I've built upon what's already in these answers and made a generic manual animator, as everything can be distilled down to a percentage float value and a block.
class ManualAnimator {
enum AnimationCurve {
case linear, parametric, easeInOut, easeIn, easeOut
func modify(_ x: CGFloat) -> CGFloat {
switch self {
case .linear:
return x
case .parametric:
return x.parametric
case .easeInOut:
return x.quadraticEaseInOut
case .easeIn:
return x.quadraticEaseIn
case .easeOut:
return x.quadraticEaseOut
}
}
}
private var displayLink: CADisplayLink?
private var start = Date()
private var total = TimeInterval(0)
private var closure: ((CGFloat) -> Void)?
private var animationCurve: AnimationCurve = .linear
func animate(duration: TimeInterval, curve: AnimationCurve = .linear, _ animations: @escaping (CGFloat) -> Void) {
guard duration > 0 else { animations(1.0); return }
reset()
start = Date()
closure = animations
total = duration
animationCurve = curve
let d = CADisplayLink(target: self, selector: #selector(tick))
d.add(to: .current, forMode: .common)
displayLink = d
}
@objc private func tick() {
let delta = Date().timeIntervalSince(start)
var percentage = animationCurve.modify(CGFloat(delta) / CGFloat(total))
//print("%:", percentage)
if percentage < 0.0 { percentage = 0.0 }
else if percentage >= 1.0 { percentage = 1.0; reset() }
closure?(percentage)
}
private func reset() {
displayLink?.invalidate()
displayLink = nil
}
}
extension CGFloat {
fileprivate var parametric: CGFloat {
guard self > 0.0 else { return 0.0 }
guard self < 1.0 else { return 1.0 }
return ((self * self) / (2.0 * ((self * self) - self) + 1.0))
}
fileprivate var quadraticEaseInOut: CGFloat {
guard self > 0.0 else { return 0.0 }
guard self < 1.0 else { return 1.0 }
if self < 0.5 { return 2 * self * self }
return (-2 * self * self) + (4 * self) - 1
}
fileprivate var quadraticEaseOut: CGFloat {
guard self > 0.0 else { return 0.0 }
guard self < 1.0 else { return 1.0 }
return -self * (self - 2)
}
fileprivate var quadraticEaseIn: CGFloat {
guard self > 0.0 else { return 0.0 }
guard self < 1.0 else { return 1.0 }
return self * self
}
}
Implementation
let initialOffset = collectionView.contentOffset.y
let delta = collectionView.bounds.size.height
let animator = ManualAnimator()
animator.animate(duration: TimeInterval(1.0), curve: .easeInOut) { [weak self] (percentage) in
guard let `self` = self else { return }
self.collectionView.contentOffset = CGPoint(x: 0.0, y: initialOffset + (delta * percentage))
if percentage == 1.0 { print("Done") }
}
It might be worth combining the animate function with an init method.. it's not a huge deal though.