UICollectionViewLayoutAttributes
UICollectionViewLayoutAttributes是一个非常重要的类，属性列表如下
@property (nonatomic) CGRect frame;
@property (nonatomic) CGPoint center;
@property (nonatomic) CGSize size;
@property (nonatomic) CATransform3D transform3D;
@property (nonatomic) CGRect bounds NS_AVAILABLE_IOS(7_0);
@property (nonatomic) CGAffineTransform transform NS_AVAILABLE_IOS(7_0);
@property (nonatomic) CGFloat alpha;
@property (nonatomic) NSInteger zIndex; // default is 0
@property (nonatomic, getter=isHidden) BOOL hidden;
UICollectionViewLayoutAttributes的实例中包含了诸如边框、中心点、大小、形状、边界、透明度、层次关系和是否隐藏等信息。当UICollectionView在获取布局时将针对每一个indexPath的部件(包括cell、SupplementaryView和DecorationView)，向其上的UICollectionViewLayout实例询问该部件的布局信息，这个布局信息，就是以UICollectionViewLayoutAttributes的实例方式给出的。
自定义UICollectionViewLayout
UICollectionViewLayout的功能是为UICollectionView提供布局信息，不仅包括cell的布局信息，也包括SupplementaryView和DecorationView的布局信息。实现一个自定义的UICollectionViewLayout的常规做法是继承UICollectionViewLayout，然后重载下列方法
// 返回collectionView的内容尺寸
-(CGSize)collectionViewContentSize
/**
* 返回rect中的所有元素的布局属性
* 返回的是包含UICollectionViewLayoutAttributes的数组
* UICollectionViewLayoutAttributes可以是cell、SupplementaryView或者DecorationView的信息通过不同的UICollectionViewLayoutAttributes初始化方法可以得到不同类型的UICollectionViewLayoutAttributes
* + (instancetype)layoutAttributesForCellWithIndexPath:(NSIndexPath *)indexPath
* + (instancetype)layoutAttributesForSupplementaryViewOfKind:(NSString *)elementKind withIndexPath:(NSIndexPath *)indexPath
* + (instancetype)layoutAttributesForDecorationViewOfKind:(NSString *)decorationViewKind withIndexPath:(NSIndexPath *)indexPath
*/
-(NSArray*)layoutAttributesForElementsInRect:(CGRect)rect
// 返回对应于indexPath的位置的cell的布局属性
- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)path
// 返回对应于indexPath位置的SupplementaryView的布局属性，如果没有SupplementaryView可不重载
- (nullable UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)elementKind atIndexPath:(NSIndexPath *)indexPath;
// 返回对应于indexPath位置的DecorationView的布局属性，如果没有DecorationView可不重载
- (nullable UICollectionViewLayoutAttributes *)layoutAttributesForDecorationViewOfKind:(NSString*)elementKind atIndexPath:(NSIndexPath *)indexPath;
// 当边界发生改变时，是否应该刷新布局。如果YES则在边界变化时，将重新计算需要的布局信息。
- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
初始化一个UICollectionViewLayout实例后，首先-(void)prepareLayout将被调用，之后-(CGSize)collectionViewContentSize被调用，以确定collectionView应该占据的尺寸。注意这里的尺寸不是指可是部分的尺寸，应该是所有内容占据的尺寸。接下来-(NSArray*)layoutAttributesForElementsInRect:(CGRect)rect被调用，获取部件的布局信息。
CircleLayout---官方示例
参数声明
@interface CircleLayout ()
@property (nonatomic, assign) CGPoint center;
@property (nonatomic, assign) CGFloat radius;
@property (nonatomic, assign) NSInteger cellCount;
// arrays to keep track of insert, delete index paths
@property (nonatomic, strong) NSMutableArray *deleteIndexPaths;
@property (nonatomic, strong) NSMutableArray *insertIndexPaths;
@end
首先布局准备中计算所需要用到的参数
-(void)prepareLayout
{
[super prepareLayout];
CGSize size = self.collectionView.frame.size;
_cellCount = [[self collectionView] numberOfItemsInSection:0];
_center = CGPointMake(size.width / 2.0, size.height / 2.0);
_radius = MIN(size.width, size.height) / 2.5;
}
然后按照UICollectionViewLayout子类的要求重载所需要的方法
// 返回collectionView的内容尺寸
-(CGSize)collectionViewContentSize
{
return [self collectionView].frame.size;
}
// 返回对应于indexPath的位置的cell的布局属性
- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)path
{
UICollectionViewLayoutAttributes* attributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:path];
attributes.size = CGSizeMake(ITEM_SIZE, ITEM_SIZE);
attributes.center = CGPointMake(_center.x + _radius * cosf(2 * path.item * M_PI / _cellCount),
_center.y + _radius * sinf(2 * path.item * M_PI / _cellCount));
return attributes;
}
-(NSArray*)layoutAttributesForElementsInRect:(CGRect)rect
