swift - NSImagerepresentations.first 返回 nil。为什么？

Question

在下面的代码中，我尝试将 NSImage 保存到磁盘。该图像是从视频资源生成的。

@IBAction func chooseVideoFrame(sender: AnyObject) {

    let playheadPosition = player2!.currentTime()
    videoThumb = generateThumnail(videoURL.stringValue, fromTime: playheadPosition)
    videoThumbnail.image = videoThumb
    if let bits = videoThumb!.representations.first as? NSBitmapImageRep {
        let data = bits.representationUsingType(.NSJPEGFileType, properties: [:])
        theTempPath = (getDocumentsDirectory() as String) + "/thumbTemp.jpg"
        data?.writeToFile(theTempPath!, atomically: false)
    } else {

        problemAlert("Video Thumbnail problem",info: "There's a problem with saving the video Thumbnail image.")
    }
}

func generateThumnail(videoURL : String, fromTime:CMTime) -> NSImage {
    let url = NSURL(string: videoURL)
    let asset :AVAsset = AVAsset(URL:url!)
    let assetImgGenerate : AVAssetImageGenerator = AVAssetImageGenerator(asset: asset)
    assetImgGenerate.appliesPreferredTrackTransform = true
    assetImgGenerate.requestedTimeToleranceAfter = kCMTimeZero;
    assetImgGenerate.requestedTimeToleranceBefore = kCMTimeZero;

    do { let thumbnail = try assetImgGenerate.copyCGImageAtTime(fromTime, actualTime: nil)

            thumb = NSImage(CGImage: thumbnail, size: NSZeroSize)

       } catch {

            problemAlert("Could Not Generate Thumbnail",info: "There was a problem with making the video Thumbnail image.")

       }

    return thumb!
}

当它运行时，位被设置为零，我不明白为什么。有人能解释一下为什么吗？

Answer 1

经过仔细地反复使用 xCode 中的编译器并仔细阅读文档，我终于得到了可以按我想要的方式工作的代码。

@IBAction func chooseVideoFrame(sender: AnyObject) {
        // Get the playhead time and grab fram from that time
        let playheadPosition = player2!.currentTime()
        videoThumb = generateThumnail(videoURL.stringValue, fromTime: playheadPosition)
        videoThumbnail.image = videoThumb
        // Save the frame as a JPEG and save it to disk so it can be saved as
        // as CKAsset later
        if let tiffRep = videoThumb?.TIFFRepresentation {
            let jpegImage = NSBitmapImageRep(data: tiffRep)!.representationUsingType(.NSJPEGFileType, properties: [:])!
            theTempPath = (getDocumentsDirectory() as String) + "/thumbTemp.jpg"
            jpegImage.writeToFile(theTempPath!, atomically: false)

        } else {

            problemAlert("Video Thumbnail problem", info: "There's a problem with saving the video Thumbnail image.")
        }
    }

    func generateThumnail(videoURL : String, fromTime:CMTime) -> NSImage {
        let url = NSURL(string: videoURL)
        let asset :AVAsset = AVAsset(URL:url!)
        let assetImgGenerate : AVAssetImageGenerator = AVAssetImageGenerator(asset: asset)
        assetImgGenerate.appliesPreferredTrackTransform = true
        assetImgGenerate.requestedTimeToleranceAfter = kCMTimeZero;
        assetImgGenerate.requestedTimeToleranceBefore = kCMTimeZero;

        do { let thumbnail = try assetImgGenerate.copyCGImageAtTime(fromTime, actualTime: nil)

            thumb = NSImage(CGImage: thumbnail, size: CGSize(width: 768.0, height: 432.0))

           } catch {

                problemAlert("Could Not Generate Thumbnail",info: "There was a problem with making the video Thumbnail image.")
           }

        return thumb!
    }