仅当函数完成创建它们时才从视频中检索 UIImage - swift - swift 以编程方式

Question

我正在使用 generateCGImagesAsynchronouslyForTimes 从视频创建 UIImages :

fileprivate func createImageFramesFromVideo(completion: @escaping ([UIImage]) -> Void) {
    guard let url = self.videoUrl else {
        return
    }
    
    let asset = AVAsset(url: url)
    let imageGenerator = AVAssetImageGenerator(asset: asset)
    let videoDuration = asset.duration
    
    //getting image snapshots more or less in half the video
    let time1 = CMTime(value: videoDuration.value/2 - videoDuration.value/10, timescale: videoDuration.timescale)
    let time2 = CMTime(value: videoDuration.value/2 - videoDuration.value/11, timescale: videoDuration.timescale)
    let time3 = CMTime(value: videoDuration.value/2 - videoDuration.value/12, timescale: videoDuration.timescale)
    let time4 = CMTime(value: videoDuration.value/2, timescale: videoDuration.timescale)
    let time5 = CMTime(value: videoDuration.value/2 + videoDuration.value/12, timescale: videoDuration.timescale)
    let time6 = CMTime(value: videoDuration.value/2 + videoDuration.value/11, timescale: videoDuration.timescale)
    let time7 = CMTime(value: videoDuration.value/2 + videoDuration.value/10, timescale: videoDuration.timescale)
    
    var imageArray : [UIImage] = []
    
    imageGenerator.generateCGImagesAsynchronously(forTimes: [NSValue(time: time1), NSValue(time: time2), NSValue(time: time3), NSValue(time: time4), NSValue(time: time5), NSValue(time: time6), NSValue(time: time7)]) { (time, image, time1, result, err) in
        if let err = err {
            print("there's an error in retrieving the images", err)
        }
       
        let theImage = UIImage(cgImage: image!)
        
        imageArray.append(theImage)
        
        if(result == .succeeded){
            completion(imageArray)
        }
    }
}

由于函数的完成处理程序被调用每次创建一个新图像，completion()被调用多次。

我想实现 completion() 仅在创建所有图像（在本例中为 7 个）后调用。我该怎么做？

Answer 1

您需要使用一个DispatchGroup来等待多个异步函数完成。您需要确保对 DispatchGroup.enter() 和 leave() 的调用是平衡的（调用相同的次数），因为当您使用 dispatchGroup.notify(queue:) 时，闭包只会执行一次 leave() 已为每个 enter().

调用

您需要为 times 数组的每个元素调用 enter（它将包含 CMTime 变量 time1 到 time7，然后在generateCGImagesAsynchronously(forTimes: 的完成块，你调用 leave()。这确保了 notify(queue:) 只会在 generateCGImagesAsynchronously 为所有图像调用它自己的完成后才执行它的关闭。

fileprivate func createImageFramesFromVideo(completion: @escaping ([UIImage]) -> Void) {
    guard let url = self.videoUrl else { return }

    let asset = AVAsset(url: url)
    let imageGenerator = AVAssetImageGenerator(asset: asset)
    let videoDuration = asset.duration

    //getting image snapshots more or less in half the video
    let time1 = CMTime(value: videoDuration.value/2 - videoDuration.value/10, timescale: videoDuration.timescale)
    let time2 = CMTime(value: videoDuration.value/2 - videoDuration.value/11, timescale: videoDuration.timescale)
    let time3 = CMTime(value: videoDuration.value/2 - videoDuration.value/12, timescale: videoDuration.timescale)
    let time4 = CMTime(value: videoDuration.value/2, timescale: videoDuration.timescale)
    let time5 = CMTime(value: videoDuration.value/2 + videoDuration.value/12, timescale: videoDuration.timescale)
    let time6 = CMTime(value: videoDuration.value/2 + videoDuration.value/11, timescale: videoDuration.timescale)
    let time7 = CMTime(value: videoDuration.value/2 + videoDuration.value/10, timescale: videoDuration.timescale)
    let times = [NSValue(time: time1), NSValue(time: time2), NSValue(time: time3), NSValue(time: time4), NSValue(time: time5), NSValue(time: time6), NSValue(time: time7)]

    var imageArray : [UIImage] = []

    let dispatchGroup = DispatchGroup()
    times.forEach { _ in
        dispatchGroup.enter()
    }


    imageGenerator.generateCGImagesAsynchronously(forTimes: times) { (time, image, time1, result, err) in
        if let err = err {
            print("there's an error in retrieving the images", err)
        }

        let theImage = UIImage(cgImage: image!)

        imageArray.append(theImage)
        dispatchGroup.leave()
    }

    dispatchGroup.notify(queue: .main) {
        completion(imageArray)
    }
}

仅当函数完成创建它们时才从视频中检索 UIImage - swift - swift 以编程方式

Retrieving UIImage from video only when function completed creating them - swift - swift programmatically

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