iOS7 Day-by-Day :: Day 5 :: UIDynamics with Collection Views

Written by Sam Davies

This post is part of a daily series of posts introducing the most exciting new parts of iOS7 for developers – #iOS7DayByDay. To see the posts you’ve missed check out the introduction page, but have a read through the rest of this post first!


Back at the beginning of this series, day 0 took a look at the new UIKit Dynamics physics engine, and used it to build a Newton’s Cradle. Although this was a lot of fun, and served as a good introduction to UIKit Dynamics, it’s not particularly obvious how this can be useful when building apps. Today’s DbD looks at how to link the physics engine with UICollectionViews – resulting in some subtle effects noticeable as the user interacts with the collection.

The demo project which accompanies today’s post is a horizontal ‘springy’ carousel, where the individual items are attached to springs. We will also show how to use the dynamics engine to animate the newly added cells.

The code for this blog post is available in the github repo which accompanies this series – at

Building a Carousel

In order to demonstrate using the physics engine with a collection view, we firstly need to make a carousel out of aUICollectionView. This post isn’t a tutorial on how to use UICollectionView, so I’ll skip briefly through this part. We’ll make the view controller the datasource and delegate for the collection view, and implement the methods we need:

#pragma mark - UICollectionViewDataSource methods
- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
    return 1;

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
    return [_collectionViewCellContent count];

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
    SCCollectionViewSampleCell *cell = (SCCollectionViewSampleCell *)[self.collectionView dequeueReusableCellWithReuseIdentifier:@"SpringyCell" forIndexPath:indexPath];
    cell.numberLabel.text = [NSString stringWithFormat:@"%d", [_collectionViewCellContent[indexPath.row] integerValue]];
    return cell;

#pragma mark - UICollectionViewDelegate methods
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
    return itemSize;

The cells are each square tiles which contain a number inside a UILabel. The numbers of the cells we are currently displaying in the collection view are stored inside an array (_colletionViewCellContent) as NSNumber objects. We do this to preserve the ordering of the cells – not important at this stage, but will be once we work out how to insert new cells.

In order to get the collection view to appear as a horizontal carousel we need to provide a custom layout. As is often the case, the flow layout has a lot of what we need, so we’ll subclass that:

@interface SCSpringyCarousel : UICollectionViewFlowLayout
- (instancetype)initWithItemSize:(CGSize)size;

In order to force all of the items into a horizontal carousel at the bottom of the view we need to know the item height – hence the constructor which requires an item size. We override the prepareLayout method to set the content inset to push the items to the bottom of the collection view:

- (void)prepareLayout
    // We update the section inset before we layout
    self.sectionInset = UIEdgeInsetsMake(CGRectGetHeight(self.collectionView.bounds) - _itemSize.height, 0, 0, 0);
    [super prepareLayout];

Setting this as the layout on the collection view will create the horizontal carousel we’re after.

- (void)viewDidLoad
    [super viewDidLoad];

    // Provide the layout
    _collectionViewLayout = [[SCSpringyCarousel alloc] initWithItemSize:itemSize];
    self.collectionView.collectionViewLayout = _collectionViewLayout;

Springy Carousel Non Springy

Adding springs

Now on to the more exciting stuff – let’s fix this up with the UIKit Dynamics physics engine.

The physical model we’re going to use has each item connected to the position it would have been fixed to in a vanilla flow layout – i.e. the we take the items from the carousel we’ve already made, and attach the them to their positions with springs. Then, as we scroll the view, the springs will stretch and we’ll get the effect we want. Well, nearly, we need to perturb the springs a distance proportional to the distance from the touch point, but we’ll come to that when the time is right.

Translating this model into a UIDynamics concept is as follows:

  • When we are preparing the layout we request the positioning information from the flow layout super class.
  • We add appropriate behaviors to these positioning objects to allow them to be animated in the physics world.
  • These behaviors and position objects are passed to the animator so that the simulation can run.
  • The methods on the UICollectionViewLayout are overridden to return the positions from the animator, instead of the flow layout superclass.

