I&#39;m trying to add `UICollectionView` to `ViewController`, and I need to have 3 cells &#39;per row&#39; without blank space between cells (it should look like a grid). Cell width should be one third of screen size, so I thought that the `layout.item` width should be the same. But then I get this:

![layout.itemSize width = screenSize.width/3][1]

If I reduce that size (by *7* or *8 pixels* e.g.), it&#39;s better, but the  third cell in row is not completely visible, and I still have that blank space (top &amp; bottom, and left &amp; right) .

![enter image description here][2]

    class ViewController: UIViewController, UICollectionViewDelegateFlowLayout, UICollectionViewDataSource {
        var collectionView: UICollectionView?
        var screenSize: CGRect!
        var screenWidth: CGFloat!
        var screenHeight: CGFloat!
        
        override func viewDidLoad() {
            super.viewDidLoad()
            
            screenSize = UIScreen.mainScreen().bounds
            screenWidth = screenSize.width
            screenHeight = screenSize.height
            
            // Do any additional setup after loading the view, typically from a nib
            let layout: UICollectionViewFlowLayout = UICollectionViewFlowLayout()
            layout.sectionInset = UIEdgeInsets(top: 20, left: 0, bottom: 10, right: 0)
            layout.itemSize = CGSize(width: screenWidth / 3, height: screenWidth / 3)
            collectionView = UICollectionView(frame: self.view.frame, collectionViewLayout: layout)
            collectionView!.dataSource = self
            collectionView!.delegate = self
            collectionView!.registerClass(CollectionViewCell.self, forCellWithReuseIdentifier: &quot;CollectionViewCell&quot;)
            collectionView!.backgroundColor = UIColor.greenColor()
            self.view.addSubview(collectionView!)
        }
    
        func numberOfSectionsInCollectionView(collectionView: UICollectionView) -&gt; Int {
            return 1
        }
        
        func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -&gt; Int {
            return 20
        }
        
        func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -&gt; UICollectionViewCell {
            let cell = collectionView.dequeueReusableCellWithReuseIdentifier(&quot;CollectionViewCell&quot;, forIndexPath: indexPath) as CollectionViewCell
            cell.backgroundColor = UIColor.whiteColor()
            cell.layer.borderColor = UIColor.blackColor().CGColor
            cell.layer.borderWidth = 0.5
            cell.frame.size.width = screenWidth / 3
            cell.frame.size.height = screenWidth / 3
            
            cell.textLabel?.text = &quot;\(indexPath.section):\(indexPath.row)&quot;
            return cell
        }
    }

  [1]: http://i.stack.imgur.com/CoMTm.png
  [2]: http://i.stack.imgur.com/5mIIG.png

How to set cell spacing and UICollectionView - UICollectionViewFlowLayout size ratio?

I have `UICollectionView` with horizontal scrolling and there are always 2 cells side-by-side per the entire screen. I need the scrolling to stop at the begining of a cell. With paging enabled, the collection view scrolls the whole page, which is 2 cells at once, and then it stops.

I need to enable scrolling by a single cell, or scrolling by multiple cells with stopping at the edge of the cell.

I tried to subclass `UICollectionViewFlowLayout` and to implement the method `targetContentOffsetForProposedContentOffset`, but so far I was only able to break my collection view and it stopped scrolling. Is there any easier way to achieve this and how, or do I really need to implement all methods of `UICollectionViewFlowLayout ` subclass? Thanks.

Paging UICollectionView by cells, not screen

I am using a UICollectionView in my project, where there are multiple cells of differing widths on a line. According to:
https://developer.apple.com/library/content/documentation/WindowsViews/Conceptual/CollectionViewPGforIOS/UsingtheFlowLayout/UsingtheFlowLayout.html

it spreads the cells out across the line with equal padding. This happens as expected, except I want to left justify them, and hard code a padding width.

I figure I need to subclass UICollectionViewFlowLayout, however after reading some of the tutorials etc online I just don&#39;t seem to get how this works.


Left Align Cells in UICollectionView

&gt; 2015-08-18 16:07:51.523 Example[16070:269647] the behavior of the
&gt; UICollectionViewFlowLayout is not defined because: 2015-08-18
&gt; 16:07:51.523   
&gt; Example[16070:269647] the item width must be less than
&gt; the width of the UICollectionView minus the section insets left and
&gt; right values, minus the content insets left and right values.   
&gt; 2015-08-18 16:07:51.524 Example[16070:269647] The relevant
&gt; UICollectionViewFlowLayout instance is &lt;UICollectionViewFlowLayout:
&gt; 0x7f24d670&gt;, and it is attached to &lt;UICollectionView: 0x7b377200;
&gt; frame = (0 0; 768 1024); clipsToBounds = YES; autoresize = W+H;
&gt; gestureRecognizers = &lt;NSArray: 0x7f24ce30&gt;; animations = {
&gt; position=&lt;CABasicAnimation: 0x7f279d60&gt;;
&gt; bounds.origin=&lt;CABasicAnimation: 0x7f279430&gt;;
&gt; bounds.size=&lt;CABasicAnimation: 0x7f279480&gt;; }; layer = &lt;CALayer:
&gt; 0x7aec9cf0&gt;; contentOffset: {0, 0}; contentSize: {1024, 770}&gt;
&gt; collection view layout: &lt;UICollectionViewFlowLayout: 0x7f24d670&gt;.
&gt; 2015-08-18 16:07:51.524 Example[16070:269647] Make a symbolic
&gt; breakpoint at UICollectionViewFlowLayoutBreakForInvalidSizes to catch
&gt; this in the debugger.


