable to do so – Pigtail

That's the correct solution! Be careful though, in prepare(forAnimatedBoundsChange oldBounds: CGRect) {} the collectionView?.bounds would be the new bounds. If you want to compute the focused indexpath manually, you should use the oldBounds and the collectionView?.contentOffset which remains unchanged by default when rotating. – Callie

Answer 5

For those using iOS 8+, willRotateToInterfaceOrientation and didRotateFromInterfaceOrientation are deprecated.

You should use the following now:

/* 
This method is called when the view controller's view's size is changed by its parent (i.e. for the root view controller when its window rotates or is resized). 
If you override this method, you should either call super to propagate the change to children or manually forward the change to children.
*/
- (void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id <UIViewControllerTransitionCoordinator>)coordinator 
{
    [super viewWillTransitionToSize:size withTransitionCoordinator:coordinator];

    [coordinator animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext> context) {
        // Update scroll position during rotation animation
        self.collectionView.contentOffset = (CGPoint){contentOffsetX, contentOffsetY};
    } completion:^(id<UIViewControllerTransitionCoordinatorContext> context) {
        // Whatever you want to do when the rotation animation is done
    }];
}

Swift 3:

override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
    super.viewWillTransition(to: size, with: coordinator)

    coordinator.animate(alongsideTransition: { (context:UIViewControllerTransitionCoordinatorContext) in
        // Update scroll position during rotation animation
    }) { (context:UIViewControllerTransitionCoordinatorContext) in
        // Whatever you want to do when the rotation animation is done
    }
}

Mineralogy

How about fullscreen cells ? When I slide to second or third cell, and rotate I see previous cell on last, and last on second :( – Sergo

Answer 6

I think the correct solution is to override targetContentOffsetForProposedContentOffset: method in a subclassed UICollectionViewFlowLayout

From the docs:

During layout updates, or when transitioning between layouts, the collection view calls this method to give you the opportunity to change the proposed content offset to use at the end of the animation. You might override this method if the animations or transition might cause items to be positioned in a way that is not optimal for your design.

Busy

Answer 7

To piggy back off troppoli's solution you can set the offset in your custom class without having to worry about remembering to implement the code in your view controller. prepareForAnimatedBoundsChange should get called when you rotate the device then finalizeAnimatedBoundsChange after its done rotating.

@interface OrientationFlowLayout ()

@property (strong)NSIndexPath* pathForFocusItem;

@end

@implementation OrientationFlowLayout

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset {
    if (self.pathForFocusItem) {
        UICollectionViewLayoutAttributes* layoutAttrs = [self layoutAttributesForItemAtIndexPath:
                                                         self.pathForFocusItem];
        return CGPointMake(layoutAttrs.frame.origin.x - self.collectionView.contentInset.left,
                           layoutAttrs.frame.origin.y - self.collectionView.contentInset.top);
    }
    else {
        return [super targetContentOffsetForProposedContentOffset:proposedContentOffset];
    }
}

- (void)prepareForAnimatedBoundsChange:(CGRect)oldBounds {
    [super prepareForAnimatedBoundsChange:oldBounds];
    self.pathForFocusItem = [[self.collectionView indexPathsForVisibleItems] firstObject];
}

- (void)finalizeAnimatedBoundsChange {
    [super finalizeAnimatedBoundsChange];
    self.pathForFocusItem = nil;
}

@end

Synonymize

is the call to super needed though? – Greaves

I implemented this in Xamarin, works great in iOS 11 – Randirandie

Answer 8

This problem bothered me for a bit as well. The highest voted answered seemed a bit too hacky for me so I just dumbed it down a bit and just change the alpha of the collection view respectively before and after rotation. I also don't animate the content offset update.

- (void)willRotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration
{
self.collectionView.alpha = 0;
[self.collectionView.collectionViewLayout invalidateLayout];

self.scrollPositionBeforeRotation = CGPointMake(self.collectionView.contentOffset.x / self.collectionView.contentSize.width,
                                                self.collectionView.contentOffset.y / self.collectionView.contentSize.height);
}

- (void)didRotateFromInterfaceOrientation:(UIInterfaceOrientation)fromInterfaceOrientation;
{
CGPoint newContentOffset = CGPointMake(self.scrollPositionBeforeRotation.x * self.collectionView.contentSize.width,
                                       self.scrollPositionBeforeRotation.y * self.collectionView.contentSize.height);

[self.collectionView setContentOffset:newContentOffset animated:NO];
self.collectionView.alpha = 1;
}

Fairly smooth and less hacky.