This all sounds a lot more complicated than it actually is – honestly! Let’s work through it in stages.

Behavior Manager

In order to keep the code nice and tidy, we’ll create a class which manages the dynamic behaviors inside the animator. It’s API should look like the following:

@interface SCItemBehaviorManager : NSObject
@property (readonly, strong) UIGravityBehavior *gravityBehavior;
@property (readonly, strong) UICollisionBehavior *collisionBehavior;
@property (readonly, strong) NSDictionary *attachmentBehaviors;
@property (readonly, strong) UIDynamicAnimator *animator;

- (instancetype)initWithAnimator:(UIDynamicAnimator *)animator;

- (void)addItem:(UICollectionViewLayoutAttributes *)item anchor:(CGPoint)anchor;
- (void)removeItemAtIndexPath:(NSIndexPath *)indexPath;
- (void)updateItemCollection:(NSArray*)items;
- (NSArray *)currentlyManagedItemIndexPaths;

The behavior of each of our cells is constructed from a shared UIGravityBehavior, a shared UICollisionBehavior and an individual UIAttachmentBehvaior. We create our behavior manager with a UIDynamicAnimator and expose methods for adding, removing items, as well as a method to update the collection to match an array.

When we create a manager object then we want to create the shared behaviors, and attach them to the animator:

- (instancetype)initWithAnimator:(UIDynamicAnimator *)animator
    self = [super init];
    if(self) {
        _animator = animator;
        _attachmentBehaviors = [NSMutableDictionary dictionary];
        [self createGravityBehavior];
        [self createCollisionBehavior];
        // Add the global behaviors to the animator
        [self.animator addBehavior:self.gravityBehavior];
        [self.animator addBehavior:self.collisionBehavior];
    return self;

with the 2 utility methods called here being very simple, and having similar composition to what we used for the Newton’s Cradle project back on day 0:

- (void)createGravityBehavior
    _gravityBehavior = [[UIGravityBehavior alloc] init];
    _gravityBehavior.magnitude = 0.3;

- (void)createCollisionBehavior
    _collisionBehavior = [[UICollisionBehavior alloc] init];
    _collisionBehavior.collisionMode = UICollisionBehaviorModeBoundaries;
    _collisionBehavior.translatesReferenceBoundsIntoBoundary = YES;
    // Need to add item behavior specific to this
    UIDynamicItemBehavior *itemBehavior = [[UIDynamicItemBehavior alloc] init];
    itemBehavior.elasticity = 1;
    // Add it as a child behavior
    [_collisionBehavior addChildBehavior:itemBehavior];

You’ll notice that we don’t add any dynamic items to the behaviors at this stage – principally because we don’t actually have any yet. The collision behavior isn’t going to be used for collisions between the individual cells, but instead within the boundary of the collection view. Hence the setting the two properties: collisionMode and translatesReferenceBoundsIntoBoundary. We also add a UIDynamicItemBehavior to specify the elasticity of the collisions, in the same way that we did with the pendula.

Now we have created these global behaviors we need to implement the addItem: and removeItem: methods. The add method will add the new item to the global behaviors and also set up the spring which attaches the cell to the background canvas:

- (void)addItem:(UICollectionViewLayoutAttributes *)item anchor:(CGPoint)anchor
    UIAttachmentBehavior *attachmentBehavior = [self createAttachmentBehaviorForItem:item anchor:anchor];
    // Add the behavior to the animator
    [self.animator addBehavior:attachmentBehavior];
    // And store it in the dictionary. Keyed by the index path
    [_attachmentBehaviors setObject:attachmentBehavior forKey:item.indexPath];

    // Also need to add this item to the global behaviors
    [self.gravityBehavior addItem:item];
    [self.collisionBehavior addItem:item];

The spring behavior is created using a utility method:

- (UIAttachmentBehavior *)createAttachmentBehaviorForItem:(id<UIDynamicItem>)item anchor:(CGPoint)anchor
    UIAttachmentBehavior *attachmentBehavior = [[UIAttachmentBehavior alloc] initWithItem:item attachedToAnchor:anchor];
    attachmentBehavior.damping = 0.5;
    attachmentBehavior.frequency = 0.8;
    attachmentBehavior.length = 0;
    return attachmentBehavior;

We also store the attachment behavior in a dictionary, keyed by the NSIndexPath. This will allow us to work out which spring we need to remove when we implement the remove method.

