IGListKit/Source/IGListCollectionViewLayout.h

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

#import <UIKit/UIKit.h>

#import <IGListKit/IGListMacros.h>

NS_ASSUME_NONNULL_BEGIN

/**
 This UICollectionViewLayout subclass is for vertically or horizontally scrolling lists of data with variable widths and
 heights. It supports an infinite number of sections and items. All work is done on the main thread, and while extremely efficient,
 care must be taken not to stall the main thread in sizing delegate methods.

 This layout piggybacks on the mechanics of UICollectionViewFlowLayout in that:

 - Your UICollectionView data source must also conform to UICollectionViewDelegateFlowLayout
 - Header support given via UICollectionElementKindSectionHeader

 All UICollectionViewDelegateFlowLayout methods are required and used by this layout:

 ```
 - (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath;
 - (UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout insetForSectionAtIndex:(NSInteger)section;
 - (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout minimumLineSpacingForSectionAtIndex:(NSInteger)section;
 - (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout minimumInteritemSpacingForSectionAtIndex:(NSInteger)section;
 - (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout referenceSizeForHeaderInSection:(NSInteger)section;
 ```

 In a vertically scrolling layout, sections and items are put into the same horizontal row until the max-x position
 of an item extends beyond the width of the collection view. When that happens, the item is "newlined" to the next row.
 The y position of that row is determined by the maximum height (including section insets) of the section/item of the previous row.

 Ex. of a section (2,0) with a large width causing a newline.
 ```
 |[ 0,0 ][ 1,0 ]         |
 |[         2,0         ]|
 ```

 A section with a non-zero height header will always cause that section to newline. Headers are always stretched to the
 width of the collection view, pinched with the section insets.

 Ex. of a section (2,0) with a header inset on the left/right.
 ```
 |[ 0,0 ][ 1,0 ]         |
 | >======header=======< |
 | [ 2,0 ]               |
 ```

 Section insets apply to items in the section no matter if they begin on a new row or are on the same row as a previous
 section.

 Ex. of a section (2) with multiple items and a left inset.
 ```
 |[ 0,0 ][ 1,0 ] >[ 2,0 ]|
 | >[ 2,1 ][ 2,2 ][ 2,3 ]|
 ```

 Interitem spacing applies to items and sections within the same row. Line spacing only applies to items within the same
 section.
 
 In a horizontally scrolling layout, sections and items are flowed vertically until they need to be "newlined" to the 
 next column. Headers, if used, are stretched to the height of the collection view, minus the section insets.

 Please see the unit tests for more configuration examples and expected output.
 */
NS_SWIFT_NAME(ListCollectionViewLayout)
@interface IGListCollectionViewLayout : UICollectionViewLayout

/**
 Direction in which layout will be scrollable; items will be flowed in the perpendicular direction, "newlining" when they
 run out of space along that axis or when a non-zero header is found.
 */
@property (nonatomic, readonly) UICollectionViewScrollDirection scrollDirection;

/**
 Set this to adjust the offset of the sticky headers in the scrolling direction. Can be used to change the sticky 
 header position as UI like the navigation bar is scrolled offscreen. In a vertically scrolling layout, changing 
 this to the height of the navigation bar will give the effect of the headers sticking to the nav as it is collapsed.

 @note Changing the value on this method will invalidate the layout every time.
 */
@property (nonatomic, assign) CGFloat stickyHeaderYOffset;

/**
 Set this to `YES` to show sticky header when a section had no item. Default is `NO`.
*/
@property (nonatomic, assign) BOOL showHeaderWhenEmpty;

/**
 Notify the layout that a specific section was modified before invalidation. Used to optimize layout re-calculation.

 @note When updating a collection view (ex: calling `-insertSections`), `-invalidateLayoutWithContext` gets called on
 the layout object. However, the invalidation context doesn't provide details on which index paths are being modified,
 which typically forces a full layout re-calculation. We can use this method to keep track of which section actually
 needs to be updated on the following `-invalidateLayoutWithContext`. See `IGListCollectionView`.

 @param modifiedSection The section that was modified.
 */
- (void)didModifySection:(NSInteger)modifiedSection;

/**
 Create and return a new collection view layout.

 @param stickyHeaders Set to `YES` to stick section headers to the top of the bounds while scrolling.
 @param scrollDirection Direction along which the collection view will be scrollable (if content size exceeds the frame size)
 @param topContentInset The content inset (top or left, depending on scrolling direction) used to offset the sticky headers. Ignored if stickyHeaders is `NO`.
 @param stretchToEdge Specifies whether to stretch width (in vertically scrolling layout) or height (horizontally scrolling) of last item to right/bottom edge when distance from last item to right/bottom edge < epsilon(1)

 @return A new collection view layout.
 */
- (instancetype)initWithStickyHeaders:(BOOL)stickyHeaders
                      scrollDirection:(UICollectionViewScrollDirection)scrollDirection
                      topContentInset:(CGFloat)topContentInset
                        stretchToEdge:(BOOL)stretchToEdge NS_DESIGNATED_INITIALIZER;

/**
 Create and return a new vertically scrolling collection view layout.
 
 @param stickyHeaders Set to `YES` to stick section headers to the top of the bounds while scrolling.
 @param topContentInset The top content inset used to offset the sticky headers. Ignored if stickyHeaders is `NO`.
 @param stretchToEdge Specifies whether to stretch width of last item to right edge when distance from last item to right edge < epsilon(1)
 
 @return A new collection view layout.
 */
- (instancetype)initWithStickyHeaders:(BOOL)stickyHeaders
                      topContentInset:(CGFloat)topContentInset
                        stretchToEdge:(BOOL)stretchToEdge;

/**
 :nodoc:
 */
- (instancetype)init NS_UNAVAILABLE;

/**
 :nodoc:
 */
+ (instancetype)new NS_UNAVAILABLE;

@end

NS_ASSUME_NONNULL_END