UIKit

animator = UIDynamicAnimator(referenceView: view)

let gravity   = UIGravityBehavior(items: [card])
let collision = UICollisionBehavior(items: [card])
collision.translatesReferenceBoundsIntoBoundary = true
let item      = UIDynamicItemBehavior(items: [card])
item.elasticity = 0.6

[gravity, collision, item].forEach { animator.addBehavior($0) }

let gravity = UIGravityBehavior(items: cards)

motionManager.startDeviceMotionUpdates(to: .main) { motion, _ in
    guard let m = motion else { return }
    // 화면 좌표계로 변환된 중력 벡터
    let v = OrientationAwareGravity.uiKitVector(
        deviceX: m.gravity.x, deviceY: m.gravity.y
    )
    gravity.gravityDirection = CGVector(dx: v.dx * 2, dy: v.dy * 2)
}

@objc func handlePan(_ gesture: UIPanGestureRecognizer) {
    guard let item = gesture.view else { return }
    switch gesture.state {
    case .began:
        // 드래그 중에는 snap 제거
        if let snap = snapBehaviors[item] { animator?.removeBehavior(snap) }
    case .changed:
        item.center = gesture.location(in: containerView)
        animator?.updateItem(usingCurrentState: item)
    case .ended:
        // 가장 가까운 그리드 포인트로 snap
        let nearest = nearestGridPoint(to: item.center)
        let snap = UISnapBehavior(item: item, snapTo: nearest)
        snap.damping = currentDamping
        animator?.addBehavior(snap)
        snapBehaviors[item] = snap
    default: break
    }
}

let collision = UICollisionBehavior(items: bubbles)
collision.translatesReferenceBoundsIntoBoundary = true

let item = UIDynamicItemBehavior(items: bubbles)
item.elasticity = 0.6
item.allowsRotation = false   // 글자 똑바로 유지

// 탭 시 주변으로 밀어내기
let push = UIPushBehavior(items: [tappedBubble], mode: .instantaneous)
push.angle = .random(in: 0..<2 * .pi)
push.magnitude = 1.0
animator?.addBehavior(push)

for ball in balls {
    let attachment = UIAttachmentBehavior(
        item: ball,
        attachedToAnchor: anchorAbove(ball)
    )
    attachment.length = stringLength
    attachment.damping = 0      // 진자 운동 무손실
    animator?.addBehavior(attachment)
}

// 모든 공이 서로 충돌
let collision = UICollisionBehavior(items: balls)
let item = UIDynamicItemBehavior(items: balls)
item.elasticity = 1.0           // 완전 탄성 충돌
animator?.addBehavior(collision)
animator?.addBehavior(item)

// 각 메뉴 아이템을 자기의 "고정 위치"에 스프링으로 매닮
for (i, item) in items.enumerated() {
    let anchor = idealPosition(for: i)
    let spring = UIAttachmentBehavior(item: item, attachedToAnchor: anchor)
    spring.length = 0
    spring.damping = 0.5
    spring.frequency = 3.0
    animator?.addBehavior(spring)
}

// 인접 아이템끼리도 약한 스프링으로 연결
for i in 0..<(items.count - 1) {
    let chain = UIAttachmentBehavior(item: items[i], attachedTo: items[i+1])
    chain.damping = 0.5
    chain.frequency = 2.0
    animator?.addBehavior(chain)
}