TLLayoutTransitioning

Enhanced transitioning between UICollectionView layouts in iOS.

##Overview

TLLayoutTransitioning provides a TLLayoutTransition transition layout subclass and a UICollectionView+TLTransitioning category that combine to solve a few problems with collection view layout transitioning:

1. UICollectionViewLayoutTransition does not handle content offset well, often leaving cells where you don't want them. TLTransitionLayout provides elegant control of content offset with Minimal, Visible, Center, Top, Left, Bottom or Right placement options relative to one or more index paths.

2. UICollectionViewLayoutTransition does not support supplementary views. TLTransitionLayout provides support for any supplementary view kinds specified in the initializer.

3. -[UICollectionView setCollectionViewLayout:animated:completion] has serious known bugs in iOS7 and does not provide any animation options. TLLayoutTransitioning provides a robust alternative to this API with support for animation duration, 30+ easing curves and content offset control. This is done by using CADisplayLink to drive an interactive TLTransitionLayout as a non-interactive animation.

Check out the demos in the Examples workspace!

###TLTransitionLayout Class

TLTransitionLayout is a subclass of UICollectionViewTransitionLayout that interpolates linearly between layouts and optionally between the current content offset and a specified final offset.

The final offset is specified by setting the toContentOffset property. The UICollectionView+TLTransitioning category provides an API for calculating Minimal, Visible, Center, Top, Left, Bottom or Right offset placements relative to one or more index paths.

The basic usage is as follows:

- (void)someViewControllerEventHandler
{
    UICollectionViewLayout *nextLayout = ...;
    self.transitionLayout = (TLTransitionLayout *)[self.collectionView startInteractiveTransitionToCollectionViewLayout:nextLayout 
                                        completion:^(BOOL completed, BOOL finish) {
	    if (finish) {
            self.collectionView.contentOffset = self.transitionLayout.toContentOffset;
            self.transitionLayout = nil;
	    }
    }];
    NSArray *indexPaths = ...;// some selection of index paths to place
    self.transitionLayout.toContentOffset = [self.collectionView toContentOffsetForLayout:self.transitionLayout indexPaths:indexPaths placement:TLTransitionLayoutIndexPathPlacementCenter];
}

- (UICollectionViewTransitionLayout *)collectionView:(UICollectionView *)collectionView transitionLayoutForOldLayout:(UICollectionViewLayout *)fromLayout newLayout:(UICollectionViewLayout *)toLayout
{
    NSArray *supplementaryKinds = ...; // optional supplementary view kinds
    return [[TLTransitionLayout alloc] initWithCurrentLayout:fromLayout nextLayout:toLayout supplementaryKinds:supplementaryKinds];
}

Note that the collection view will reset contentOffset after the transition is finalized, but as illustrated above, this can be negated by setting it back to toContentOffset in the completion block.

###UICollectionView+TLTransitioning Category

The UICollectionView+TLTransitioning category provides some of useful methods for calculating for interactive transitions. In particular, the toContentOffsetForLayout:indexPaths:placement API calculates final content offset values to achieve Minimal, Visible, Center, Top, Left, Bottom or Right placements for one or more index paths. The expanded version of this API provides for even further fine-tuning and supports transitioning to a different collection view size and content inset:

- (CGPoint)toContentOffsetForLayout:(UICollectionViewTransitionLayout *)layout
                         indexPaths:(NSArray *)indexPaths
                          placement:(TLTransitionLayoutIndexPathPlacement)placement
                    placementAnchor:(CGPoint)placementAnchor
                     placementInset:(UIEdgeInsets)placementInset
                             toSize:(CGSize)toSize
                     toContentInset:(UIEdgeInsets)toContentInset

UICollectionView+TLTransitioning also provides an alternative to -[UICollectionView setCollectionViewLayout:animated:completion] for non-interactive animation between layouts with support for animation duration, 30 built-in easing curves (courtesy of Warren Moore's AHEasing library), user defined easing curves (by defining custom AHEasingFunctions) and content offset control. The basic transition call is as follows:

TLTransitionLayout *layout = (TLTransitionLayout *)[collectionView transitionToCollectionViewLayout:toLayout duration:2 easing:QuarticEaseInOut completion:nil];
CGPoint toOffset = [collectionView toContentOffsetForLayout:layout indexPaths:@[indexPath] placement:TLTransitionLayoutIndexPathPlacementCenter];
layout.toContentOffset = toOffset;

where the view controller is configured to provide an instance of TLTransitionLayout as described above. Check out the Resize sample project in the Examples workspace to see this in action.

##Installation

If you're not using CocoaPods, copy the following files into your project:

TLTransitionLayout.h
TLTransitionLayout.m
UICollectionView+TLTransitionAnimator.h    
UICollectionView+TLTransitionAnimator.m

As well as the following files from AHEasing:

easing.h
easing.c

##Examples

Open the Examples workspace (not the project) to run the sample app. The following examples are included:

###Resize

The Resize example combines TLTransitionLayout and -[UICollectionView+TLTransitioning transitionToCollectionViewLayout:duration:easing:completion:] as a better alternative to -[UICollectionView setCollectionViewLayout:animated:completion]. Experiment with different durations, easing curves and content offset options on the settings panel. Toggle "show section headers" to see transitioning supplementary views.

###Pinch

The Pinch example uses demonstrates a simple pinch-driven interactive transition using TLTransitionLayout. The destination contentOffset is selected such that the initial visible cells remain centered. Or if a cell is tapped, the contentOffset the cell is centered.