iOS UIKit Dynamics with UICollectionView Creating a Custom Dragging Behavior with UIDynamicAnimator

Example

This example shows how to create a custom dragging behavior by Subclassing UIDynamicBehavior and subclassing UICollectionViewFlowLayout. In the example, we have UICollectionView that allows for the selection of multiple items. Then with a long press gesture those items can be dragged in an elastic, "springy" animation driven by a UIDynamicAnimator.

The dragging behavior is produced by combining a low-level behavior that adds a UIAttachmentBehavior to the for corners of a UIDynamicItem and a high-level behavior that manages the low-level behavior for a number of UIDynamicItems.

We can begin by creating this low-level behavior, we'll call RectangleAttachmentBehavior

Swift

final class RectangleAttachmentBehavior: UIDynamicBehavior
{
    init(item: UIDynamicItem, point: CGPoint)
    {
        // Higher frequency more "ridged" formation
        let frequency: CGFloat = 8.0
        
        // Lower damping longer animation takes to come to rest
        let damping: CGFloat = 0.6
        
        super.init()
        
        // Attachment points are four corners of item
        let points = self.attachmentPoints(for: point)
        
        let attachmentBehaviors: [UIAttachmentBehavior] = points.map
        {
            let attachmentBehavior = UIAttachmentBehavior(item: item, attachedToAnchor: $0)
            attachmentBehavior.frequency = frequency
            attachmentBehavior.damping = damping
            return attachmentBehavior
        }
        
        attachmentBehaviors.forEach
        {
            addChildBehavior($0)
        }
    }
    
    func updateAttachmentLocation(with point: CGPoint)
    {
        // Update anchor points to new attachment points
        let points = self.attachmentPoints(for: point)
        let attachments = self.childBehaviors.flatMap { $0 as? UIAttachmentBehavior }
        let pairs = zip(points, attachments)
        pairs.forEach { $0.1.anchorPoint = $0.0 }
    }
    
    func attachmentPoints(for point: CGPoint) -> [CGPoint]
    {
        // Width and height should be close to the width and height of the item
        let width: CGFloat = 40.0
        let height: CGFloat = 40.0
        
        let topLeft = CGPoint(x: point.x - width * 0.5, y: point.y - height * 0.5)
        let topRight = CGPoint(x: point.x + width * 0.5, y: point.y - height * 0.5)
        let bottomLeft = CGPoint(x: point.x - width * 0.5, y: point.y + height * 0.5)
        let bottomRight = CGPoint(x: point.x + width * 0.5, y: point.y + height * 0.5)
        let points = [topLeft, topRight, bottomLeft, bottomRight]
        return points
    }
}

Objective-C

@implementation RectangleAttachmentBehavior

- (instancetype)initWithItem:(id<UIDynamicItem>)item point:(CGPoint)point
{
    CGFloat frequency = 8.0f;
    CGFloat damping = 0.6f;
    self = [super init];
    if (self)
    {
        NSArray <NSValue *> *pointValues = [self attachmentPointValuesForPoint:point];
        for (NSValue *value in pointValues)
        {
            UIAttachmentBehavior *attachment = [[UIAttachmentBehavior alloc]initWithItem:item attachedToAnchor:[value CGPointValue]];
            attachment.frequency = frequency;
            attachment.damping = damping;
            [self addChildBehavior:attachment];
        }
    }
    return self;
}

- (void)updateAttachmentLocationWithPoint:(CGPoint)point
{
    NSArray <NSValue *> *pointValues = [self attachmentPointValuesForPoint:point];
    for (NSInteger i = 0; i < pointValues.count; i++)
    {
        NSValue *pointValue = pointValues[i];
        UIAttachmentBehavior *attachment = self.childBehaviors[i];
        attachment.anchorPoint = [pointValue CGPointValue];
    }
}

- (NSArray <NSValue *> *)attachmentPointValuesForPoint:(CGPoint)point
{
    CGFloat width = 40.0f;
    CGFloat height = 40.0f;
    
    CGPoint topLeft = CGPointMake(point.x - width * 0.5, point.y - height * 0.5);
    CGPoint topRight = CGPointMake(point.x + width * 0.5, point.y - height * 0.5);
    CGPoint bottomLeft = CGPointMake(point.x - width * 0.5, point.y + height * 0.5);
    CGPoint bottomRight = CGPointMake(point.x + width * 0.5, point.y + height * 0.5);
    
    NSArray <NSValue *> *pointValues = @[[NSValue valueWithCGPoint:topLeft], [NSValue valueWithCGPoint:topRight], [NSValue valueWithCGPoint:bottomLeft], [NSValue valueWithCGPoint:bottomRight]];
    return pointValues;
}

@end

Next we can create the high-level behavior that will combine a number of RectangleAttachmentBehavior.