Once we created the attachment behavior we add it to the animator, and add the provided item to the shared gravity and collision behaviors.

The remove method performs exactly the opposite operation – remove the attachment behavior from the animator and the item from the shared gravity and collision behaviors:

- (void)removeItemAtIndexPath:(NSIndexPath *)indexPath
    // Remove the attachment behavior from the animator
    UIAttachmentBehavior *attachmentBehavior = self.attachmentBehaviors[indexPath];
    [self.animator removeBehavior:attachmentBehavior];

    // Remove the item from the global behaviors
    for(UICollectionViewLayoutAttributes *attr in [self.gravityBehavior.items copy])
        if([attr.indexPath isEqual:indexPath]) {
            [self.gravityBehavior removeItem:attr];
    for (UICollectionViewLayoutAttributes *attr in [self.collisionBehavior.items copy])
        if([attr.indexPath isEqual:indexPath]) {
            [self.collisionBehavior removeItem:attr];

    // And remove the entry from our dictionary
    [_attachmentBehaviors removeObjectForKey:indexPath];

This method is slightly more complicated than we would like. Removing the attachment behavior is as we would expect, but removing the item from the shared behaviors is a little more complicated. The item objects have been copied, and have different references. Therefore we need to search though all of the items the gravity behavior is acting upon, and remove the one with the same index path. Hence we loop through the items searching for the item with the same index path.

There is one more method on the API of the behavior manager – updateItemCollection:. This method takes a collection of items and then calls the addItem:anchor: and removeItem: methods with the correct arguments to ensure that the manager is currently managing the correct items. We’ll see very soon why this is useful, but let’s take a look at the implementation:

- (void)updateItemCollection:(NSArray *)items
    // Let's find the ones we need to remove. We work in indexPaths here
    NSMutableSet *toRemove = [NSMutableSet setWithArray:[self.attachmentBehaviors allKeys]];
    [toRemove minusSet:[NSSet setWithArray:[items valueForKeyPath:@"indexPath"]]];

    // Let's remove any we no longer need
    for (NSIndexPath *indexPath in toRemove) {
        [self removeItemAtIndexPath:indexPath];

    // Find the items we need to add springs to. A bit more complicated =(
    // Loop through the items we want
    NSArray *existingIndexPaths = [self currentlyManagedItemIndexPaths];
    for(UICollectionViewLayoutAttributes *attr in items) {
        // Find whether this item matches an existing index path
        BOOL alreadyExists = NO;
        for(NSIndexPath *indexPath in existingIndexPaths) {
            if ([indexPath isEqual:attr.indexPath]) {
                alreadyExists = YES;
        // If it doesn't then let's add it
        if(!alreadyExists) {
            // Need to add
            [self addItem:attr];

It’s a very simple method – we first find the items we need to remove – using some simple set operations ({Items we currently have} / {Items we should have}). Then we loop through the resultant set and call the removeItem: method.

To work out the items we need to add we loop try to find each item in the collection we’ve been sent in our dictionary of managed items. If we can’t find it then we need to start managing the behavior for it, so we call the addItem:anchor: method. Importantly, the anchor point is the current center position provided in the UIDynamicItem object. In terms of the UICollectionView, this means that we want our item to be anchored to the position the flow layout would like to place them.

Using the manager in the collection view layout

Now we’ve created the behavior manager, we’ve actually implemented nearly all of the UIDynamics code we need. All that remains is to wire it up to the collection view layout. This wiring up takes the form of overriding several methods in ourUICollectionViewFlowLayout subclass: SCSpringyCarousel.

