UIkit动力学是UIkit框架中模拟真实世界的一些特性。
UIDynamicAnimator
主要有UIDynamicAnimator类，通过这个类中的不同行为来实现一些动态特性。
它一般有两种初始化方法，先讲常见的第一种
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animator= [[UIDynamicAnimator alloc] initWithReferenceView:self.view]; |
动态特性的实现主要依靠它所添加的行为，通过以下方法进行添加和移除，
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[animator addBehavior:attachmentBehavior]; [animator removeAllBehaviors]; |
接下来介绍五个不同的行为，UIAttachmentBehavior（吸附），UICollisionBehavior（碰撞），UIGravityBehavior（重力），UIPushBehavior（推动），UISnapBehavior（捕捉）。另外还有一个辅助的行为UIDynamicItemBehavior，用来在item层级设定一些参数，比如item的摩擦，阻力，角阻力，弹性密度和可允许的旋转等等。
UIAttachmentBehavior（吸附）
先讲吸附行为，
它的初始化方法
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attachmentBehavior = [[UIAttachmentBehavior alloc] initWithItem:iv offsetFromCenter:centerOffset attachedToAnchor:location]; |
item是实现UIDynamicItem协议的id类型，这里设置吸附一个UIImageView的实例iv。offset可以设置吸附的偏移，anchor是设置锚点。
UIAttachmentBehavior有几个属性，例如damping，frequency。damping是阻尼数值，frequency是震动频率
直接上代码，实现一个pan手势，让一个image跟着手势跑
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-(void)gesture:(UIPanGestureRecognizer *)gesture{ CGPoint location = [gesture locationInView:self.view]; CGPoint boxLocation = [gesture locationInView:iv]; switch (gesture.state) { case UIGestureRecognizerStateBegan:{ NSLog(@"you touch started position %@",NSStringFromCGPoint(location)); NSLog(@"location in image started is %@",NSStringFromCGPoint(boxLocation)); [animator removeAllBehaviors]; // Create an attachment binding the anchor point (the finger's current location) // to a certain position on the view (the offset) UIOffset centerOffset = UIOffsetMake(boxLocation.x - CGRectGetMidX(iv.bounds), boxLocation.y - CGRectGetMidY(iv.bounds)); attachmentBehavior = [[UIAttachmentBehavior alloc] initWithItem:iv offsetFromCenter:centerOffset attachedToAnchor:location]; attachmentBehavior.damping=0.5; attachmentBehavior.frequency=0.8; // Tell the animator to use this attachment behavior [animator addBehavior:attachmentBehavior]; break; } case UIGestureRecognizerStateEnded: { [animator removeBehavior:attachmentBehavior]; break; } default: [attachmentBehavior setAnchorPoint:[gesture locationInView:self.view]]; break; }} |
UIPushBehavior（推动）
UIPushBehavior 可以为一个UIView施加一个力的作用，这个力可以是持续的，也可以只是一个冲量。我们可以指定力的大小，方向和作用点等等信息。
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pushBehavior = [[UIPushBehavior alloc] initWithItems:@[iv] mode:UIPushBehaviorModeInstantaneous]; |
UIPushBehavior 有pushDirection、magnitude等属性，
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//1 CGPoint velocity = [gesture velocityInView:self.view]; CGFloat magnitude = sqrtf((velocity.x * velocity.x) + (velocity.y * velocity.y)); if (magnitude > ThrowingThreshold) { //2 pushBehavior = [[UIPushBehavior alloc] initWithItems:@[iv] mode:UIPushBehaviorModeInstantaneous]; pushBehavior.pushDirection = CGVectorMake((velocity.x / 10) , (velocity.y / 10)); pushBehavior.magnitude = magnitude / ThrowingvelocityPadding; [animator addBehavior:pushBehavior]; //3// UIDynamicItemBehavior 其实是一个辅助的行为，用来在item层级设定一些参数，比如item的摩擦，阻力，角阻力，弹性密度和可允许的旋转等等 NSInteger angle = arc4random_uniform(20) - 10; itemBehavior = [[UIDynamicItemBehavior alloc] initWithItems:@[iv]]; itemBehavior.friction = 0.2; itemBehavior.allowsRotation = YES; [itemBehavior addAngularVelocity:angle forItem:iv]; [animator addBehavior:itemBehavior]; //4 [self performSelector:@selector(resetDemo) withObject:nil afterDelay:0.4]; } |
UIGravityBehavior（重力）
直接上代码，实现随机掉落一张图片的代码
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// Set up self.animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view]; self.gravityBeahvior = [[UIGravityBehavior alloc] initWithItems:nil]; [self.animator addBehavior:self.gravityBeahvior]; - (void)tapped:(UITapGestureRecognizer *)gesture { NSUInteger num = arc4random() % 40 + 1; NSString *filename = [NSString stringWithFormat:@"m%lu", (unsigned long)num]; UIImage *image = [UIImage imageNamed:filename]; UIImageView *imageView = [[UIImageView alloc] initWithImage:image]; [self.view addSubview:imageView]; CGPoint tappedPos = [gesture locationInView:gesture.view]; imageView.center = tappedPos; [self.gravityBeahvior addItem:imageView]; } |
UICollisionBehavior（碰撞）
继续上面的代码，当图片快掉落出边界的时候有 碰撞效果，这个就是UICollisionBehavior实现的。
