带计时器的绘图应用程序。绘图不到 20 秒后计时器开始滞后

Question

我在一些朋友的帮助下制作了这个应用程序。我真的不知道代码是如何工作的。

它基本上使用苹果铅笔记录数据（平板电脑上的时间、苹果铅笔的速度、笔划数等）。然而，随着时间的流逝和绘图的增多，计时器与实际时间不同步。

这个应用程序的目的是用于痴呆症研究，我让患者在平板电脑上画画，然后我收集相关信息。如果计时器发臭，我就无法进行研究了。

我试过禁用所有计时器，但延迟仍然一样。我有一种感觉，它与笔画的采样方式有关。我只需要一个笔画计数，我不需要它来显示每分钟的笔画（这是它目前正在做的）。我认为中风计数器可能是原因？？？

这是程序： https://drive.google.com/open?id=1lwzKwG7NLcX1qmE5yoxsdq5HICV2TNHm

class StrokeSegment {
    var sampleBefore: StrokeSample?
    var fromSample: StrokeSample!
    var toSample: StrokeSample!
    var sampleAfter: StrokeSample?
    var fromSampleIndex: Int

    var segmentUnitNormal: CGVector {
        return segmentStrokeVector.normal!.normalized!
    }

    var fromSampleUnitNormal: CGVector {
        return interpolatedNormalUnitVector(between: previousSegmentStrokeVector, and: segmentStrokeVector)
    }

    var toSampleUnitNormal: CGVector {
        return interpolatedNormalUnitVector(between: segmentStrokeVector, and: nextSegmentStrokeVector)
    }

    var previousSegmentStrokeVector: CGVector {
        if let sampleBefore = self.sampleBefore {
            return fromSample.location - sampleBefore.location
        } else {
            return segmentStrokeVector
        }
    }

    var segmentStrokeVector: CGVector {
        return toSample.location - fromSample.location
    }

    var nextSegmentStrokeVector: CGVector {
        if let sampleAfter = self.sampleAfter {
            return sampleAfter.location - toSample.location
        } else {
            return segmentStrokeVector
        }
    }

    init(sample: StrokeSample) {
        self.sampleAfter = sample
        self.fromSampleIndex = -2
    }

    @discardableResult
    func advanceWithSample(incomingSample: StrokeSample?) -> Bool {
        if let sampleAfter = self.sampleAfter {
            self.sampleBefore = fromSample
            self.fromSample = toSample
            self.toSample = sampleAfter
            self.sampleAfter = incomingSample
            self.fromSampleIndex += 1
            return true
        }
        return false
    }
}

class StrokeSegmentIterator: IteratorProtocol {
    private let stroke: Stroke
    private var nextIndex: Int
    private let sampleCount: Int
    private let predictedSampleCount: Int
    private var segment: StrokeSegment!

    init(stroke: Stroke) {
        self.stroke = stroke
        nextIndex = 1
        sampleCount = stroke.samples.count
        predictedSampleCount = stroke.predictedSamples.count
        if (predictedSampleCount + sampleCount) > 1 {
            segment = StrokeSegment(sample: sampleAt(0)!)
            segment.advanceWithSample(incomingSample: sampleAt(1))
        }
    }

    func sampleAt(_ index: Int) -> StrokeSample? {
        if index < sampleCount {
            return stroke.samples[index]
        }
        let predictedIndex = index - sampleCount
        if predictedIndex < predictedSampleCount {
            return stroke.predictedSamples[predictedIndex]
        } else {
            return nil
        }
    }

    func next() -> StrokeSegment? {
        nextIndex += 1
        if let segment = self.segment {
            if segment.advanceWithSample(incomingSample: sampleAt(nextIndex)) {
                return segment
            }
        }
        return nil
    }
}

例如 true 25 秒，应用程序显示总时间为 20 秒。

Answer 1

A Timer 不是计算经过时间的东西。它是一种用于在经过一段时间后触发执行的工具。但是只是 "after" 过去了一段时间，而不是 "exactly after" 过去了一段时间。因此，例如做类似的事情：

var secondsElapsed: TimeInterval = 0.0
let timeInitiated = Date()

Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { _ in
    secondsElapsed += 1
    print("\(secondsElapsed) seconds should have passed but in reality \(Date().timeIntervalSince(timeInitiated)) second have passed")
}

你会发现两者并不相同，但非常接近。但是一旦我像这样添加一些额外的工作：

var secondsElapsed：TimeInterval = 0.0 让 timeInitiated = Date()

func countTo(_ end: Int) {
    var string = ""
    for i in 1...end {
        string += String(i)
    }
    print("Just counted to string of lenght \(string.count)")
}

Timer.scheduledTimer(withTimeInterval: 1.0/60.0, repeats: true) { _ in
    countTo(100000)
}

Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { _ in
    secondsElapsed += 1
    print("\(secondsElapsed) seconds should have passed but in reality \(Date().timeIntervalSince(timeInitiated)) second have passed")
}

我们遇到类似“14.0 秒应该过去但实际上已经过去 19.17617702484131 秒”的情况。

我们让应用程序很忙，所以它没有时间正确计数。

在您的情况下，您需要使用以下两种解决方案之一：

如果您对经过的时间感兴趣，只需使用 timeIntervalSince，如第一个代码片段中所示。
如果你需要确保每 N 秒触发一次，你应该优化你的代码，考虑多线程......但主要记住你只能接近 "every N seconds"，它不应该可以保证每 N 秒执行一次。

带计时器的绘图应用程序。绘图不到 20 秒后计时器开始滞后

Drawing app with timer. Timers begins to lag after drawing for less than 20 seconds

ios

swift

ipad