我一直在复制2013 WWDC Session 217＆＃34;在iOS 7＆＃34;上探索滚动视图。我使用的是Xcode 7 beta 2，我的项目仅限iOS 9。
我正在尝试使用UIDynamicAnimator与我的UICollectionViewLayout以类似于会话217中提供的方式来模仿Messages.app的感觉。我的UICollectionViewLayout是自定义的，出于某种原因，我的细胞似乎在我的项目中以圆周运动反弹。
这是我的自定义布局代码。
// Didn't write this code myself, but should be pretty simple to follow. @Goles
#import "VVSpringCollectionViewFlowLayout.h"
@interface VVSpringCollectionViewFlowLayout()
@property (nonatomic, strong) UIDynamicAnimator *animator;
@end
@implementation VVSpringCollectionViewFlowLayout
-(id)init {
if (self = [super init]) {
_springDamping = 0.5;
_springFrequency = 0.8;
_resistanceFactor = 500;
}
return self;
}
- (id)initWithCoder:(nonnull NSCoder *)aDecoder {
self = [super initWithCoder:aDecoder];
if (self) {
_springDamping = 0.5;
_springFrequency = 0.8;
_resistanceFactor = 500;
}
return self;
}
-(void)prepareLayout {
[super prepareLayout];
if (!_animator) {
_animator = [[UIDynamicAnimator alloc] initWithCollectionViewLayout:self];
CGSize contentSize = [self collectionViewContentSize];
NSArray *items = [super layoutAttributesForElementsInRect:CGRectMake(0, 0, contentSize.width, contentSize.height)];
for (UICollectionViewLayoutAttributes *item in items) {
UIAttachmentBehavior *spring = [[UIAttachmentBehavior alloc] initWithItem:item attachedToAnchor:item.center];
spring.length = 0;
spring.damping = self.springDamping;
spring.frequency = self.springFrequency;
[_animator addBehavior:spring];
}
}
}
-(NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
return [_animator itemsInRect:rect];
}
- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
return [_animator layoutAttributesForCellAtIndexPath:indexPath];
}
-(BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {
UIScrollView *scrollView = self.collectionView;
CGFloat scrollDelta = newBounds.origin.y - scrollView.bounds.origin.y;
CGPoint touchLocation = [scrollView.panGestureRecognizer locationInView:scrollView];
for (UIAttachmentBehavior *spring in _animator.behaviors) {
CGPoint anchorPoint = spring.anchorPoint;
CGFloat distanceFromTouch = fabs(touchLocation.y - anchorPoint.y);
CGFloat scrollResistance = distanceFromTouch / self.resistanceFactor;
id<UIDynamicItem> item = [spring.items firstObject];
CGPoint center = item.center;
if (scrollDelta > 0) {
center.y += MIN(scrollDelta, scrollDelta * scrollResistance);
}
item.center = center;
[_animator updateItemUsingCurrentState:item];
}
return NO;
}
@end
这可能会导致这个循环动作？我只是更改UIAttachmentAttributes中心的Y轴属性。
center.y += MIN(scrollDelta, scrollDelta * scrollResistance);
我在这里缺少什么？ （在其他项目中尝试过这种确切的布局，似乎有效）。
编辑：
我上传了一个示例项目（已删除），自定义集合视图布局类称为VVSpringCollectionViewFlowLayout.m，因为我有很多东西，所以我没有多少时间来研究这个问题最近在工作。
当示例项目运行时（Xcode 7 beta或更高版本），您将被提示使用滑块，一直向右拖动以显示Collection View Cells。
答案 0 :(得分：5)
以下代码可以帮助您指出正确的方向。它还有一些附加功能 - 比如清理不必要的动画师行为，如果不在视野中，跟踪触摸这样的动画师行为。剥离旧项目所以应该工作。包含演示创建视频的Github示例项目 - https://github.com/serendipityapps/SpringyCollectionView
@interface VVSpringCollectionViewFlowLayout ()
@property (nonatomic, strong) UIDynamicAnimator *dynamicAnimator;
@property (nonatomic, strong) NSMutableSet *visibleIndexPathsSet;
@property (nonatomic, assign) CGFloat latestDelta;
@end
@implementation VVSpringCollectionViewFlowLayout
- (id)init {
if (self = [super init]) {
self.dynamicAnimator = [[UIDynamicAnimator alloc] initWithCollectionViewLayout:self];
self.visibleIndexPathsSet = [NSMutableSet set];
}
return self;
}
- (id)initWithCoder:(nonnull NSCoder *)aDecoder {
if (self = [super initWithCoder:aDecoder]) {
self.dynamicAnimator = [[UIDynamicAnimator alloc] initWithCollectionViewLayout:self];
self.visibleIndexPathsSet = [NSMutableSet set];
}
return self;
}
-(NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
return [self.dynamicAnimator itemsInRect:rect];
}
-(UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
return [self.dynamicAnimator layoutAttributesForCellAtIndexPath:indexPath];
}
- (UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath {
return [self.dynamicAnimator layoutAttributesForSupplementaryViewOfKind:kind atIndexPath:indexPath];
}
-(void)prepareLayout {
[super prepareLayout];
// Need to enlarge visible rect slightly to avoid flickering.
CGRect visibleRect = CGRectInset((CGRect){.origin = self.collectionView.bounds.origin, .size = self.collectionView.frame.size}, -100, -100);
NSArray *itemsInVisibleRectArray = [super layoutAttributesForElementsInRect:visibleRect];
NSArray *cells = [itemsInVisibleRectArray filteredArrayUsingPredicate:[NSPredicate predicateWithBlock:^BOOL(UICollectionViewLayoutAttributes *item, NSDictionary *bindings) {
return !item.representedElementKind;
}]];
NSSet *itemsIndexPathsInVisibleRectSet = [NSSet setWithArray:[cells valueForKey:@"indexPath"]];
// Remove any behaviours that are no longer visible.