We had already overridden prepareLayout to force the flow layout to take the form of a horizontal carousel. We now add more to that method to ensure that the dynamic animator has all the relevant items under its control:

- (void)prepareLayout
    // We update the section inset before we layout
    self.sectionInset = UIEdgeInsetsMake(CGRectGetHeight(self.collectionView.bounds) - _itemSize.height, 0, 0, 0);
    [super prepareLayout];

    // Get a list of the objects around the current view
    CGRect expandedViewPort = self.collectionView.bounds;
    expandedViewPort.origin.x -= 2 * _itemSize.width;
    expandedViewPort.size.width += 4 * _itemSize.width;
    NSArray *currentItems = [super layoutAttributesForElementsInRect:expandedViewPort];

    // We update our behavior collection to just contain the objects we can currently (almost) see
    [_behaviorManager updateItemCollection:currentItems];   

The first few lines of code are exactly as before. We then work out an expanded viewport bounds. This involves taking the current viewport and expanding it to the left and right, ensuring that the items which are soon to appear on screen are under the control of our dynamic animator. Once we have the viewport we ask our superclass for the layout attributes for all the items which would appear within this rectangle – i.e. all the items which would have appeared within that range had we been using a vanilla flow layout. Like UIView these UICollectionViewLayoutAttributes objects all adopt the UIDynamicItem protocol, and hence can be animated by our UIDynamicAnimator. We pass this collection of objects through to our behavior manager to ensure that we are managing the behavior of the correct items.

The next method we need to override is shouldInvalidateLayoutForBoundsChange:. We don’t actually want to change the behavior of this method (the default returns NO and we won’t change this), but it gets called whenever the bounds of our collection view changes. In the world of scroll views, the bounds property represents the current viewport position – i.e. the x and y values are not necessarily 0 as they usually are. Therefore, a ‘bounds change’ event in a UIScrollView subclass actually occurs as the scrollview is scrolled.

This method is the most complicated part of this demo project, so we’ll step through it bit-by-bit.

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
    CGFloat scrollDelta = newBounds.origin.x - self.collectionView.bounds.origin.x;

    CGPoint touchLocation = [self.collectionView.panGestureRecognizer locationInView:self.collectionView];

    for (UIAttachmentBehavior *behavior in [_behaviorManager.attachmentBehaviors allValues]) {
        CGPoint anchorPoint = behavior.anchorPoint;
        CGFloat distFromTouch = ABS(anchorPoint.x - touchLocation.x);

        UICollectionViewLayoutAttributes *attr = [behavior.items firstObject];
        CGPoint center =;
        CGFloat scrollFactor = MIN(1, distFromTouch / 500);

        center.x += scrollDelta * scrollFactor; = center;

        [_dynamicAnimator updateItemUsingCurrentState:attr];

    return NO;
  1. Firstly we find out how much we have just scrolled the scroll view – since we were last called, and hence last updated our springs.
  2. We can then find the location of the current touch within the collection view, since we have access to thepanGestureRecognizer of the underlying scrollview.
  3. Now we need to loop through each of the springs in the behavior manager, updating them.
  4. Firstly we find out how far our item’s rest position (i.e. the behavior’s anchor point) is from the touch. This is because we’re going to stretch the springs proportionally to how far they are from our touch point.
  5. Then we work out the new position of the current cell – using a magic scrollFactor and the actual scrollDelta.
  6. We tell the dynamic animator that it should refresh its understanding of the item’s state. When an item is added to a dynamic animator it makes an internal copy of the item’s state and then animates that. In order to push new state in we update theUIDynamicItem properties and then tell the animator that it should reload the state of this item.
  7. Finally we return NO – we are letting the dynamic animator manage the positions of our cells, we don’t need the collection view to re-request it from the layout.

