AVAssetWriterInput和readyForMoreMediaData

Question

AVAssetWriterInput的readyForMoreMediaData是否在后台线程中更新？如果readyForMoreMediaData为NO，我是否可以在主线程中阻塞并等待，直到值变为YES？

我通过向AVAssetWriterInput推送数据来使用它(即不使用requestMediaDataWhenReadyOnQueue)，并且我已经设置了expectsMediaDataInRealTime，99.9%的时间我可以在它上面调用appendSampleBuffer (或appendPixelBuffer)，就像我的应用程序生成框架一样快。

除非您在AVAssetWriter会话过程中将设备(iPhone 3GS)设置为休眠15分钟左右，否则这种方法工作得很好。在唤醒设备后，appendPixelBuffer有时会收到一个错误消息，“当readyForMoreMediaData为NO时，无法追加像素缓冲区”。因此我的问题是-如何最好地响应readyForMoreMediaData=NO，如果我可以像这样在主线程中等待一小段时间：

while ( ![assetWriterInput readyForMoreMediaData] )
{
    Sleep for a few milliseconds
}

Answer 1

注意不要只是阻塞线程，这是我在没有工作之前所做的事情：

while (adaptor.assetWriterInput.readyForMoreMediaData == FALSE) {
  [NSThread sleepForTimeInterval:0.1];
}

在我的iPad2上，上述方法有时会失败。这样做反而解决了问题：

while (adaptor.assetWriterInput.readyForMoreMediaData == FALSE) {
  NSDate *maxDate = [NSDate dateWithTimeIntervalSinceNow:0.1];
  [[NSRunLoop currentRunLoop] runUntilDate:maxDate];
}

Answer 2

找不到类似的东西，所以我把它留在了这里。Swift 4解决方案。最好使用精确的技术来解决这个问题。F.e.使用NSCondition：

func startRecording() {
        // start recording code goes here 

    readyForMediaCondition = NSCondition()
    readyForMediaObservation = pixelBufferInput?.assetWriterInput.observe(\.isReadyForMoreMediaData, options: .new, changeHandler: { [weak self](_, change) in
        guard let isReady = change.newValue else {
            return
        }

        if isReady {
            self?.readyForMediaCondition?.lock()
            self?.readyForMediaCondition?.signal()
            self?.readyForMediaCondition?.unlock()
        }
    })
}

接下来：

func grabFrame(time: CMTime? = nil) {
    readyForMediaCondition?.lock()
    while !pixelBufferInput!.assetWriterInput.isReadyForMoreMediaData {
        readyForMediaCondition?.wait()
    }
    readyForMediaCondition?.unlock()

    // append your framebuffer here
}

最后别忘了让观察者失效

readyForMediaObservation?.invalidate()

Answer 3

        int waitTime = 300;
        while (weakSelf.input.readyForMoreMediaData == NO) {
            NSLog(@"readyForMoreMediaData is NO");
            NSTimeInterval waitIntervale = 0.001 * waitTime;
            NSDate *maxDate = [NSDate dateWithTimeIntervalSinceNow:waitIntervale];
            [[NSRunLoop currentRunLoop] runUntilDate:maxDate];
            waitTime += 200; // add 200ms every time
        }