Swift

final class DragBehavior: UIDynamicBehavior
{
    init(items: [UIDynamicItem], point: CGPoint)
    {
        super.init()
        items.forEach
        {
            let rectAttachment = RectangleAttachmentBehavior(item: $0, point: point)
            self.addChildBehavior(rectAttachment)
        }
    }
    
    func updateDragLocation(with point: CGPoint)
    {
        // Tell low-level behaviors location has changed
        self.childBehaviors.flatMap { $0 as? RectangleAttachmentBehavior }.forEach { $0.updateAttachmentLocation(with: point) }
    }
}

Objective-C

@implementation DragBehavior

- (instancetype)initWithItems:(NSArray <id<UIDynamicItem>> *)items point: (CGPoint)point
{
    self = [super init];
    if (self)
    {
        for (id<UIDynamicItem> item in items)
        {
            RectangleAttachmentBehavior *rectAttachment = [[RectangleAttachmentBehavior alloc]initWithItem:item point:point];
            [self addChildBehavior:rectAttachment];
        }
    }
    return self;
}

- (void)updateDragLocationWithPoint:(CGPoint)point
{
    for (RectangleAttachmentBehavior *rectAttachment in self.childBehaviors)
    {
        [rectAttachment updateAttachmentLocationWithPoint:point];
    }
}

@end

Now with our behaviors in place, the next step is to add them to our collection view when. Because normally we want a standard grid layout we can subclass UICollectionViewFlowLayout and only change attributes when dragging. We do this mainly through overriding layoutAttributesForElementsInRect and using the UIDynamicAnimator's convenience method itemsInRect.

Swift

final class DraggableLayout: UICollectionViewFlowLayout
{
    // Array that holds dragged index paths
    var indexPathsForDraggingElements: [IndexPath]?
    
    // The dynamic animator that will animate drag behavior
    var animator: UIDynamicAnimator?
    
    // Custom high-level behavior that dictates drag animation
    var dragBehavior: DragBehavior?
    
    // Where dragging starts so can return there once dragging ends
    var startDragPoint = CGPoint.zero
    
    // Bool to keep track if dragging has ended
    var isFinishedDragging = false
    
    
    // Method to inform layout that dragging has started
    func startDragging(indexPaths selectedIndexPaths: [IndexPath], from point: CGPoint)
    {
        indexPathsForDraggingElements = selectedIndexPaths
        animator = UIDynamicAnimator(collectionViewLayout: self)
        animator?.delegate = self
        
        // Get all of the draggable attributes but change zIndex so above other cells
        let draggableAttributes: [UICollectionViewLayoutAttributes] = selectedIndexPaths.flatMap {
            let attribute = super.layoutAttributesForItem(at: $0)
            attribute?.zIndex = 1
            return attribute
        }
        
        startDragPoint = point
        
        // Add them to high-level behavior
        dragBehavior = DragBehavior(items: draggableAttributes, point: point)
        
        // Add high-level behavior to animator
        animator?.addBehavior(dragBehavior!)
    }
    
    func updateDragLocation(_ point: CGPoint)
    {
        // Tell high-level behavior that point has updated
        dragBehavior?.updateDragLocation(with: point)
    }
    
    func endDragging()
    {
        isFinishedDragging = true
        
        // Return high-level behavior to starting point
        dragBehavior?.updateDragLocation(with: startDragPoint)
    }
    
    func clearDraggedIndexPaths()
    {
        // Reset state for next drag event
        animator = nil
        indexPathsForDraggingElements = nil
        isFinishedDragging = false
    }
    
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]?
    {
        let existingAttributes: [UICollectionViewLayoutAttributes] = super.layoutAttributesForElements(in: rect) ?? []
        var allAttributes = [UICollectionViewLayoutAttributes]()
        
        // Get normal flow layout attributes for non-drag items
        for attributes in existingAttributes
        {
            if (indexPathsForDraggingElements?.contains(attributes.indexPath) ?? false) == false
            {
                allAttributes.append(attributes)
            }
        }
        
