##Making Supplementary Views Float

This example will be using a subclass of UICollectionViewFlowLayout. You can achieve a similar effect with a purely custom UICollectionViewLayout subclass, but for simplicity we will be using flow layout.

First you will need to make sure that your UICollectionView instance is using your custom layout. You can do this by selecting the layout object in your nib or storyboard within the collection view object, or by setting the collectionViewLayout property on your collection view in your view controller's viewDidLoad: method.

Next add a prototype header or footer view, or register a class for the kind of supplementary view you wish to float with the appropriate reuse identifier. If you provide a header or footer view in your nib or storyboard file, then the kind will be UICollectionElementKindSectionHeader or UICollectionElementKindSectionFooter respectively.

Then subclass UICollectionViewFlowLayout and override the following methods:

You should override this so that as the view scrolls you will get a chance to update the position of the floating view(s). This gets queried every time the bounds changes.

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
{
    return YES;
}

This method is the master list of all elements that should be visible in your collection view. Make sure that it contains a UICollectionViewLayoutAttributes object for each of your floating views.

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSMutableArray *allItems = [[super layoutAttributesForElementsInRect:rect] mutableCopy];
    
    __block BOOL headerFound = NO;
    __block BOOL footerFound = NO;
    [allItems enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        if ([[obj representedElementKind] isEqualToString:UICollectionElementKindSectionHeader]) {
            headerFound = YES;
            [self updateHeaderAttributes:obj];
        } else if ([[obj representedElementKind] isEqualToString:UICollectionElementKindSectionFooter]) {
            footerFound = YES;
            [self updateFooterAttributes:obj];
        }
    }];
    
    
    // Flow layout will remove items from the list if they are supposed to be off screen, so we add them
    // back in in those cases.
    if (!headerFound) {
        [allItems addObject:[self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader atIndexPath:[NSIndexPath indexPathForItem:[allItems count] inSection:0]]];
    }
    if (!footerFound) {
        [allItems addObject:[self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionFooter atIndexPath:[NSIndexPath indexPathForItem:[allItems count] inSection:0]]];
    }
    
    return allItems;
}

Finally, override this method to make sure that you provide the correct layout attributes for a single supplementary view (also, if you were paying close attention we used this method in layoutAttributesForElementsInRect:).

- (UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewLayoutAttributes *attributes = [UICollectionViewLayoutAttributes layoutAttributesForSupplementaryViewOfKind:kind withIndexPath:indexPath];
    attributes.size = CGSizeMake(self.collectionView.bounds.size.width, 44);
    if ([kind isEqualToString:UICollectionElementKindSectionHeader]) {
        [self updateHeaderAttributes:attributes];
    } else {
        [self updateFooterAttributes:attributes];
    }
    return attributes;
}

And then implement the following helper methods

- (void)updateHeaderAttributes:(UICollectionViewLayoutAttributes *)attributes
{
    CGRect currentBounds = self.collectionView.bounds;
    attributes.zIndex = 1;
    attributes.hidden = NO;
    CGFloat yCenterOffset = currentBounds.origin.y + attributes.size.height/2.f;
    attributes.center = CGPointMake(CGRectGetMidX(currentBounds), yCenterOffset);
}

- (void)updateFooterAttributes:(UICollectionViewLayoutAttributes *)attributes
{
    CGRect currentBounds = self.collectionView.bounds;
    attributes.zIndex = 1;
    attributes.hidden = NO;
    CGFloat yCenterOffset = currentBounds.origin.y + currentBounds.size.height - attributes.size.height/2.0f;
    attributes.center = CGPointMake(CGRectGetMidX(currentBounds), yCenterOffset);
}

License

Copyright (c) 2014 Derrick Hathaway

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

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