Cultivation

Answer 9

I use a variant of fz. answer (iOS 7 & 8) :

Before rotation :

1. Store the current visible index path
2. Create a snapshot of the collectionView
3. Put an UIImageView with it on top of the collection view

After rotation :

1. Scroll to the stored index

2. Remove the image view.

   @property (nonatomic) NSIndexPath *indexPath;
   
   - (void)willRotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation
                                   duration:(NSTimeInterval)duration {
       self.indexPathAfterRotation = [[self.collectionView indexPathsForVisibleItems] firstObject];
   
       // Creates a temporary imageView that will occupy the full screen and rotate.
       UIGraphicsBeginImageContextWithOptions(self.collectionView.bounds.size, YES, 0);
       [self.collectionView drawViewHierarchyInRect:self.collectionView.bounds afterScreenUpdates:YES];
       UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
       UIGraphicsEndImageContext();
   
       UIImageView *imageView = [[UIImageView alloc] initWithImage:image];
       [imageView setFrame:[self.collectionView bounds]];
       [imageView setTag:kTemporaryImageTag];
       [imageView setBackgroundColor:[UIColor blackColor]];
       [imageView setContentMode:UIViewContentModeCenter];
       [imageView setAutoresizingMask:0xff];
       [self.view insertSubview:imageView aboveSubview:self.collectionView];
   
       [[self.collectionView collectionViewLayout] invalidateLayout];
   }
   
   - (void)didRotateFromInterfaceOrientation:(UIInterfaceOrientation)fromInterfaceOrientation {
       [self.collectionView scrollToItemAtIndexPath:self.indexPath atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:NO];
   
       [[self.view viewWithTag:kTemporaryImageTag] removeFromSuperview];
   }
   

Saladin

I ended up using something like this as well, wish I'd seen yours first – Suppletion

Answer 10

After rotate interface orientation the UICollectionViewCell usually move to another position, because we won't update contentSize and contentOffset.

So the visible UICollectionViewCell always not locate at expected position.

The visible UICollectionView which we expected image as follow

Orientation which we expected

UICollectionView must delegate the function [collectionView sizeForItemAtIndexPath] of『UICollectionViewDelegateFlowLayout』.

And you should calculate the item Size in this function.

The custom UICollectionViewFlowLayout must override the functions as follow.

1. -(void)prepareLayout

   ． Set itemSize, scrollDirection and others.

2. -(CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity

   ． Calculate page number or calculate visible content offset.

3. -(CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset

   ． Return visual content offset.

4. -(CGSize)collectionViewContentSize

   ． Return the total content size of collectionView.

Your viewController must override 『willRotateToInterfaceOrientation』and in this function you should call the function [XXXCollectionVew.collectionViewLayout invalidateLayout];

But 『willRotateToInterfaceOrientation』 is deprecated in iOS 9, or you could call the function [XXXCollectionVew.collectionViewLayout invalidateLayout] in difference way.

There's an example as follow : https://github.com/bcbod2002/CollectionViewRotationTest

Tryptophan

Please describe what you have done or tell anything about your project! – Stridulous

Answer 11

in Swift 3.

you should track which cell item(Page) is being presented before rotate by indexPath.item, the x coordinate or something else. Then, in your UICollectionView:

override func collectionView(_ collectionView: UICollectionView, targetContentOffsetForProposedContentOffset proposedContentOffset: CGPoint) -> CGPoint {

    let page:CGFloat = pageNumber // your tracked page number eg. 1.0
    return CGPoint(x: page * collectionView.frame.size.width, y: -(topInset))
    //the 'y' value would be '0' if you don't have any top EdgeInset
}

In my case I invalidate the layout in viewDidLayoutSubviews() so the collectionView.frame.size.width is the width of the collectionVC's view that has been rotated.