        // Add dragged item attributes by asking animator for them
        if let animator = self.animator
        {
            let animatorAttributes: [UICollectionViewLayoutAttributes] = animator.items(in: rect).flatMap { $0 as? UICollectionViewLayoutAttributes }
            allAttributes.append(contentsOf: animatorAttributes)
        }
        return allAttributes
    }
}
extension DraggableLayout: UIDynamicAnimatorDelegate
{
    func dynamicAnimatorDidPause(_ animator: UIDynamicAnimator)
    {
        // Animator has paused and done dragging; reset state
        guard isFinishedDragging else { return }
        clearDraggedIndexPaths()
    }
}

Objective-C

@interface DraggableLayout () <UIDynamicAnimatorDelegate>
@property (nonatomic, strong) NSArray <NSIndexPath *> *indexPathsForDraggingElements;
@property (nonatomic, strong) UIDynamicAnimator *animator;
@property (nonatomic, assign) CGPoint startDragPoint;
@property (nonatomic, assign) BOOL finishedDragging;
@property (nonatomic, strong) DragBehavior *dragBehavior;
@end

@implementation DraggableLayout

- (void)startDraggingWithIndexPaths:(NSArray <NSIndexPath *> *)selectedIndexPaths fromPoint:(CGPoint)point
{
    self.indexPathsForDraggingElements = selectedIndexPaths;
    self.animator = [[UIDynamicAnimator alloc]initWithCollectionViewLayout:self];
    self.animator.delegate = self;
    NSMutableArray *draggableAttributes = [[NSMutableArray alloc]initWithCapacity:selectedIndexPaths.count];
    for (NSIndexPath *indexPath in selectedIndexPaths)
    {
        UICollectionViewLayoutAttributes *attributes = [super layoutAttributesForItemAtIndexPath:indexPath];
        attributes.zIndex = 1;
        [draggableAttributes addObject:attributes];
    }
    self.startDragPoint = point;
    self.dragBehavior = [[DragBehavior alloc]initWithItems:draggableAttributes point:point];
    [self.animator addBehavior:self.dragBehavior];
}

- (void)updateDragLoactionWithPoint:(CGPoint)point
{
    [self.dragBehavior updateDragLocationWithPoint:point];
}

- (void)endDragging
{
    self.finishedDragging = YES;
    [self.dragBehavior updateDragLocationWithPoint:self.startDragPoint];
}

- (void)clearDraggedIndexPath
{
    self.animator = nil;
    self.indexPathsForDraggingElements = nil;
    self.finishedDragging = NO;
}

- (void)dynamicAnimatorDidPause:(UIDynamicAnimator *)animator
{
    if (self.finishedDragging)
    {
        [self clearDraggedIndexPath];
    }
}

- (NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSArray *existingAttributes = [super layoutAttributesForElementsInRect:rect];
    NSMutableArray *allAttributes = [[NSMutableArray alloc]initWithCapacity:existingAttributes.count];
    for (UICollectionViewLayoutAttributes *attributes in existingAttributes)
    {
        if (![self.indexPathsForDraggingElements containsObject:attributes.indexPath])
        {
            [allAttributes addObject:attributes];
        }
    }
    [allAttributes addObjectsFromArray:[self.animator itemsInRect:rect]];
    return allAttributes;
}

@end

Finally, we'll create a view controller that will create our UICollectionView and handle our long press gesture.

Swift

final class ViewController: UIViewController
{
    // Collection view that displays cells
    lazy var collectionView: UICollectionView =
    {
        let collectionView = UICollectionView(frame: .zero, collectionViewLayout: DraggableLayout())
        collectionView.backgroundColor = .white
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        self.view.addSubview(collectionView)
        collectionView.topAnchor.constraint(equalTo: self.topLayoutGuide.bottomAnchor).isActive = true
        collectionView.leadingAnchor.constraint(equalTo: self.view.leadingAnchor).isActive = true
        collectionView.trailingAnchor.constraint(equalTo: self.view.trailingAnchor).isActive = true
        collectionView.bottomAnchor.constraint(equalTo: self.bottomLayoutGuide.topAnchor).isActive = true
        
        return collectionView
    }()
    
    // Gesture that drives dragging
    lazy var longPress: UILongPressGestureRecognizer =
    {
        let longPress = UILongPressGestureRecognizer(target: self, action: #selector(self.handleLongPress(sender:)))
        return longPress
    }()
    