NSArray *noLongerVisibleBehavioursCells = [self.dynamicAnimator.behaviors filteredArrayUsingPredicate:[NSPredicate predicateWithBlock:^BOOL(UIAttachmentBehavior *behaviour, NSDictionary *bindings) {
UICollectionViewLayoutAttributes *item= (UICollectionViewLayoutAttributes*)[[behaviour items] firstObject];
if (!item.representedElementKind) {
BOOL currentlyVisible = [itemsIndexPathsInVisibleRectSet member:[item indexPath]] != nil;
return !currentlyVisible;
}
else {
return NO;
}
}]];
[noLongerVisibleBehavioursCells enumerateObjectsUsingBlock:^(UIAttachmentBehavior *behaviour, NSUInteger index, BOOL *stop) {
UICollectionViewLayoutAttributes *item = (UICollectionViewLayoutAttributes*)[[behaviour items] firstObject];
[self.dynamicAnimator removeBehavior:behaviour];
[self.visibleIndexPathsSet removeObject:[item indexPath]];
}];
// Add any newly visible behaviours.
CGPoint touchLocation = [self.collectionView.panGestureRecognizer locationInView:self.collectionView];
// A "newly visible" item is one that is in the itemsInVisibleRect(Set|Array) but not in the visibleIndexPathsSet
NSArray *newlyVisibleItems = [cells filteredArrayUsingPredicate:[NSPredicate predicateWithBlock:^BOOL(UICollectionViewLayoutAttributes *item, NSDictionary *bindings) {
BOOL currentlyVisible = [self.visibleIndexPathsSet member:item.indexPath] != nil;
return !currentlyVisible;
}]];
[newlyVisibleItems enumerateObjectsUsingBlock:^(UICollectionViewLayoutAttributes *item, NSUInteger idx, BOOL *stop) {
CGPoint center = item.center;
UIAttachmentBehavior *springBehaviour = [[UIAttachmentBehavior alloc] initWithItem:item attachedToAnchor:center];
springBehaviour.length = 0.0f;
springBehaviour.damping = 0.8f;
springBehaviour.frequency = 1.0f;
// If our touchLocation is not (0,0), we'll need to adjust our item's center "in flight"
if (!CGPointEqualToPoint(CGPointZero, touchLocation)) {
CGFloat yDistanceFromTouch = fabs(touchLocation.y - springBehaviour.anchorPoint.y);
CGFloat xDistanceFromTouch = fabs(touchLocation.x - springBehaviour.anchorPoint.x);
CGFloat scrollResistance = (yDistanceFromTouch + xDistanceFromTouch) / 1500.0f;
if (self.latestDelta < 0) {
center.y += MAX(self.latestDelta, self.latestDelta*scrollResistance);
}
else {
center.y += MIN(self.latestDelta, self.latestDelta*scrollResistance);
}
item.center = center;
}
[self.dynamicAnimator addBehavior:springBehaviour];
[self.visibleIndexPathsSet addObject:item.indexPath];
}];
}
-(BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {
UIScrollView *scrollView = self.collectionView;
CGFloat delta = newBounds.origin.y - scrollView.bounds.origin.y;
self.latestDelta = delta;
CGPoint touchLocation = [self.collectionView.panGestureRecognizer locationInView:self.collectionView];
__block UIDynamicAnimator *weakDynamicAnimator = self.dynamicAnimator;
[self.dynamicAnimator.behaviors enumerateObjectsUsingBlock:^(UIAttachmentBehavior *springBehaviour, NSUInteger idx, BOOL *stop) {
CGFloat yDistanceFromTouch = fabs(touchLocation.y - springBehaviour.anchorPoint.y);
CGFloat xDistanceFromTouch = fabs(touchLocation.x - springBehaviour.anchorPoint.x);
CGFloat scrollResistance = (yDistanceFromTouch + xDistanceFromTouch) / 1500.0f;
UICollectionViewLayoutAttributes *item = (UICollectionViewLayoutAttributes*)[springBehaviour.items firstObject];
CGPoint center = item.center;
if (delta < 0) {
center.y += MAX(delta, delta*scrollResistance);
}
else {
center.y += MIN(delta, delta*scrollResistance);
}
item.center = center;
[weakDynamicAnimator updateItemUsingCurrentState:item];
}];
return NO;
}
@end
回答SampleCode： 示例代码显示这种奇怪的摆动效果，因为项目的大小是以编程方式生成的并且没有舍入 - 计算的精度会给UIDynamics和物理引擎带来问题，并且它永远无法达到平衡。简单地舍入生成的项目大小为物理提供了机会。请参阅NoteCollectionViewController.swift第77行。
func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {
let w = round(CellAspectRatio.width * collectionView.frame.width)
let h = round(CellAspectRatio.height * collectionView.frame.height)
return CGSizeMake(w, h)
}
答案 1 :(得分：0)
接受的答案大多是正确的。它实际上可以在iPhone +上以相同的症状再次失败，屏幕比例为3.以中心处于中心处于一些无理数的1/3或2/3点将再次引起永久的圆周运动。
舍入到偶数
2.0 * floorf((number/2.0) + 0.5) - 从其他帖子修改。
这将确保中心是整数而不是非理性。
然后通过仅在一个维度上拉中心将使bug消失。