循环的Swift 4完成处理程序不起作用

Question

In swift 4 I have this method that does a loop with a completion handler:

func loopImages(qa: QAClass, assets: [DKAsset], completion: ([PhotoClass]) -> Void) {

        var imageCellHolder = [PhotoClass]()

            for x in assets
            {

                x.fetchOriginalImage(completeBlock: { (image, info) in

                     let compressedImage = image?.resizeWithWidth(width: 800)

                     let imageData:Data = compressedImage!.jpegData(compressionQuality: 0.5)!

                     imageCellHolder.append(PhotoClass(job: String(self.selectedCommunity! + self.selectedLot!), photo: imageData, itemId: qa.itemId))

                 })
            }

            completion(imageCellHolder)        
    }

And here is the method its suppose to call when the loop is done:

func loopFinished(imageCellHolder: [PhotoClass])
    {
        self.saveQAPhotos(cellHolder: imageCellHolder) { result in

            print(result)

        }
    }

And this is how I am calling the loop method:

self.loopImages(qa: qa, assets: assets, completion: self.loopFinished)

My problem is the completion handler method (loopFinished) is called before the loop is finished.

In case you are wondering, fetchOriginalImage is from the library DKImagePickerController https://github.com/zhangao0086/DKImagePickerController

Answer 1

You need a DispatchGroup for multiple asunchounous tasks finish completion

func loopImages(qa: QAClass, assets: [DKAsset], completion: ([PhotoClass]) -> Void) {

    var imageCellHolder = [PhotoClass]()

    let dispa = DispatchGroup()

    for x in assets
    {
        dispa.enter()

        x.fetchOriginalImage(completeBlock: { (image, info) in

            let compressedImage = image?.resizeWithWidth(width: 800)

            let imageData:Data = compressedImage!.jpegData(compressionQuality: 0.5)!

            imageCellHolder.append(PhotoClass(job: String(self.selectedCommunity! + self.selectedLot!), photo: imageData, itemId: qa.itemId))

            dispa.leave()

        })
    }

    dispa.notify(queue: .main) {
         completion(imageCellHolder)
    }

}

Answer 2

The reason for this is that fetching the images takes time, the actual for loop completes very quickly and returns an empty array because the images haven't been fetched yet.

Consider using a DispatchGroup

func loopImages(qa: QAClass, assets: [DKAsset], completion: ([PhotoClass]) -> Void) {

    var imageCellHolder = [PhotoClass]()
    let group = DispatchGroup()

    for x in assets
    {
        group.enter()
        x.fetchOriginalImage(completeBlock: { (image, info) in

            let compressedImage = image?.resizeWithWidth(width: 800)

            let imageData:Data = compressedImage!.jpegData(compressionQuality: 0.5)!

            imageCellHolder.append(PhotoClass(job: String(self.selectedCommunity! + self.selectedLot!), photo: imageData, itemId: qa.itemId))
            group.leave()

        })

        group.notify(queue: .main) {
            completion(imageCellHolder)
        }
    }
}

The DispatchGroup will wait for all images to finish downloading and will call the completion when they finish

Answer 3

Your tasks are asynchronous. That means, you don't have data immediately after you request them. You need to wait then every image is fetched.

You can avoid using DispatchGroup and you can track if every image has been fetched by increasing some count variable. After count of fetched images is the same as count of your assets array, you can call completion

func loopImages(qa: QAClass, assets: [DKAsset], completion: ([PhotoClass]) -> Void) {

    var imageCellHolder = [PhotoClass]()

    var count = 0 // <--

    for x in assets {

        x.fetchOriginalImage { image, info in

            let compressedImage = image?.resizeWithWidth(width: 800)

            let imageData:Data = compressedImage!.jpegData(compressionQuality: 0.5)!

            imageCellHolder.append(PhotoClass(job: String(self.selectedCommunity! + self.selectedLot!), photo: imageData, itemId: qa.itemId))

  /* -> */  count += 1
            if count == assets.count {
                completion(imageCellHolder)
            }

        }
    }

}

Answer 4

这是因为fetchOriginalImage是异步的，您应该使用DispatchGroup之类的东西来跟踪所有异步调用何时完成