{
NSMutableArray* attributes = [NSMutableArray array];
for (NSInteger i=0 ; i < self.cellCount; i++) {
NSIndexPath* indexPath = [NSIndexPath indexPathForItem:i inSection:0];
[attributes addObject:[self layoutAttributesForItemAtIndexPath:indexPath]];
}
return attributes;
}
增加删除cell的动画
- (void)prepareForCollectionViewUpdates:(NSArray *)updateItems
{
// Keep track of insert and delete index paths
[super prepareForCollectionViewUpdates:updateItems];
self.deleteIndexPaths = [NSMutableArray array];
self.insertIndexPaths = [NSMutableArray array];
for (UICollectionViewUpdateItem *update in updateItems)
{
if (update.updateAction == UICollectionUpdateActionDelete)
{
[self.deleteIndexPaths addObject:update.indexPathBeforeUpdate];
}
else if (update.updateAction == UICollectionUpdateActionInsert)
{
[self.insertIndexPaths addObject:update.indexPathAfterUpdate];
}
}
}
- (void)finalizeCollectionViewUpdates
{
[super finalizeCollectionViewUpdates];
// release the insert and delete index paths
self.deleteIndexPaths = nil;
self.insertIndexPaths = nil;
}
// Note: name of method changed
// Also this gets called for all visible cells (not just the inserted ones) and
// even gets called when deleting cells!
- (UICollectionViewLayoutAttributes *)initialLayoutAttributesForAppearingItemAtIndexPath:(NSIndexPath *)itemIndexPath
{
// Must call super
UICollectionViewLayoutAttributes *attributes = [super initialLayoutAttributesForAppearingItemAtIndexPath:itemIndexPath];
if ([self.insertIndexPaths containsObject:itemIndexPath])
{
// only change attributes on inserted cells
if (!attributes)
attributes = [self layoutAttributesForItemAtIndexPath:itemIndexPath];
// Configure attributes ...
attributes.alpha = 0.0;
attributes.center = CGPointMake(_center.x, _center.y);
}
return attributes;
}
// Note: name of method changed
// Also this gets called for all visible cells (not just the deleted ones) and
// even gets called when inserting cells!
- (UICollectionViewLayoutAttributes *)finalLayoutAttributesForDisappearingItemAtIndexPath:(NSIndexPath *)itemIndexPath
{
// So far, calling super hasn't been strictly necessary here, but leaving it in
// for good measure
UICollectionViewLayoutAttributes *attributes = [super finalLayoutAttributesForDisappearingItemAtIndexPath:itemIndexPath];
if ([self.deleteIndexPaths containsObject:itemIndexPath])
{
// only change attributes on deleted cells
if (!attributes)
attributes = [self layoutAttributesForItemAtIndexPath:itemIndexPath];
// Configure attributes ...
attributes.alpha = 0.0;
attributes.center = CGPointMake(_center.x, _center.y);
attributes.transform3D = CATransform3DMakeScale(0.1, 0.1, 1.0);
}
return attributes;
}
在ViewController中为collectionView增加手势识别
UITapGestureRecognizer* tapRecognizer = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(handleTapGesture:)];
[self.collectionView addGestureRecognizer:tapRecognizer];
手势响应方法
#pragma mark - event response
- (void)handleTapGesture:(UITapGestureRecognizer *)sender {
if (sender.state == UIGestureRecognizerStateEnded)
{
CGPoint initialPinchPoint = [sender locationInView:self.collectionView];
NSIndexPath* tappedCellPath = [self.collectionView indexPathForItemAtPoint:initialPinchPoint];
if (tappedCellPath!=nil)
{
self.cellCount = self.cellCount - 1;
[self.collectionView performBatchUpdates:^{
[self.collectionView deleteItemsAtIndexPaths:@[tappedCellPath]];
} completion:nil];
}
else
{
self.cellCount = self.cellCount + 1;
[self.collectionView performBatchUpdates:^{
[self.collectionView insertItemsAtIndexPaths:@[[NSIndexPath indexPathForItem:0 inSection:0]]];
} completion:nil];
}
}
}
CircleLayout 中cell都分布在圆周上，点击cell的话会将其从collectionView中移除，点击空白处会加入一个cell，加入和移除cell都会有动画效果.
效果图如下
CircleLayout.png
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