核心音频渲染回调中的时间戳可以为null吗？

Question

我有一个iOS锻炼应用，需要在后台保持运行（和播放音频）。可悲的是，我知道，实现此目标的唯一方法是通过Core Audio播放听不见的声音。

一个用户遇到了重复发生的崩溃，我无法在模拟器中重现。 （他们有iPhone 5，iOS10。）Xcode中的崩溃报告指向了我从未见过的崩溃行。 timestamp在渲染回调中可以为空吗？

private func render(ctx: UnsafeMutableRawPointer,
                    ioActionFlags: UnsafeMutablePointer<AudioUnitRenderActionFlags>,
                    timeStamp: UnsafePointer<AudioTimeStamp>,
                    bus: UInt32,
                    nFrames: UInt32,
                    ioData: UnsafeMutablePointer<AudioBufferList>?) -> OSStatus
{
    guard let ioData = ioData else { return noErr }
    let abl = UnsafeMutableAudioBufferListPointer(ioData)
    guard let bufferData = abl[0].mData else { return noErr }

    let impl = Unmanaged<MyClass>.fromOpaque(ctx).takeUnretainedValue()

    let buf: UnsafeMutablePointer<Float32> = bufferData.assumingMemoryBound(to: Float32.self)
    // **** Crash appears to pointing to this next line. 
    var t: Int = Int(timeStamp.pointee.mSampleTime) % impl.samples.count
    for i in 0..<Int(nFrames) {
        buf[i] = impl.samples[t]
        t += 1
        if t >= impl.samples.count { t = 0 }
    }
    ...