There are 2 more methods we need to override, the purpose of both is to remove the responsibility of item layout from the flow layout class, and give it instead to the dynamic animator:

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
    return [_dynamicAnimator itemsInRect:rect];

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
    return [_dynamicAnimator layoutAttributesForCellAtIndexPath:indexPath];

The dynamic animator has 2 helper methods for precisely this purpose, which plug nicely into the collection view layout class. These methods are used by the collection view to position the cells. We simply get the dynamic animator to return the positions of the relevant cells – either by indexPath or for the cells which are visible in the specified rectangle.

Test run

Well, if you run this project up now you should have a horizontal carousel, which as you drag items around you get a springy effect – where cells ahead of the drag direction bunch up, and those behind spread out.

Springy Carousel Scrolling

Inserting items

Now that we’ve got this springy carousel working, we’re going to see how difficult it is to govern adding new cells using the dynamic animator as well as scrolling. We’ve actually done a lot of the work, so let’s see what we need to add.

With a standard UICollectionView, the layout provides the layout attributes for an appearing item, and then the item will be animated to its final position within the collection – i.e. the position returned by layoutAttributesForItemAtIndexPath:. However, we are going to perform the animation using our UIDynamicAnimator, and therefore need to prevent UIView animations. To do this add the following line to prepareLayout:

[UIView setAnimationsEnabled:NO];

This will ensure that we don’t have 2 different animation processes fighting against each other.

As mentioned, the UICollectionViewLayout will get called to ask for where a new item should be positioned, using the snappily named initialLayoutAttributesForAppearingItemAtIndexPath: method. We are going to let our animator handle this:

- (UICollectionViewLayoutAttributes *)initialLayoutAttributesForAppearingItemAtIndexPath:(NSIndexPath *)itemIndexPath
    return [_dynamicAnimator layoutAttributesForCellAtIndexPath:itemIndexPath];

Now we actually need to do let the animator know that it a new item arriving, update the positions of the existing items appropriately, and position the new one. To do this we override the prepareForCollectionViewUpdates: method on the SCSpringyCarouselclass:

- (void)prepareForCollectionViewUpdates:(NSArray *)updateItems
    for (UICollectionViewUpdateItem *updateItem in updateItems) {
        if(updateItem.updateAction == UICollectionUpdateActionInsert) {
            // Reset the springs of the existing items
            [self resetItemSpringsForInsertAtIndexPath:updateItem.indexPathAfterUpdate];

            // Where would the flow layout like to place the new cell?
            UICollectionViewLayoutAttributes *attr = [super initialLayoutAttributesForAppearingItemAtIndexPath:updateItem.indexPathAfterUpdate];
            CGPoint center =;
            CGSize contentSize = [self collectionViewContentSize];
            center.y -= contentSize.height - CGRectGetHeight(attr.bounds);

            // Now reset the center of insertion point for the animator
            UICollectionViewLayoutAttributes *insertionPointAttr = [self layoutAttributesForItemAtIndexPath:updateItem.indexPathAfterUpdate];
   = center;
            [_dynamicAnimator updateItemUsingCurrentState:insertionPointAttr];

This is a long method, but can break it down into simple chunks:

  1. This method gets called for inserts, removals and moves. We’re only interested in insertions for this project, so we’re only going to do something if our update is of type UICollectionUpdateActionInsert.
  2. When an insert happens, the collection view will re-assign the layout attributes of those cells above the insertion index to their nextmost neighbor – i.e. if inserting at index 4, then the cell currently at 5 will be updated to have the layout attributes of the cell currently at 6 etc. In our scenario we want to keep the anchor point of the behavior associated with the layout attributes of our neighbor, but the position should be our current position – not that of our neighbor. We perform this with a utility methodresetItemSpringsForInsertAtIndexPath:, which we’ll look at later.
  3. Now we deal with the new cell which is being inserted. We ask the flow layout where it would like to position it. We want it to appear at the top of the collection view, so that the animator will drop it down using the gravity behavior. We use this to work out where the center of the inserted cell should be.
  4. Now we ask the animator for the layout attributes for the index path we’re inserting at, and then update the position to match the one we’ve just calculated.

