Is there any waitUntil operator or something similar in RXSwift to wait a value?

Question

I am using RXSwift to handle with a lot of async AVAssetWriterInput write operations, but I need to wait for isReadyForMoreMediaData before writing more buffers inside the file, how can I handle this?

Basically, the observable receives a lot of buffers emitted by the asyncWriterSubject and I want to write all of them in the order that I am receiving.

I have this subject:

private var asyncWriter = ReplaySubject<(AVAssetWriterInput,CMSampleBuffer)>.create(bufferSize: 1)

I emit the values for it using this code:

asyncWriter.onNext((videoWriterInput, buffer))

And I am subscribing it here to listen:

disposable = asyncWriter.asObservable()
    .takeWhile {
        (writerPointer, _) in
            writerPointer.isReadyForMoreMediaData
        }.observeOn(MainScheduler.asyncInstance)
        .subscribe(onNext: { (writerPointer, buffer) in
            writerPointer.append(buffer)
    })

Don't know about RxSwift, but `AVAssetWriterInput ` itself has [requestMediaDataWhenReady(on:using:)](https://developer.apple.com/documentation/avfoundation/avassetwriterinput/1387508-requestmediadatawhenready) which is exactly for cases where you need to wait for `isReadyForMoreMediaData` turning from false to true one or multiple times... — timbre timbre, Jun 22 '22 at 23:08

Daniel T. · Accepted Answer · 2022-06-22T23:51:21.200

Here's some general information on how to handle back pressure.

This will write a CMSampleBuffer to the writerPointer up to 100 times per second. When isReadyForMoreMediaData is false, it will store sample buffers until the Bool is true again.

func example(asyncWriter: Observable<CMSampleBuffer>, writerPointer: AVAssetWriterInput) -> Disposable {
    enum Action {
        case buffer(CMSampleBuffer)
        case isReady(Bool)
    }
    var isReadyForMoreMediaData: Observable<Bool> {
        Observable<Int>.interval(.milliseconds(10), scheduler: MainScheduler.instance)
            .flatMap { [writerPointer] _ in Observable.just(writerPointer.isReadyForMoreMediaData) }
    }
    return Observable.merge(
        isReadyForMoreMediaData.map { Action.isReady($0) },
        asyncWriter.map { Action.buffer($0) }
    )
        .scan(into: (buffer: [CMSampleBuffer](), trigger: false, emission: CMSampleBuffer?.none), accumulator: { current, new in
            switch new {
            case let .buffer(buff):
                if current.trigger {
                    if current.buffer.isEmpty {
                        current.emission = buff
                    }
                    else {
                        current.emission = current.buffer[0]
                        current.buffer.removeFirst()
                        current.buffer.append(buff)
                    }
                }
                else {
                    current.buffer.append(buff)
                }
            case let .isReady(trig):
                current.trigger = trig
                if trig && !current.buffer.isEmpty {
                    current.emission = current.buffer[0]
                    current.buffer.removeFirst()
                }
            }
        })
        .compactMap { $0.emission }
        .observe(on: MainScheduler.instance)
        .subscribe(onNext: { buffer in
            writerPointer.append(buffer)
        })
}

Is there any waitUntil operator or something similar in RXSwift to wait a value?

1 Answers1