FrameTimeintervalCollector.swift

import Foundation

class FrameTimeintervalCollector {
    private(set) lazy var displayLink: CADisplayLink = { //< 1.
        let displayLink = CADisplayLink(target: self, selector: #selector(tick(displayLink:)))
        displayLink.add(to: .main, forMode: .default)
        return displayLink
    }()
    private(set) var frameTickTimeSet = Set<FrameTickTime>() //< 2.
    private var lastTimestamp: TimeInterval = 0
    private let serialQueue = DispatchQueue(label: "frameTickQueue")
    
    init() {
        displayLink.isPaused = false
    }

    @objc private func tick(displayLink: CADisplayLink) {
        serialQueue.async { //< 3.
            guard self.lastTimestamp != 0 else {
                self.lastTimestamp = displayLink.timestamp
                return
            }

            let frameTimeinterval = displayLink.timestamp - self.lastTimestamp
            self.lastTimestamp = displayLink.timestamp
            self.frameTickTimeSet.insert(FrameTickTime(frameTimeinterval: frameTimeinterval))
        }
    }
    
    func reset() {
        frameTickTimeSet.removeAll()
    }
}

struct FrameTickTime: Hashable {
    let frameTimeinterval: TimeInterval
    let date: Date = Date()
}

enum FPSMetricError: Error {
    case frameTickTimeIsEmpty
}