    // Array that holds selected index paths
    var selectedIndexPaths = [IndexPath]()
    
    override func viewDidLoad()
    {
        super.viewDidLoad()
        collectionView.delegate = self
        collectionView.dataSource = self
        collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: "Cell")
        collectionView.addGestureRecognizer(longPress)
    }
    
    func handleLongPress(sender: UILongPressGestureRecognizer)
    {
        guard let draggableLayout = collectionView.collectionViewLayout as? DraggableLayout else { return }
        let location = sender.location(in: collectionView)
        switch sender.state
        {
        case .began:
            draggableLayout.startDragging(indexPaths: selectedIndexPaths, from: location)
        case .changed:
            draggableLayout.updateDragLocation(location)
        case .ended, .failed, .cancelled:
            draggableLayout.endDragging()
        case .possible:
            break
        }
    }
}
extension ViewController: UICollectionViewDelegate, UICollectionViewDataSource
{
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int
    {
        return 1000
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell
    {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath)
        cell.backgroundColor = .gray
        if selectedIndexPaths.contains(indexPath) == true
        {
            cell.backgroundColor = .red
        }
        return cell
    }
    
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath)
    {
        // Bool that determines if cell is being selected or unselected
        let isSelected = !selectedIndexPaths.contains(indexPath)
        let cell = collectionView.cellForItem(at: indexPath)
        cell?.backgroundColor = isSelected ? .red : .gray
        if isSelected
        {
            selectedIndexPaths.append(indexPath)
        }
        else
        {
            selectedIndexPaths.remove(at: selectedIndexPaths.index(of: indexPath)!)
        }
    }
}

Objective-C

@interface ViewController () <UICollectionViewDelegate, UICollectionViewDataSource>
@property (nonatomic, strong) UICollectionView *collectionView;
@property (nonatomic, strong) UILongPressGestureRecognizer *longPress;
@property (nonatomic, strong) NSMutableArray <NSIndexPath *> *selectedIndexPaths;
@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.collectionView.delegate = self;
    self.collectionView.dataSource = self;
    [self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"Cell"];
    [self.collectionView addGestureRecognizer:self.longPress];
    self.selectedIndexPaths = [[NSMutableArray alloc]init];
}

- (UICollectionView *)collectionView
{
    if (!_collectionView)
    {
        _collectionView = [[UICollectionView alloc]initWithFrame:CGRectZero collectionViewLayout:[[DraggableLayout alloc]init]];
        _collectionView.backgroundColor = [UIColor whiteColor];
        _collectionView.translatesAutoresizingMaskIntoConstraints = NO;
        [self.view addSubview:_collectionView];
        [_collectionView.topAnchor constraintEqualToAnchor:self.topLayoutGuide.bottomAnchor].active = YES;
        [_collectionView.leadingAnchor constraintEqualToAnchor:self.view.leadingAnchor].active = YES;
        [_collectionView.trailingAnchor constraintEqualToAnchor:self.view.trailingAnchor].active = YES;
        [_collectionView.bottomAnchor constraintEqualToAnchor:self.bottomLayoutGuide.topAnchor].active = YES;
    }
    return _collectionView;
}

- (UILongPressGestureRecognizer *)longPress
{
    if (!_longPress)
    {
        _longPress = [[UILongPressGestureRecognizer alloc]initWithTarget:self action:@selector(handleLongPress:)];
    }
    return _longPress;
}

- (void)handleLongPress:(UILongPressGestureRecognizer *)sender
{
    DraggableLayout *draggableLayout = (DraggableLayout *)self.collectionView.collectionViewLayout;
    CGPoint location = [sender locationInView:self.collectionView];
    if (sender.state == UIGestureRecognizerStateBegan)
    {
        [draggableLayout startDraggingWithIndexPaths:self.selectedIndexPaths fromPoint:location];
    }
    else if(sender.state == UIGestureRecognizerStateChanged)
    {
        [draggableLayout updateDragLoactionWithPoint:location];
    }
    else if(sender.state == UIGestureRecognizerStateEnded ||  sender.state == UIGestureRecognizerStateCancelled || sender.state == UIGestureRecognizerStateFailed)
    {
        [draggableLayout endDragging];
    }
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return 1000;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
    cell.backgroundColor = [UIColor grayColor];
    if ([self.selectedIndexPaths containsObject:indexPath])
    {
        cell.backgroundColor = [UIColor redColor];
    }
    return cell;
}

- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
    BOOL isSelected = ![self.selectedIndexPaths containsObject:indexPath];
    UICollectionViewCell *cell = [collectionView cellForItemAtIndexPath:indexPath];
    if (isSelected)
    {
        cell.backgroundColor = [UIColor redColor];
        [self.selectedIndexPaths addObject:indexPath];
    }
    else
    {
        cell.backgroundColor = [UIColor grayColor];
        [self.selectedIndexPaths removeObject:indexPath];
    }
}

@end

For more information 2013 WWDC Session "Advanced Techniques with UIKit Dynamics"