That is what I get and what I do

    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -&gt; CGSize {
            return CGSizeMake(self.collectionView!.frame.size.width - 20, 66)
        }

I rotate from landscape to portrait then the console shows this error message only in iOS 9 if anyone knows what happens and if there is a fix for this?

[![Screenshot][1]][1]


  [1]: http://i.stack.imgur.com/QZKAX.png

the behavior of the UICollectionViewFlowLayout is not defined, because the cell width is greater than collectionView width

Running my app in a device with iOS 10 I get this error:

*UICollectionView received layout attributes for a cell with an index path that does not exist*

In iOS 8 and 9 works fine. I have been researching and I have found that is something related to invalidate the collection view layout. I tried to implement that solution with no success, so I would like to ask for direct help. This is my hierarchy view:

    -&gt;Table view 
        -&gt;Each cell of table is a custom collection view [GitHub Repo][1]
            -&gt;Each item of collection view has another collection view

What I have tried is to insert

        [self.collectionView.collectionViewLayout invalidateLayout];

In the

    - (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView

of both collection views. 

Also I have tried to invalidate layout before doing a reload data, does not work...

Could anyone give me some directions to take?

  [1]: https://github.com/apploft/APLExpandableCollectionView

iOS 10 bug: UICollectionView received layout attributes for a cell with an index path that does not exist

I have a `UIScrollView` that lays out a grid of icons.  If you were to imagine the layout for the iOS Springboard, you&#39;d be pretty close to correct.  It has a horizontal, paged scroll (just like Springboard).  However, it appears that the layout is not quite right.  It appears as though it is laying out the items from top to bottom.  As a result, my last column only has 2 rows in it, due to the number of items to be displayed.  I&#39;d rather have my last row on the last page have 2 items, like you would see in the Springboard.

How can this be accomplished with `UICollectionView` and its related classes?  Do I have to write a custom `UICollectionViewFlowLayout`?

UICollectionView - Horizontal scroll, horizontal layout?

this is a pretty straightforward question, but I haven&#39;t been able to find a definitive answer to it on SO (if I missed it, please correct me). 

Basically, my question is: ***Is it possible to align `UICollectionView` row contents from right to left instead of from left to right?***

In my research I&#39;ve seen answers suggesting subclassing `UICollectionViewFlowLayout`, but I haven&#39;t been able to find an example where one was created for right-alignment.

My goal is to have 2 collection views set up like this:

![Example][1]

Any help is greatly appreciated!


  [1]: http://i.stack.imgur.com/brayL.png

Aligning right to left on UICollectionView

I&#39;m going to create a book shelf in one of my projects. The basic requirements are as follows:

- similar to iBooks bookshelf
- supports both orientations well
- supports all kinds of iOS devices well (different resolutions)
- supports deleting and inserting items
- supports reordering of items by long press gesture
- shows a hidden logo when first row is pulled down

The UICollectionView is the first option that occurred to me. It easily supports grid cells. So I googled it and found some very useful tutorials:

[Bryan Hansen&#39;s UICollectionView custom layout tutorial][1]

[Mark Pospesel&#39;s How to Add a Decoration View to a UICollectionView][2]

[LXReorderableCollectionViewFlowLayout][3]

And here is the result: (Please ignore the color inconsistency problem because of the graphics I chose are not perfect yet.)

![enter image description here][4]

What I did:

1. Created a custom layout by creating a class inherited from ```LXReorderableCollectionViewFlowLayout``` (for reordering purposes) which inherited from UICollectionFlowLayout
2. Added a decoration view for showing logo
3. Added a decoration view for showing the bookshelfs

But I ran into a few problems:

**1. I can&#39;t scroll at all if the items can be shown in one screen**

Then I added the following code, to make the contentsize bigger

    - (CGSize)collectionViewContentSize
    {
        return CGSizeMake(self.collectionView.bounds.size.width,      self.collectionView.bounds.size.height+100);
    }

Then I can scroll now. Here I pull down the first row:

![enter image description here][5]

You can see the the decoration view for logo is working.

