Animate a UICollectionView cell on selection

Question

I have a basic grid in a UICollectionView. It's a simple 2 column, multiple row layout using the UICollectionViewDelegateFlowLayout. When a cell is selected, I want to dim the background, float the cell to the center of the screen and then have a workflow based on the selected cell. I'm fairly new to UICollectionViews, and I'm not sure of the best way to go about this.

Should I have a custom layout of the UICollectionView for when a cell is selected?

Or is there a way I can animate the selected cell without having to create a new layout

If anyone can just get me going on the right direction, I think I'll be good to research how to execute it.

Answer 1

After trying several (and rather hacky) solutions, I solved this by writing my own UICollectionView layout. Previous solutions I attempted:

• Add a custom UIView overtop the UICollectionView and attempt to fake the original cell moving by overlaying it, dimming the background, and animating the new UIView
• Animate the cell without creating a brand new layout

I was trying to avoid it, but after I got into coding it, it was a lot less painful than I originally thought.

Here is my code, for anyone interested:

.h:

@interface FMStackedLayout : UICollectionViewLayout

@property(nonatomic,assign) CGPoint     center;
@property(nonatomic,assign) NSInteger   cellCount;

-(id)initWithSelectedCellIndexPath:(NSIndexPath *)indexPath;

@end

.m:

#define ITEM_WIDTH  128.0f
#define ITEM_HEIGHT 180.0f

static NSUInteger       const RotationCount         = 32;
static NSUInteger       const RotationStride        = 3;
static NSUInteger       const PhotoCellBaseZIndex   = 100;

@interface FMStackedLayout ()

@property(strong,nonatomic) NSArray     *rotations;
@property(strong,nonatomic) NSIndexPath *selectedIndexPath;

@end

@implementation FMStackedLayout

#pragma mark - Lifecycle
-(id)initWithSelectedCellIndexPath:(NSIndexPath *)indexPath{
    self = [super init];
    if (self) {
        self.selectedIndexPath = indexPath;
        [self setup];
    }
    return self;
}

-(id)initWithCoder:(NSCoder *)aDecoder{
    self = [super init];
    if (self){
        [self setup];
    }
    return self;
}

-(void)setup{
    NSMutableArray *rotations = [NSMutableArray arrayWithCapacity:RotationCount];
    CGFloat percentage = 0.0f;

    for (NSUInteger i = 0; i < RotationCount; i++) {
        // Ensure that each angle is different enough to be seen
        CGFloat newPercentage = 0.0f;
        do {
            newPercentage = ((CGFloat)(arc4random() % 220) - 110) * 0.0001f;
        } while (fabsf(percentage - newPercentage) < 0.006);

        percentage = newPercentage;

        CGFloat angle = 2 * M_PI * (1.0f + percentage);
        CATransform3D transform = CATransform3DMakeRotation(angle, 0.0f, 0.0f, 1.0f);
        [rotations addObject:[NSValue valueWithCATransform3D:transform]];
    }

    self.rotations = rotations;
}

#pragma mark - Layout
-(void)prepareLayout{
    [super prepareLayout];

    CGSize size = self.collectionView.frame.size;
    self.cellCount = [self.collectionView numberOfItemsInSection:0];
    self.center = CGPointMake(size.width / 2.0, size.height / 2.0);
}

-(CGSize)collectionViewContentSize{
    return self.collectionView.frame.size;
}

-(UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath{
    UICollectionViewLayoutAttributes *attributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];

    attributes.size = CGSizeMake(ITEM_WIDTH, ITEM_HEIGHT);
    attributes.center = self.center;
    if (indexPath.item == self.selectedIndexPath.item) {
        attributes.zIndex = 100;
    }else{
        attributes.transform3D = [self transformForPersonViewAtIndex:indexPath];
    }

    return attributes;
}

-(NSArray *)layoutAttributesForElementsInRect:(CGRect)rect{
    NSMutableArray *attributes = [NSMutableArray array];
    for (NSInteger i = 0; i < self.cellCount; i++) {
        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:i
                                                     inSection:0];
        [attributes addObject:[self layoutAttributesForItemAtIndexPath:indexPath]];
    }
    return attributes;
}

#pragma mark - Private
-(CATransform3D)transformForPersonViewAtIndex:(NSIndexPath *)indexPath{
    NSInteger offset = (indexPath.section * RotationStride + indexPath.item);
    return [self.rotations[offset % RotationCount] CATransform3DValue];
}

@end

Then, on your UICollectionView, you call

MyLayout *stackedLayout = [[MyLayout alloc] initWithSelectedCellIndexPath:indexPath];
[stackedLayout invalidateLayout];
[self.collectionView setCollectionViewLayout:stackedLayout
                                    animated:YES];

Animations between cells will be handled by the custom layout.