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// Set up self.animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view]; self.gravityBeahvior = [[UIGravityBehavior alloc] initWithItems:nil]; self.collisionBehavior = [[UICollisionBehavior alloc] initWithItems:nil]; self.collisionBehavior.translatesReferenceBoundsIntoBoundary = YES; self.itemBehavior = [[UIDynamicItemBehavior alloc] initWithItems:nil]; self.itemBehavior.elasticity = 0.6; self.itemBehavior.friction = 0.5; self.itemBehavior.resistance = 0.5; [self.animator addBehavior:self.gravityBeahvior]; [self.animator addBehavior:self.collisionBehavior]; [self.animator addBehavior:self.itemBehavior]; - (void)tapped:(UITapGestureRecognizer *)gesture { NSUInteger num = arc4random() % 40 + 1; NSString *filename = [NSString stringWithFormat:@"m%lu", (unsigned long)num]; UIImage *image = [UIImage imageNamed:filename]; UIImageView *imageView = [[UIImageView alloc] initWithImage:image]; [self.view addSubview:imageView]; CGPoint tappedPos = [gesture locationInView:gesture.view]; imageView.center = tappedPos; [self.gravityBeahvior addItem:imageView]; [self.collisionBehavior addItem:imageView]; [self.itemBehavior addItem:imageView];} |
另外，UICollisionBehavior有它的代理，其中列举两个方法，它们表示行为开始和结束的时候的代理。
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- (void)collisionBehavior:(UICollisionBehavior*)behavior beganContactForItem:(id <UIDynamicItem>)item withBoundaryIdentifier:(id <NSCopying>)identifier atPoint:(CGPoint)p;- (void)collisionBehavior:(UICollisionBehavior*)behavior endedContactForItem:(id <UIDynamicItem>)item withBoundaryIdentifier:(id <NSCopying>)identifier; |
UISnapBehavior（捕捉）
UISnapBehavior 将UIView通过动画吸附到某个点上。
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- (void) handleTap:(UITapGestureRecognizer *)paramTap{ CGPoint tapPoint = [paramTap locationInView:self.view]; if (self.snapBehavior != nil){ [self.animator removeBehavior:self.snapBehavior]; } self.snapBehavior = [[UISnapBehavior alloc] initWithItem:self.squareView snapToPoint:tapPoint]; self.snapBehavior.damping = 0.5f; //剧列程度 [self.animator addBehavior:self.snapBehavior];} |
UICollectionView与UIDynamicAnimator
文章开头说到UIDynamicAnimator有两种初始化方法，这里介绍它与UICollectionView的完美结合，让UICollectionView产生各种动态特性的行为。
你是否记得iOS系统中信息应用中的附有弹性的消息列表，他就是加入了UIAttachmentBehavior吸附行为，这里通过一个UICollectionView实现类似效果。
主要是复写UICollectionViewFlowLayout，在layout中为每一个布局属性元素加上吸附行为就可以了。
关于复写layout，可以参考onevcat的博客
http://www.onevcat.com/2012/08/advanced-collection-view/
下面就直接上代码了
首先遍历每个 collection view layout attribute 来创建和添加新的 dynamic animator
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-(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 *attachment = [[UIAttachmentBehavior alloc] initWithItem:item attachedToAnchor:item.center]; attachment.length = 0; attachment.damping = self.damping; attachment.frequency = self.frequency; [_animator addBehavior:attachment]; } }} |
接下来我们现在需要实现 layoutAttributesForElementsInRect: 和 layoutAttributesForItemAtIndexPath: 这两个方法，UIKit 会调用它们来询问 collection view 每一个 item 的布局信息。我们写的代码会把这些查询交给专门做这些事的dynamic animator
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-(NSArray *)layoutAttributesForElementsInRect:(CGRect)rect { return [_animator itemsInRect:rect];} - (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath { return [_animator layoutAttributesForCellAtIndexPath:indexPath];} |
然后是响应滚动事件的方法
这个方法会在 collection view 的 bound 发生改变的时候被调用，根据最新的 content offset 调整我们的 dynamic animator 中的 behaviors 的参数。在重新调整这些 behavior 的 item 之后，我们在这个方法中返回 NO；因为 dynamic animator 会关心 layout 的无效问题，所以在这种情况下，它不需要去主动使其无效
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-(BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds { UIScrollView *scrollView = self.collectionView; CGFloat scrollDelta = newBounds.origin.y - scrollView.bounds.origin.y; NSLog(@" %f %f",newBounds.origin.y,scrollView.bounds.origin.y); CGPoint touchLocation = [scrollView.panGestureRecognizer locationInView:scrollView]; for (UIAttachmentBehavior *behavior in _animator.behaviors) { CGPoint anchorPoint = behavior.anchorPoint; CGFloat distanceFromTouch = fabsf(touchLocation.y - anchorPoint.y); CGFloat scrollResistance = distanceFromTouch / self.resistanceFactor; UICollectionViewLayoutAttributes *item = [behavior.items firstObject]; CGPoint center = item.center; center.y += (scrollDelta > 0) ? MIN(scrollDelta, scrollDelta * scrollResistance) : MAX(scrollDelta, scrollDelta * scrollResistance); item.center = center; [_animator updateItemUsingCurrentState:item]; } return NO;} |
让我们仔细查看这个代码的细节。首先我们得到了这个 scroll view（就是我们的 collection view ），然后计算它的 content offset 中 y 的变化（在这个例子中，我们的 collection view 是垂直滑动的）。一旦我们得到这个增量，我们需要得到用户接触的位置。这是非常重要的，因为我们希望离接触位置比较近的那些物体能移动地更迅速些，而离接触位置比较远的那些物体则应该滞后些。
对于 dynamic animator 中的每个 behavior，我们将接触点到该 behavior 物体的 y 的距离除以 500。分母越小，这个 collection view 的的交互就越有弹簧的感觉。