**2. But I got the second set of problems when I pull up the last row:**

![enter image description here][6]

You can see the decoration view is not added at the green box part.

**3. The background of decoration view for bookshelf is getting darker and darker. (Please refer to [same problem here][7]**
 
**4. The book shelf bar sometimes moves when I reorder the items**

![enter image description here][8]

I list some of the important code here:

    - (void)prepareLayout
    {
        // call super so flow layout can do all the math for cells, headers, and footers
        [super prepareLayout];
        
        NSMutableDictionary *dictionary = [NSMutableDictionary dictionary];
        NSMutableDictionary *shelfLayoutInfo = [NSMutableDictionary dictionary];
        
        // decoration view - emblem
        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:0 inSection:0];
        UICollectionViewLayoutAttributes *emblemAttributes =
            [UICollectionViewLayoutAttributes layoutAttributesForDecorationViewOfKind:[EmblemView kind]
                withIndexPath:indexPath];
        emblemAttributes.frame = [self frameForEmblem:YES];
        dictionary[[EmblemView kind]] = @{indexPath: emblemAttributes};
    
        // Calculate where shelves go in a vertical layout
        int sectionCount = [self.collectionView numberOfSections];
        
        CGFloat y = 0;
        CGFloat availableWidth = self.collectionViewContentSize.width - (self.sectionInset.left + self.sectionInset.right);
        int itemsAcross = floorf((availableWidth + self.minimumInteritemSpacing) / (self.itemSize.width + self.minimumInteritemSpacing));
        
        for (int section = 0; section &lt; sectionCount; section++)
        {
            y += self.headerReferenceSize.height;
            //y += self.sectionInset.top;
            
            int itemCount = [self.collectionView numberOfItemsInSection:section];
            int rows = ceilf(itemCount/(float)itemsAcross)+1; // add 2 more empty row which doesn&#39;t have any data
            for (int row = 0; row &lt; rows; row++)
            {
                indexPath = [NSIndexPath indexPathForItem:row inSection:section];
                shelfLayoutInfo[indexPath] = [NSValue valueWithCGRect:CGRectMake(0,y, self.collectionViewContentSize.width, self.itemSize.height + DECORATION_HEIGHT)];
                y += self.itemSize.height;
                
                if (row &lt; rows - 1)
                    y += self.minimumLineSpacing;
            }
            
            y += self.sectionInset.bottom;
            y += self.footerReferenceSize.height;
        }
        
        dictionary[[ShelfView kind]] = shelfLayoutInfo;
        
        self.shelfLayoutInfo = dictionary;
    }


    - (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
    {
        NSArray *attributesArrayInRect = [super layoutAttributesForElementsInRect:rect];
        
        // cell layout info
        for (BookShelfLayoutAttributes *attribs in attributesArrayInRect)
        {
            
            attribs.zIndex = 1;
            CATransform3D t = CATransform3DIdentity;
            t = CATransform3DTranslate(t, 0, 0, 40);
            attribs.transform3D = CATransform3DRotate(t, 15 * M_PI / 180, 1, 0, 0);
        }
        
        // Add our decoration views (shelves)
        NSMutableDictionary* shelfDictionary = self.shelfLayoutInfo[[ShelfView kind]];
        NSMutableArray *newArray = [attributesArrayInRect mutableCopy];
        
        [shelfDictionary enumerateKeysAndObjectsUsingBlock:^(id key, NSValue* obj, BOOL *stop) {
            
            if (CGRectIntersectsRect([obj CGRectValue], rect))
            {
                UICollectionViewLayoutAttributes *attributes = [UICollectionViewLayoutAttributes layoutAttributesForDecorationViewOfKind:[ShelfView kind] withIndexPath:key];
                attributes.frame = [obj CGRectValue];
                
                NSLog(@&quot;decorationView rect = %@&quot;,NSStringFromCGRect(attributes.frame));
                attributes.zIndex = 0;
                //attributes.alpha = 0.5; // screenshots
                [newArray addObject:attributes];
            }
        }];
        
        attributesArrayInRect = [NSArray arrayWithArray:newArray];
        
        NSMutableDictionary* emblemDictionary = self.shelfLayoutInfo[[EmblemView kind]];
        NSMutableArray *newArray2 = [attributesArrayInRect mutableCopy];
        [emblemDictionary enumerateKeysAndObjectsUsingBlock:^(NSIndexPath *indexPath, UICollectionViewLayoutAttributes *attributes, BOOL *innerStop) {
            if (CGRectIntersectsRect(rect, attributes.frame)) {
                [newArray2 addObject:attributes];
            }
        }];
        
        attributesArrayInRect = [NSArray arrayWithArray:newArray2];
        
        return attributesArrayInRect;
    }


I&#39;ll appreciate if you&#39;re patient enough to read this post and provide any advice or suggestions. And I&#39;ll post the complete source code if I can fix all the issues. Thank you in advance.

