使用 MPMoviePlayer 请求视频缩略图的问题

Question

我正在尝试解析包含视频网址和标题的 JSON 数据。解析数据后，我在 UITableviewCell 中显示视频缩略图。问题是当数据被解析时，当它请求缩略图时，NSNotification 选择器方法永远不会被调用。这是我的代码：

override func viewDidLoad() { 
super.viewDidLoad() 
getJson_results() 
}

override func viewWillDisappear(animated: Bool) {
NSNotificationCenter.defaultCenter().removeObserver(self)
}

func videoThumbnailIsAvailable(notification: NSNotification) {
    println("Flag1") //the control never reaches here
    var thumbnail = notification.userInfo?[MPMoviePlayerThumbnailImageKey] as UIImage
    self.temp_image = thumbnail
}


func videodurationIsAvailable(notification: NSNotification) {
    var val = moviePlayer?.duration
    self.temp_duration = val!
}

override func tableView(tableView: UITableView, numberOfRowsInSection section:    Int) -> Int { 
    return length_array; 
}

override func numberOfSectionsInTableView(tableView: UITableView) -> Int {
return 1
}

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath) as WPR_Main
    cell.imgView?.image =           get_Media_thumbnail(NSURL(string:self.url[indexPath.row])!)
    cell.titleLabel?.text = self.titles[indexPath.row]
    return cell
    }


func get_Media_thumbnail(url: NSURL) -> UIImage {
    moviePlayer? = MPMoviePlayerController(contentURL: url)
    NSNotificationCenter.defaultCenter().addObserver(self,
    selector: "videoThumbnailIsAvailable:",
    name: MPMoviePlayerThumbnailImageRequestDidFinishNotification,
    object: nil)
    moviePlayer?.requestThumbnailImagesAtTimes([1.0], timeOption:.NearestKeyFrame )
   return self.temp_image
}

Answer 1

实现这一点的最佳方法是使用 AVAssetImageGenerator 而不是使用 requestThumbnailImagesAtTimes。 AVAsset 还允许异步下载图像，这非常快并且适用于需求。代码如下：

var urlAsset = AVURLAsset(URL: NSURL(string:self.url[indexPath.row])!, options: nil)
        if(urlAsset.tracksWithMediaType(AVMediaTypeVideo).count > 0){
            var imageGenerator = AVAssetImageGenerator(asset: urlAsset)
            imageGenerator.appliesPreferredTrackTransform=true
            let durationSeconds = CMTimeGetSeconds(urlAsset.duration)
            let midPoint = CMTimeMakeWithSeconds(durationSeconds/2, 1)
            var error:NSError? = nil
            var actualTime = CMTimeMake(0, 0)
            imageGenerator.generateCGImagesAsynchronouslyForTimes( [ NSValue(CMTime:midPoint) ], completionHandler: {
                (requestTime, thumbnail, actualTime, result, error) -> Void in

                if result == AVAssetImageGeneratorResult.Succeeded {

                    dispatch_async(dispatch_get_main_queue()) {
                        var thumbnailImage = UIImage(CGImage: thumbnail)
                        cell.imgView?.image =  thumbnailImage
                    }//dispatch
                }//if
                else {
                    var thumbnailImage = UIImage(named: "video-backup.png")
                    cell.imageView?.contentMode = .ScaleAspectFit
                    cell.imgView?.image =  thumbnailImage
                }
            })