ios - Xcode - iOS - AWS S3 - 下载的图像在首次下载时不显示

Question

这是我用来从 AWS S3 服务器下载图像并将其指定为 ImageView 中显示的图像的代码:

    let s3BucketName = "bucketName"
    let fileName = Globals.currAuthorName.filter { $0 != Character(" ") } + ".jpg"

    let downloadFilePath = NSTemporaryDirectory().stringByAppendingPathComponent(fileName)
    let downloadingFileURL = NSURL.fileURLWithPath(downloadFilePath)

    // Create a credential provider for AWS requests
    let credentialsProvider = AWSCognitoCredentialsProvider(
        regionType: AWSRegionType.USEast1,
        identityPoolId: "us-east-1:********-****-****-****-************")

    // Create a service configuration for AWS requests
    let defaultServiceConfiguration = AWSServiceConfiguration(
        region: AWSRegionType.USEast1,
        credentialsProvider: credentialsProvider)

    // Create a new download request to S3, and set its properties
    AWSServiceManager.defaultServiceManager().defaultServiceConfiguration = defaultServiceConfiguration

    let downloadRequest = AWSS3TransferManagerDownloadRequest()
    downloadRequest.bucket = s3BucketName
    downloadRequest.key  = "folderName/" + fileName
    downloadRequest.downloadingFileURL = downloadingFileURL

    let transferManager = AWSS3TransferManager.defaultS3TransferManager()
    transferManager.download(downloadRequest)

    // Set the UIImageView to show the file that was downloaded
    let image = UIImage(contentsOfFile: downloadFilePath)
    authorImage.contentMode = UIViewContentMode.ScaleAspectFit
    authorImage.image = image

问题是，当我运行 iOS 模拟器并导航到相应的页面时，第一次访问该页面时图像永远不会加载。但是，当我离开页面然后返回页面时，它总是被加载，并且每次我访问该页面时。当我停止运行该应用程序然后再次启动它并再次访问该页面时，图像仍在加载。仅当我第一次访问某个 iOS 模拟器的页面时，图像才未加载(即在 iPhone 6 模拟器、iPhone 5 模拟器等中发生过一次)

我尝试通过添加这段代码来解决问题:

while (image == nil) {
  transferManager.download(downloadRequest)
  let image = UIImage(contentsOfFile: downloadFilePath)
}

但这只会导致我第一次访问该页面时出现无限循环。控制台最终也输出此错误:

2015-07-06 23:26:28.529 CCBF[19839:92323] 19839: CFNetwork internal error (0xc01a:/SourceCache/CFNetwork_Sim/CFNetwork-711.3.18/Foundation/NSURLRequest.mm:798) 2015-07-06 23:26:28.564 AuthorProject[19839:92323] AWSiOSSDKv2 [Error] AWSURLSessionManager.m line:240 | __41-[AWSURLSessionManager taskWithDelegate:]_block_invoke222 | Invalid AWSURLSessionTaskType.

编辑:

我尝试实现以下 continuewithblock:

var image : UIImage!

let task = transferManager.download(downloadRequest)
task.continueWithBlock({
    if task.error != nil {
        println(task.error)
    } else {
        dispatch_async(dispatch_get_main_queue(), {
            image = UIImage(contentsOfFile: downloadFilePath)
            self.authorImage.contentMode = UIViewContentMode.ScaleAspectFit
            self.authorImage.image = image
        })
    }
    return nil
}())

但是，当我运行它并尝试加载图像时，我收到一个运行时错误，该错误将我指向“continueWithExecutor”方法的代码。具体来说，代码行:

result = block(self);

消息是:

EXC_BAD_ACCESS(code: 1, address=0x10)

编辑 2:

我似乎找到了一个有效的解决方案:

var image : UIImage!

    let taskTODO = transferManager.download(downloadRequest)
    taskTODO.continueWithBlock{ (task: AWSTask!) -> AnyObject! in
        if task.error != nil {
            println(task.error)
        } else {
            dispatch_async(dispatch_get_main_queue(), {
                image = UIImage(contentsOfFile: downloadFilePath)
                self.authorImage.contentMode = UIViewContentMode.ScaleAspectFit
                self.authorImage.image = image
            })
        }

        return nil
    }

这看起来是正确的方法吗？

Answer 1

transferManager.download(downloadRequest) 是异步的，这意味着下载图像大约需要 20 毫秒。但是，第一次进入屏幕时，您会尝试立即分配图像(尚未下载)。在随后的屏幕加载中——图像已经被下载和缓存——所以它可以立即显示，。您需要的是一个完成 block ，指示图像何时完成下载并准备好显示，您将在该 block 中分配图像。请参见下面的示例:

let readRequest1 : AWSS3TransferManagerDownloadRequest = AWSS3TransferManagerDownloadRequest()
    readRequest1.bucket = "shrikar-picbucket"
    readRequest1.key =  "bingo"
    readRequest1.downloadingFileURL = downloadingFileURL1

    let task = transferManager.download(readRequest1)
    task.continueWithBlock { (task) ->; AnyObject! in
        println(task.error)
        if task.error != nil {
        } else {
            dispatch_async(dispatch_get_main_queue()
                , { () ->; Void in
                    self.selectedImage.image = UIImage(contentsOfFile: downloadingFilePath1)
                    self.selectedImage.setNeedsDisplay()
                    self.selectedImage.reloadInputViews()

            })
            println("Fetched image")
        }
        return nil
    }