semaphore_loop.swift

let writerReadySemaphore = DispatchSemaphore(value: 0)
let frameWrittenSemaphore = DispatchSemaphore(value: 0)
let isFinished = Atomic<Bool>(false)
let queue = DispatchQueue(label: "gif_convertor")
videoWriterInput.requestMediaDataWhenReady(on: queue) {
    while !isFinished.load(ordering: .sequentiallyConsistent),
            videoWriterInput.isReadyForMoreMediaData
    {
        writerReadySemaphore.signal()
        frameWrittenSemaphore.wait()
    }
}

// ...

while index < gif.frameCount {
    writerReadySemaphore.wait()

    // ...write next frame

    index+=1
    if index == gif.frameCount {
        isFinished.store(true, ordering: .sequentiallyConsistent)
    }

    frameWrittenSemaphore.signal()
}