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+// Copyright (c) 2014 Giovanni Lodi
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+//
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+// Permission is hereby granted, free of charge, to any person obtaining a copy of
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+// this software and associated documentation files (the "Software"), to deal in
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+// the Software without restriction, including without limitation the rights to
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+// use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
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+// the Software, and to permit persons to whom the Software is furnished to do so,
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+// subject to the following conditions:
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+//
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+// The above copyright notice and this permission notice shall be included in all
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+// copies or substantial portions of the Software.
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+//
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+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
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+// FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
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+// COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
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+// IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
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+// CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
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+
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+#import "UICollectionViewLeftAlignedLayout.h"
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+
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+@interface UICollectionViewLayoutAttributes (LeftAligned)
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+
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+- (void)leftAlignFrameWithSectionInset:(UIEdgeInsets)sectionInset;
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+
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+@end
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+
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+@implementation UICollectionViewLayoutAttributes (LeftAligned)
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+
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+- (void)leftAlignFrameWithSectionInset:(UIEdgeInsets)sectionInset
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+{
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+ CGRect frame = self.frame;
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+ frame.origin.x = sectionInset.left;
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+ self.frame = frame;
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+}
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+
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+@end
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+
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+#pragma mark -
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+
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+@implementation UICollectionViewLeftAlignedLayout
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+
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+#pragma mark - UICollectionViewLayout
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+
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+- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
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+ NSArray *originalAttributes = [super layoutAttributesForElementsInRect:rect];
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+ NSMutableArray *updatedAttributes = [NSMutableArray arrayWithArray:originalAttributes];
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+ for (UICollectionViewLayoutAttributes *attributes in originalAttributes) {
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+ if (!attributes.representedElementKind) {
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+ NSUInteger index = [updatedAttributes indexOfObject:attributes];
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+ updatedAttributes[index] = [self layoutAttributesForItemAtIndexPath:attributes.indexPath];
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+ }
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+ }
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+
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+ return updatedAttributes;
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+}
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+
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+- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
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+ UICollectionViewLayoutAttributes* currentItemAttributes = [[super layoutAttributesForItemAtIndexPath:indexPath] copy];
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+ UIEdgeInsets sectionInset = [self evaluatedSectionInsetForItemAtIndex:indexPath.section];
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+
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+ BOOL isFirstItemInSection = indexPath.item == 0;
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+ CGFloat layoutWidth = CGRectGetWidth(self.collectionView.frame) - sectionInset.left - sectionInset.right;
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+
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+ if (isFirstItemInSection) {
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+ [currentItemAttributes leftAlignFrameWithSectionInset:sectionInset];
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+ return currentItemAttributes;
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+ }
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+
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+ NSIndexPath* previousIndexPath = [NSIndexPath indexPathForItem:indexPath.item-1 inSection:indexPath.section];
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+ CGRect previousFrame = [self layoutAttributesForItemAtIndexPath:previousIndexPath].frame;
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+ CGFloat previousFrameRightPoint = previousFrame.origin.x + previousFrame.size.width;
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+ CGRect currentFrame = currentItemAttributes.frame;
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+ CGRect strecthedCurrentFrame = CGRectMake(sectionInset.left,
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+ currentFrame.origin.y,
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+ layoutWidth,
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+ currentFrame.size.height);
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+ // if the current frame, once left aligned to the left and stretched to the full collection view
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+ // widht intersects the previous frame then they are on the same line
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+ BOOL isFirstItemInRow = !CGRectIntersectsRect(previousFrame, strecthedCurrentFrame);
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+
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+ if (isFirstItemInRow) {
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+ // make sure the first item on a line is left aligned
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+ [currentItemAttributes leftAlignFrameWithSectionInset:sectionInset];
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+ return currentItemAttributes;
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+ }
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+
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+ CGRect frame = currentItemAttributes.frame;
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+ frame.origin.x = previousFrameRightPoint + [self evaluatedMinimumInteritemSpacingForSectionAtIndex:indexPath.section];
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+ currentItemAttributes.frame = frame;
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+ return currentItemAttributes;
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+}
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+
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+- (CGFloat)evaluatedMinimumInteritemSpacingForSectionAtIndex:(NSInteger)sectionIndex
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+{
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+ if ([self.collectionView.delegate respondsToSelector:@selector(collectionView:layout:minimumInteritemSpacingForSectionAtIndex:)]) {
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+ id<UICollectionViewDelegateLeftAlignedLayout> delegate = (id<UICollectionViewDelegateLeftAlignedLayout>)self.collectionView.delegate;
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+
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+ return [delegate collectionView:self.collectionView layout:self minimumInteritemSpacingForSectionAtIndex:sectionIndex];
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+ } else {
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+ return self.minimumInteritemSpacing;
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+ }
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+}
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+
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+- (UIEdgeInsets)evaluatedSectionInsetForItemAtIndex:(NSInteger)index
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+{
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+ if ([self.collectionView.delegate respondsToSelector:@selector(collectionView:layout:insetForSectionAtIndex:)]) {
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+ id<UICollectionViewDelegateLeftAlignedLayout> delegate = (id<UICollectionViewDelegateLeftAlignedLayout>)self.collectionView.delegate;
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+
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+ return [delegate collectionView:self.collectionView layout:self insetForSectionAtIndex:index];
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+ } else {
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+ return self.sectionInset;
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+ }
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+}
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+
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+@end
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南鑫林