ios - UIDynamicAnimator + 自定义 UICollectionViewLayout 导致永久圆周运动

Question

我一直在复制 2013 WWDC Session 217“Exploring Scroll Views on iOS 7”。我使用的是 Xcode 7 beta 2，我的项目仅适用于 iOS 9。

我正在尝试将 UIDynamicAnimator 与我的 UICollectionViewLayout 结合使用，其方式类似于 session 217 中呈现的方式，以模仿 Messages.app 的感觉。我的 UICollectionViewLayout 是一个自定义布局，出于某种原因，我的单元格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  

是什么导致了这种圆周运动？我只是更改了我的 UIAttachmentAttributes 中心的 Y 轴属性。

center.y += MIN(scrollDelta, scrollDelta * scrollResistance);  

我在这里错过了什么？ (在其他项目中尝试过这种确切的布局并且似乎有效)。

编辑:

我上传了一个示例项目(已删除)，自定义 Collection View 布局类称为 VVSpringCollectionViewFlowLayout.m，自从我有一个最近有很多工作要做。

当示例项目运行时(Xcode 7 beta 或更高版本)，系统会提示您使用 slider ，一直拖动到右侧以可视化 Collection View Cells。

Answer 1

下面的代码应该可以帮助您/为您指明正确的方向。它也有一些额外的功能——比如清理不必要的动画师行为(如果不在 View 中)、跟踪触摸以便从那个点开始动画师的行为。从一个旧项目中剥离出来，所以应该可以正常工作。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

SampleCode 的答案:示例代码显示出这种奇怪的摇摆效果，因为项目的大小是以编程方式生成的，并且没有四舍五入——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。简单地四舍五入生成的项目大小给物理一个机会。参见 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)
}