Tgroup

Answer 12

What does the job for me is this:

1. Set the size of your my cells from your my UICollectionViewDelegateFlowLayout method

   func collectionView(collectionView: UICollectionView!, layout collectionViewLayout: UICollectionViewLayout!, sizeForItemAtIndexPath indexPath: NSIndexPath!) -> CGSize
   {
       return collectionView.bounds.size
   }
   

2. After that I implement willRotateToInterfaceOrientationToInterfaceOrientation:duration: like this

   override func willRotateToInterfaceOrientation(toInterfaceOrientation: UIInterfaceOrientation, duration: NSTimeInterval) 
   {
       let currentPage = Int(collectionView.contentOffset.x / collectionView.bounds.size.width)
   
       var width = collectionView.bounds.size.height
       UIView.animateWithDuration(duration) {
           self.collectionView.setContentOffset(CGPointMake(width * CGFloat(currentPage), 0.0), animated: false)
           self.collectionView.collectionViewLayout.invalidateLayout()
       }
   }
   

The above code is in Swift but you get the point and it's easy to "translate"

Yachtsman

Answer 13

This work like a charm:

-(CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath {
    return self.view.bounds.size;
}

-(void)willRotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration {

    int currentPage = collectionMedia.contentOffset.x / collectionMedia.bounds.size.width;
    float width = collectionMedia.bounds.size.height;

    [UIView animateWithDuration:duration animations:^{
        [self.collectionMedia setContentOffset:CGPointMake(width * currentPage, 0.0) animated:NO];
        [[self.collectionMedia collectionViewLayout] invalidateLayout];
}];
}

Casemate

Answer 14

If found that using targetContentOffsetForProposedContentOffset does not work in all scenarios and the problem with using didRotateFromInterfaceOrientation is that it gives visual artifacts. My perfectly working code is as follows:

- (void)willRotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration
{
    [super willRotateToInterfaceOrientation:toInterfaceOrientation duration:duration];
    _indexPathOfFirstCell = [self indexPathsForVisibleItems].firstObject;
}

- (void)willAnimateRotationToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration {
    [super willAnimateRotationToInterfaceOrientation:toInterfaceOrientation duration:duration];
    if (_indexPathOfFirstCell) {
        [UIView performWithoutAnimation:^{
            [self scrollToItemAtIndexPath:self->_indexPathOfFirstCell atScrollPosition:UICollectionViewScrollPositionTop animated:NO];
        }];
        _indexPathOfFirstCell = nil;
    }
}

The key is to use the willRotateToInterfaceOrientation method to determine the part in the view that you want to scroll to and willAnimationRotationToInterfaceOrientation to recalculate it when the view has changed its size (the bounds have already changed when this method is called by the framework) and to actually scroll to the new position without animation. In my code I used the index path for the first visual cell to do that, but a percentage of contentOffset.y/contentSize.height would also do the job in slightly different way.

Finance

don't forget that the rotation API was deprecated as of iOS 8] – Dislocate

Answer 15

You might want to hide the collectionView during it's (incorrect) animation and show a placeholder view of the cell that rotates correctly instead.

For a simple photo gallery I found a way to do it that looks quite good. See my answer here: How to rotate a UICollectionView similar to the photos app and keep the current view centered?

Rafi

Answer 16

My way is to use a UICollectionViewFlowlayout object.

Set the ojbect line spacing if it scrolls horizontally.

[flowLayout setMinimumLineSpacing:26.0f];

Set its interitem spacing if it scrolls vertically.

[flowLayout setMinimumInteritemSpacing:0.0f];

Notice it behaves different when you rotate the screen. In my case, I have it scrolls horizontally so minimumlinespacing is 26.0f. Then it seems horrible when it rotates to landscape direction. I have to check rotation and set minimumlinespacing for that direction 0.0f to make it right.

That's it! Simple.

Rai

Answer 17

I had the issue with my project,i used two different layout for the UICollectionView.

mCustomCell *cell = [cv dequeueReusableCellWithReuseIdentifier:@"LandScapeCell" forIndexPath:indexPath];

theCustomCell *cell = [cv dequeueReusableCellWithReuseIdentifier:@"PortraitCell" forIndexPath:indexPath];

Then Check it for each orientation and use your configuration for each orientation.

