zoom entire UICollectionView

Question

I have an iPad App where I'm using a UICollectionView and each UICollectionViewCell contains just a single UIImage. Currently I'm displaying per 9 UIImages (3 rows * 3 columns) per page, I have several pages.

I would like to use Pinch Gesture to zoom on the entire UICollectionView to increase/decrease the number of row/columns displayed per page and the best would be to have beautiful zoom animation during the Pinch gesture!

Currently, I have added a Pinch Gesture on my UICollectionView. I catch the Pinch Gesture event to compute the number of rows/columns using the scale factor, if it has changed then I update the full UICollectionView using:

[_theCollectionView performBatchUpdates:^{
     [_theCollectionView deleteSections:[NSIndexSet indexSetWithIndex:0]];
     [_theCollectionView insertSections:[NSIndexSet indexSetWithIndex:0]];
 } completion:nil];

It works but I don't have smooth animation during the transition.

Any idea? UICollectionView inherits from UIScrollView, is there a possibility to re-use the UIScrollView Pinch gesture feature to reach my goal?

Answer 1

I'm assuming you're using the default UICollectionViewDelegateFlowLayout, right? Then make sure you respond accordingly to the delegate methods, and when the pinch gesture occurs, simply invalidate the layout.

For example, if I want to adjust the size of every item, while pinching:

@interface ViewController () <UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout>

@property (nonatomic,assign) CGFloat scale;
@property (nonatomic,weak)   IBOutlet UICollectionView *collectionView;

@end

@implementation ViewController
- (void)viewDidLoad
{
    [super viewDidLoad];

    self.scale = 1.0;

    [self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"cell"];

    UIPinchGestureRecognizer *gesture = [[UIPinchGestureRecognizer alloc] initWithTarget:self action:@selector(didReceivePinchGesture:)];
    [self.collectionView addGestureRecognizer:gesture];

}

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
    return CGSizeMake(50*self.scale, 50*self.scale);
}

- (void)didReceivePinchGesture:(UIPinchGestureRecognizer*)gesture
{
    static CGFloat scaleStart;

    if (gesture.state == UIGestureRecognizerStateBegan)
    {
        scaleStart = self.scale;
    }
    else if (gesture.state == UIGestureRecognizerStateChanged)
    {
        self.scale = scaleStart * gesture.scale;
        [self.collectionView.collectionViewLayout invalidateLayout];
    }
}

The property self.scale is just for show, you can apply this same concept to any other attribute, this doesn't require a beginUpdates/endUpdates because the user himself is carrying the timing of the scale.

Here's a running project, in case you want to see it in action.

Answer 2

Sorry for my 2 cents question, I have found the solution, very simple.

In my PinchGesture callback I have just done the following:

void (^animateChangeWidth)() = ^() {
    _theFlowLayout.itemSize = cellSize;
};

[UIView transitionWithView:self.theCollectionView 
                  duration:0.1f 
                   options:UIViewAnimationOptionCurveLinear 
                animations:animateChangeWidth 
                completion:nil];

All cells of my UICollectionView are successfully changed and with a nice transition.