The final piece of the puzzle is the aforementioned method which is used to update the springs of the items moved to make space for the new item:

- (void)resetItemSpringsForInsertAtIndexPath:(NSIndexPath *)indexPath
    // Get a list of items, sorted by their indexPath
    NSArray *items = [_behaviorManager currentlyManagedItemIndexPaths];
    // Now loop backwards, updating centers appropriately.
    // We need to get 2 enumerators - copy from one to the other
    NSEnumerator *fromEnumerator = [items reverseObjectEnumerator];
    // We want to skip the lastmost object in the array as we're copying left to right
    [fromEnumerator nextObject];
    // Now enumarate the array - through the 'to' positions
    [items enumerateObjectsWithOptions:NSEnumerationReverse usingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        NSIndexPath *toIndex = (NSIndexPath*)obj;
        NSIndexPath *fromIndex = (NSIndexPath *)[fromEnumerator nextObject];

        // If the 'from' cell is after the insert then need to reset the springs
        if(fromIndex && fromIndex.item >= indexPath.item) {
            UICollectionViewLayoutAttributes *toItem = [self layoutAttributesForItemAtIndexPath:toIndex];
            UICollectionViewLayoutAttributes *fromItem = [self layoutAttributesForItemAtIndexPath:fromIndex];
            [_dynamicAnimator updateItemUsingCurrentState:toItem];

We have already explained the concept above, and the implementation is pretty simple to follow. We use 2 reverse iterators, and copy the position of the cell from one to the other. Then, when the collection view updates the layout attributes of the cells, the springs will be set to pull them from their old position to their new one.

We just need to add a button and method to the view controller to manage the item additions. We add the button in the StoryBoard, and attach it to the following method:

- (IBAction)newViewButtonPressed:(id)sender {
    // What's the new number we're creating?
    NSNumber *newTile = @([_collectionViewCellContent count]);

    // We want to place it in at the correct position
    NSIndexPath *rightOfCenter = [self indexPathOfItemRightOfCenter];

    // Insert the new item content
    [_collectionViewCellContent insertObject:newTile atIndex:rightOfCenter.item];

    // Redraw
    [self.collectionView insertItemsAtIndexPaths:@[rightOfCenter]];

There’s a utility method to work out the index which is the right hand side of the center of the currently visible items:

- (NSIndexPath *)indexPathOfItemRightOfCenter
    // Find all the currently visible items
    NSArray *visibleItems = [self.collectionView indexPathsForVisibleItems];

    // Calculate the middle of the current collection view content
    CGFloat midX = CGRectGetMidX(self.collectionView.bounds);
    NSUInteger indexOfItem;
    CGFloat curMin = CGFLOAT_MAX;

    // Loop through the visible cells to find the left of center one
    for (NSIndexPath *indexPath in visibleItems) {
        UICollectionViewCell *cell = [self.collectionView cellForItemAtIndexPath:indexPath];
        if (ABS(CGRectGetMidX(cell.frame) - midX) < ABS(curMin)) {
            curMin = CGRectGetMidX(cell.frame) - midX;
            indexOfItem = indexPath.item;

    // If min is -ve then we have left of centre. If +ve then we have right of centre.
    if(curMin < 0) {
        indexOfItem += 1;

    // And now get the index path to pass back
    return [NSIndexPath indexPathForItem:indexOfItem inSection:0];


And with that we’re done. Fire up the app and try adding cells – they drop nicely in and then bounce – really cool. Try pressing adding cells whilst the carousel is scrolling – this shows how awesome the dynamic animator really is!

Springy Carousel Inserting


In day 0 we showed how easy the UIKit Dynamics physics engine is to use, but with today’s post we’ve really got to grips with a real-world example – using it to animate the cells in a collection view. This has some excellent applications, and despite its apparent complexity, is actually pretty easy to get your head around. I encourage you to investigate adding subtle animations you collection views, which will delight users, albeit subconsciously.

Don’t forget that you can get the code for this project on github at If you have any feedback/comments then feel free to use the comments box below, or hit me up on twitter – @iwantmyrealname.