Sjambok

Answer 18

-(CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath {
    CGSize pnt = CGSizeMake(70, 70);
    return pnt; }

-(UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout insetForSectionAtIndex:(NSInteger)section {

//    UIEdgeInsetsMake(<#CGFloat top#>, <#CGFloat left#>, <#CGFloat bottom#>, <#CGFloat right#>)
    return UIEdgeInsetsMake(3, 0, 3, 0); }

This way you can adjust the content offset and the size of your cell.

Crossindex

Answer 19

Use <CollectionViewDelegateFlowLayout> and in the method didRotateFromInterfaceOrientation: reload data of the CollectionView.

Implement collectionView:layout:sizeForItemAtIndexPath: method of <CollectionViewDelegateFlowLayout> and in the method verify the Interface orientation and apply your custom size of each cell.

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{

    UIInterfaceOrientation orientation = [[UIApplication sharedApplication] statusBarOrientation];

    if (UIInterfaceOrientationIsPortrait(orientation)) {

        return CGSizeMake(CGFloat width, CGFloat height);

    } else {

        return CGSizeMake(CGFloat width, CGFloat height);

    }

}

Headwaiter

Answer 20

I have a similar case in which i use this

- (void)setFrame:(CGRect)frame
{
    CGFloat currentWidth = [self frame].size.width;
    CGFloat offsetModifier = [[self collectionView] contentOffset].x / currentWidth;

    [super setFrame:frame];

    CGFloat newWidth = [self frame].size.width;

    [[self collectionView] setContentOffset:CGPointMake(offsetModifier * newWidth, 0.0f) animated:NO];
}

This is a view that contains a collectionView. In the superview I also do this

- (void)setFrame:(CGRect)frame
{    
    UICollectionViewFlowLayout *collectionViewFlowLayout = (UICollectionViewFlowLayout *)[_collectionView collectionViewLayout];

    [collectionViewFlowLayout setItemSize:frame.size];

    [super setFrame:frame];
}

This is to adjust the cell sizes to be full screen (full view to be exact ;) ). If you do not do this here a lot of error messages may appear about that the cell size is bigger than the collectionview and that the behaviour for this is not defined and bla bla bla.....

These to methods can off course be merged into one subclass of the collectionview or in the view containing the collectionview but for my current project was this the logical way to go.

Basal

Answer 21

The "just snap" answer is the right approach and doesn't require extra smoothing with snapshot overlays IMO. However there's an issue which explains why some people see that the correct page isn't scrolled to in some cases. When calculating the page, you'd want to use the height and not the width. Why? Because the view geometry has already rotated by the time targetContentOffsetForProposedContentOffset is called, and so what was the width is now the height. Also rounding is more sensible than ceiling. So:

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset
{
    NSInteger page = round(proposedContentOffset.x / self.collectionView.bounds.size.height);
    return CGPointMake(page * self.collectionView.bounds.size.width, 0);
}

Earflap

Answer 22

I solved this problem with Following Steps:

1. Calculate currently scrolled NSIndexPath
2. Disable Scrolling and Pagination in UICollectionView
3. Apply new Flow Layout to UICollectionView
4. Enable Scrolling and Pagination in UICollectionView
5. Scroll UICollectionView to current NSIndexPath

Here is the Code Template demonstrating the Above Steps:

- (void)willRotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation
                            duration:(NSTimeInterval)duration;
{
     //Calculating Current IndexPath
     CGRect visibleRect = (CGRect){.origin = self.yourCollectionView.contentOffset, .size = self.yourCollectionView.bounds.size};
     CGPoint visiblePoint = CGPointMake(CGRectGetMidX(visibleRect), CGRectGetMidY(visibleRect));
     self.currentIndexPath = [self.yourCollectionView indexPathForItemAtPoint:visiblePoint];

     //Disable Scrolling and Pagination
     [self disableScrolling];

     //Applying New Flow Layout
     [self setupNewFlowLayout];

     //Enable Scrolling and Pagination
     [self enableScrolling];
}

- (void)didRotateFromInterfaceOrientation:(UIInterfaceOrientation)fromInterfaceOrientation;
{
     //You can also call this at the End of `willRotate..` method.
     //Scrolling UICollectionView to current Index Path
     [self.yourCollectionView scrollToItemAtIndexPath:self.currentIndexPath atScrollPosition:UICollectionViewScrollPositionCenteredVertically animated:NO];
}

