ios - 循环的 Swift 4 完成处理程序不起作用

Question

在 swift 4 中，我有这个方法，它使用完成处理程序进行循环:

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)        
    }

这里是循环完成时它应该调用的方法:

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

            print(result)

        }
    }

这就是我调用循环方法的方式:

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

我的问题是在循环完成之前调用了完成处理程序方法 (loopFinished)。

如果您想知道，fetchOriginalImage 来自库 DKImagePickerController https://github.com/zhangao0086/DKImagePickerController

Answer 1

您需要一个 DispatchGroup 来完成多个非连续任务 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)
    }

}