SlowFrameMetric.swift

import Foundation
import XCTest
import QuartzCore

// 1.
@available(iOS 11.2.0, *)
class SlowFrameMetric: NSObject, XCTMetric {
    private static let slowFrameThresholdInMiliSecond = 0.017
    private let frameTimeintervalCollector = FrameTimeintervalCollector()
    
    func reportMeasurements(from startTime: XCTPerformanceMeasurementTimestamp,
                            to endTime: XCTPerformanceMeasurementTimestamp) throws -> [XCTPerformanceMeasurement] {

        // Note: When setting invocationOptions of XCTMeasureOptions in test cases,
        // the `startTime` and the `endTime` would be nil.
        let filteredFrameTickTimes: Set<FrameTickTime>
        let frameTickTimeSet = frameTimeintervalCollector.frameTickTimeSet
        if startTime == nil || endTime == nil { //< 2.
            filteredFrameTickTimes = frameTickTimeSet
            frameTimeintervalCollector.reset() //< 3.
        } else {
            // 4.
            filteredFrameTickTimes = frameTickTimeSet.filter({ frameTickTime -> Bool in
                endTime.date >= frameTickTime.date && frameTickTime.date >= startTime.date
            })
        }

        guard !filteredFrameTickTimes.isEmpty else { throw FPSMetricError.frameTickTimeIsEmpty }

        // 5.
        let slowFrameTickTimes = filteredFrameTickTimes.filter {
            $0.frameTimeinterval > SlowFrameMetric.slowFrameThresholdInMiliSecond
        }
        
        let slowFramePrecentage = Double(slowFrameTickTimes.count) / Double(filteredFrameTickTimes.count)

        return [XCTPerformanceMeasurement(identifier: "com.grindrguy.grindrx_SlowFrameMetric",
                                          displayName: "Slow Frame",
                                          doubleValue: slowFramePrecentage,
                                          unitSymbol: "%")]
    }
    
    //Mark - Optional functions

    func didStopMeasuring() {
        frameTimeintervalCollector.displayLink.isPaused = true
    }

    func willBeginMeasuring() {
        frameTimeintervalCollector.displayLink.isPaused = false
    }
}

extension SlowFrameMetric: NSCopying {
    func copy(with zone: NSZone? = nil) -> Any {
        return SlowFrameMetric()
    }
}