Swift（iOS），等待所有图像完成下载后再返回 [英] Swift (iOS), waiting for all images to finish downloading before returning
问题描述
我正在编写一个Swift iOS应用程序（首先，请耐心等待），在这里我使用Swifter HTTP服务器处理各种请求。这样的请求之一就是带有JSON数组的HTTP POST，该JSON数组指定要从网络下载的图像（并执行其他操作，与当前问题无关）。
I am writing a Swift iOS app (my first, so please bear with me) where I use Swifter HTTP server to process various requests. One such request is an HTTP POST with a JSON array specifying images to download from the web (and do some other stuff, not pertinent to the issue at hand).
I使用Alamofire下载图片（效果很好），但是我正在寻找一种好的（最好是简单的）方式来等待所有图片完成下载，然后返回对以上POST请求的响应（因为该响应必须包含指示JSON的信息）
I use Alamofire to download the images (this works fine), but I am looking for good (preferably simple) way to wait for all the images to finish downloading before returning a response to the POST request above (since the response has to contain JSON indicating the result, including any failed downloads).
什么是完成此操作的好方法（最好不阻塞主线程）？
What is a good way to accomplish this (preferably w/o blocking the main thread)?
下面是一些片段来说明：
Here are some snippets to illustrate:
public func webServer(publicDir: String?) -> HttpServer {
let server = HttpServer()
server.POST["/images/update"] = { r in
let images = ...(from JSON array in body)
let updateResult = ImageUtil.updateImages(images)
let resultJson: String = Mapper().toJSONString(updateResult, prettyPrint: true)!
if updateResult.success {
return .OK(.Text(resultJson))
}
return HttpResponse.RAW(500, "Error", nil, { $0.write([UInt8](updateResult.errorMessage.utf8)) })
}
}
static func updateImages(images: [ImageInfo]) -> UpdateResult {
let updateResult = UpdateResult()
for image in images {
Alamofire.download(.GET, serverFile.imageUrl) { temporaryURL, response in return destinationPath }
.validate()
.response{_, _, _, error in
if let error = error {
Log.error?.message("Error downloading file \(image.imageUrl) to \(image.fileName): \(error)")
} else {
updateResult.filesDownloaded++
Log.info?.message("Downloaded file \(image.imageUrl) to \(image.fileName)")
}}
}
return updateResult // It obviously returns before any images finish downloading. I need to wait until all images have downloaded before I can return an accurate result.
}
使用每bbum的分发程序更新2016年1月23日
这是尝试使用调度程序机制，但对updateImages的调用仍会立即返回（即使使用dispatch_sync时）。
This is an attempt to use the dispatcher mechanism, but the call to updateImages still return right away (even when using dispatch_sync).
如何在将HTTP响应返回给调用方之前等待所有下载完成？
public func webServer(publicDir: String?) -> HttpServer {
let server = HttpServer()
server.POST["/images/update"] = { r in
let imageDownloader = ImageDownloader()
imageDownloader.updateimageFiles(adFilesOnServer)
let resultJson: String = Mapper().toJSONString(imageDownloader.updateResult, prettyPrint: true)!
if imageDownloader.updateResult.success {
return .OK(.Text(resultJson))
}
return HttpResponse.RAW(500, "Error", nil, { $0.write([UInt8](imageDownloader.updateResult.errorMessage.utf8)) })
}
}
class ImageDownloader {
var updateResult = AdUpdateResult()
private var imageFilesOnServer = [ImageFile]()
private let fileManager = NSFileManager.defaultManager()
private let imageDirectoryURL = NSURL(fileURLWithPath: Settings.imageDirectory, isDirectory: true)
private let semaphore = dispatch_semaphore_create(4)
private let downloadQueue = dispatch_queue_create("com.acme.downloader", DISPATCH_QUEUE_SERIAL)
func updateimageFiles(imageFilesOnServer: [ImageFile]) {
self.imageFilesOnServer = imageFilesOnServer
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)
for serverFile in imageFilesOnServer {
downloadImageFileFromServer(serverFile)
}
dispatch_sync(downloadQueue) {
dispatch_sync(dispatch_get_main_queue()) {
print("done") // It gets here before images have downloaded.
}
}
}
private func downloadImageFileFromServer(serverFile: ImageFile) {
let destinationPath = imageDirectoryURL.URLByAppendingPathComponent(serverFile.fileName)
Alamofire.download(.GET, serverFile.imageUrl) { temporaryURL, response in return destinationPath }
.validate()
.response { _, _, _, error in
if let error = error {
Log.error?.message("Error downloading file \(serverFile.imageUrl) to \(serverFile.fileName): \(error)")
} else {
self.updateResult.filesDownloaded++
Log.info?.message("Downloaded file \(serverFile.imageUrl) to \(serverFile.fileName)")
}
dispatch_semaphore_signal(self.semaphore)
}
}
}
推荐答案
首先，您真的不想开除ar每张图片的每次请求数没有某种限制。
First, you really don't want to be firing off a request-per-image without some kind of a throttle. Semaphores work well for that sort of thing.
第二，您需要从根本上计算未完成的操作数，然后在完成操作后触发完成处理程序。或者，如果可以随时开始新操作，则可能要对操作进行分组。
Secondly, you need to basically count the number of operations outstanding and then fire a completion handler when they are all done. Or, if new operations can be started at any time, you'll probably want to group operations.
因此，伪代码：
sema = dispatch_semaphore_create(4) // 4 being # of concurrent operations allowed
serialQ = dispatch_queue_create(.., SERIAL)
dispatch_async(serialQ) {
dispatch_semaphore_wait(sema, FOREVER) // will block if there are 4 in flight already
for image in images {
downloader.downloadAsync(image, ...) { // completion
dispatch_semaphore_signal(sema) // signal that we are done with one
... handle downloaded image or error ...
... add downloaded images to downloadedImages ...
}
}
}
dispatch_async(serialQ) {
// since serialQ is serial, this will be executed after the downloads are done
dispatch_async(main_queue()) {
yo_main_queue_here_be_yer_images(... downloadedImages ...)
}
}
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