  [1]: http://skeuo.com/uicollectionview-custom-layout-tutorial
  [2]: http://markpospesel.wordpress.com/author/mpospese/
  [3]: https://github.com/lxcid/LXReorderableCollectionViewFlowLayout
  [4]: http://i.stack.imgur.com/M1EaC.jpg
  [5]: http://i.stack.imgur.com/EZKk9.jpg
  [6]: http://i.stack.imgur.com/0DuSX.jpg
  [7]: https://stackoverflow.com/questions/14882837/decorationviews-on-uicollectionview-are-duplicated-on-every-insert
  [8]: http://i.stack.imgur.com/RWCbn.jpg



Create a book shelf by using UICollectionView

I am using `UICollectionView` using the flow layout.
I have made a custom `UICollectionViewCell` for the same.
But on running the project the console keeps on throwing this error- 

    The behavior of the UICollectionViewFlowLayout is not defined because:
     the item height must be less that the height of the UICollectionView minus the section insets top and bottom values.

I have made sure that the size of the cell is correct

Has anyone been able to resolve this issue.


UICollectionView Layout Issue

I am have a lot of difficulty trying to create a UICollectionView like in Spotify&#39;s Player that acts like this:

![a busy cat](https://media.giphy.com/media/l0NwH3XDSYhQ6l4TC/giphy.gif)

The problem for me is two fold. 

1) How do I center the cells so that you can see the middle cell as well as the one of the left and right.

- If I create cells that are square and add spacing between each cell, the cells are correctly displayed but are not centered. 

2) With pagingEnabled = YES, the collectionview correctly swipes from one page to another. However, without the cells being centered, it simply moves the collection view over a page which is the width of the screen. So the question is how do you make the pages move so you get the effect above.

3) how do you animate the size of the cells as they move

- I don&#39;t want to worry about this too much. If I can get that to work it would be great, but the harder problems are 1 and 2.
 

The code I have currently is a simple UICollectionView with normal delegate setup and custom UICollectionview cells that are squares. Maybe I neeed to subclass UICollectionViewFlowLayout? Or maybe I need to turn pagingEnabled  to NO and then use custom swipe events? Would love any help!

How to create a centered UICollectionView like in Spotify&#39;s Player

![enter image description here][1]


1) How can i achieve as shown in image with `UICollectionView`?

2) I&#39;ve tried `-(CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath` method &amp; passed different sizes but it leaves out spaces between cells. Can i empty those spaces? I want different height &amp; width for every cell

**Current output:** 
![enter image description here][2].

  [1]: http://i.stack.imgur.com/QnRtn.png
  [2]: http://i.stack.imgur.com/FCeEq.jpg

Dynamic size UICollectionView cell

I have a UICollectionView with a custom UICollectionViewFlowLayout. It scrolls horizontally and only shows one item at a time (full height). I have pagingEnabled = YES so that it sticks to pages. 

The UICollectionView shows photos within a particular day. I initially had buttons to change the day +/- 1 day, but I thought it would be neat if the user could pull on the collection view past the first or last image (past a predetermined threshold) to change the day. I implemented this using the UIScrollView delegate methods and it works great...... if there are 2 or more items in the collection view. 

This is hard to describe, but if there is only 1 item, it doesn&#39;t allow me to pull the item past it&#39;s bounds. I.E. there is no bounce to it. 

I&#39;d like to have this behavior, but I need to overcome this problem first. I&#39;m thinking that if I removed pagingEnabled = YES and used targetContentOffsetForProposedContentOffset:proposedContentOffset:withScrollingVelocity, that I may be able to get it to work but I really enjoy the snappy behavior that paging enables. (They are mutually exclusive). 

Any ideas on this?

UICollectionView doesn&#39;t bounce when datasource has only 1 item

I keep getting &#39;Unable to simultaneously satisfy constraints&#39; exceptions (Xcode 5, iOS 7, both device and simulator), where one of the constraints in the list is something like this:

    &quot;&lt;NSLayoutConstraint:0x165b23d0 &#39;UIView-Encapsulated-Layout-Width&#39;
    H:[StoryPlayerCell:0x165affc0(0)]&gt;&quot;

I did not set this constraint myself. It&#39;s also not an `NSAutoresizingMaskLayoutConstraint`.
But where does it come from? And how can I get rid of it?

I have no idea.
And I can&#39;t find anything about `&#39;UIView-Encapsulated-Layout-Width&#39;` in the Apple&#39;s documentation.
Even the Google search returns nothing at all.

Any suggestions?

PS: I&#39;m using this cell in a `UICollectionView` with a custom subclass of `UICollectionViewFlowLayout`. Maybe the `&#39;Encapsulated-Layout&#39;` part has something to do with this?

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