- (void) disableScrolling
{
    self.yourCollectionView.scrollEnabled   = false;
    self.yourCollectionView.pagingEnabled   = false;
}

- (void) enableScrolling
{
    self.yourCollectionView.scrollEnabled   = true;
    self.yourCollectionView.pagingEnabled   = true;
}

- (void) setupNewFlowLayout
{
    UICollectionViewFlowLayout* flowLayout = [[UICollectionViewFlowLayout alloc] init];
    flowLayout.sectionInset = UIEdgeInsetsMake(0, 0, 0, 0);
    flowLayout.scrollDirection = UICollectionViewScrollDirectionHorizontal;
    flowLayout.minimumInteritemSpacing = 0;
    flowLayout.minimumLineSpacing = 0;
    [flowLayout setItemSize:CGSizeMake(EXPECTED_WIDTH, EXPECTED_HEIGHT)];

    [self.yourCollectionView setCollectionViewLayout:flowLayout animated:YES];
    [self.yourCollectionView.collectionViewLayout invalidateLayout];
}

I hope this helps.

Demonstrate

Answer 23

I had got some troubles with animateAlongsideTransition block in animateAlongsideTransition (see the code below).

Pay attention, that it is called during (but not before) the animation My task was update the table view scroll position using scrolling to the top visible row (I’ve faced with the problem on iPad when table view cells shifted up when the device rotation, therefore I was founding the solution for that problem). But may be it would be useful for contentOffset too.

I tried to solve the problem by the following way:

- (void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator {
    [super viewWillTransitionToSize:size withTransitionCoordinator:coordinator];

    __weak TVChannelsListTableViewController *weakSelf = self;

    [coordinator animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext>  _Nonnull context) {
        weakSelf.topVisibleRowIndexPath = [[weakSelf.tableView indexPathsForVisibleRows] firstObject];
    } completion:^(id<UIViewControllerTransitionCoordinatorContext>  _Nonnull context) {
        [weakSelf.tableView scrollToRowAtIndexPath:weakSelf.topVisibleRowIndexPath atScrollPosition:UITableViewScrollPositionTop animated:NO];
    }];
}

But it didn’t work. For instance, index path of the top cel was (0, 20). But when the device rotation animateAlongsideTransition block was called and [[weakSelf.tableView indexPathsForVisibleRows] firstObject] returned index path (0, 27).

I thought the problem was in retrieving index paths to weakSelf. Therefore to solve the problem I’ve moved self.topVisibleRowIndexPath before [coordinator animateAlongsideTransition: completion] method calling:

- (void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator {
    [super viewWillTransitionToSize:size withTransitionCoordinator:coordinator];

    __weak TVChannelsListTableViewController *weakSelf = self;
    self.topVisibleRowIndexPath = [[weakSelf.tableView indexPathsForVisibleRows] firstObject];

    [coordinator animateAlongsideTransition:nil completion:^(id<UIViewControllerTransitionCoordinatorContext>  _Nonnull context) {
        [weakSelf.tableView scrollToRowAtIndexPath:weakSelf.topVisibleRowIndexPath atScrollPosition:UITableViewScrollPositionTop animated:NO];
    }];
}

And the other interesting thing that I’ve discovered is that the deprecated methods willRotateToInterfaceOrientation and willRotateToInterfaceOrientation are still successful called in iOS later 8.0 when method viewWillTransitionToSize is not redefined.

So the other way to solve the problem in my case was to use deprecated method instead of new one. I think it would be not right solution, but it is possible to try if other ways don’t work :)

Onitaonlooker

Answer 24

You might want to try this untested code:

- (void) willRotateToInterfaceOrientation: (UIInterfaceOrientation) toInterfaceOrientation
                                 duration: (NSTimeInterval)         duration
{
    [UIView animateWithDuration: duration
                      animation: ^(void)
     {
       CGPoint newContentOffset = CGPointMake(self.scrollPositionBeforeRotation.x *
                                              self.collectionView.contentSize.height,
                                              self.scrollPositionBeforeRotation.y *
                                              self.collectionView.contentSize.width);
       [self.collectionView setContentOffset: newContentOffset
                                    animated: YES];
     }];
}

Torpedo