ManualAnimator.swift

import UIKit
import Foundation

// https://github.com/ekazaev/ChatLayout

final 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))
        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
    }
}