Creating an Asset Object

To create an asset to represent any resource that you can identify using a URL, you use AVURLAsset. The simplest case is creating an asset from a file:

NSURL *url = <#A URL that identifies an audiovisual asset such as a movie file#>;

AVURLAsset *anAsset = [[AVURLAsset alloc] initWithURL:url options:nil];

ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];

// Enumerate just the photos and videos group by using ALAssetsGroupSavedPhotos.

[library enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *group, BOOL *stop) {

// Within the group enumeration block, filter to enumerate just videos.

[group setAssetsFilter:[ALAssetsFilter allVideos]];

// For this example, we're only interested in the first item.

[group enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndex:0]

                        options:0

                     usingBlock:^(ALAsset *alAsset, NSUInteger index, BOOL *innerStop) {

                         // The end of the enumeration is signaled by asset == nil.

                         if (alAsset) {

                             ALAssetRepresentation *representation = [alAsset defaultRepresentation];

                             NSURL *url = [representation url];

                             AVAsset *avAsset = [AVURLAsset URLAssetWithURL:url options:nil];

                             // Do something interesting with the AV asset.

                         }

                     }];

                 }

                 failureBlock: ^(NSError *error) {

                     // Typically you should handle an error more gracefully than this.

                     NSLog(@"No groups");

                 }];

Preparing an Asset for Use

Initializing an asset (or track) does not necessarily mean that all the information that you might want to retrieve for that item is immediately available. It may require some time to calculate even the duration of an item (an MP3 file, for example, may not contain summary information). Rather than blocking the current thread while a value is being calculated, you should use the AVAsynchronousKeyValueLoading  protocol to ask for values and get an answer back later through a completion handler you define using a block. (AVAsset and AVAssetTrack conform to the AVAsynchronousKeyValueLoading protocol.)

You test whether a value is loaded for a property using statusOfValueForKey:error:. When an asset is first loaded, the value of most or all of its properties is AVKeyValueStatusUnknown. To load a value for one or more properties, you invoke loadValuesAsynchronouslyForKeys:completionHandler:. In the completion handler, you take whatever action is appropriate depending on the property’s status. You should always be prepared for loading to not complete successfully, either because it failed for some reason such as a network-based URL being inaccessible, or because the load was canceled.

NSURL *url = <#A URL that identifies an audiovisual asset such as a movie file#>;

AVURLAsset *anAsset = [[AVURLAsset alloc] initWithURL:url options:nil];

NSArray *keys = @[@"duration"];

[asset loadValuesAsynchronouslyForKeys:keys completionHandler:^() {

    NSError *error = nil;

    AVKeyValueStatus tracksStatus = [asset statusOfValueForKey:@"duration" error:&error];

    switch (tracksStatus) {

        case AVKeyValueStatusLoaded:

            [self updateUserInterfaceForDuration];

            break;

        case AVKeyValueStatusFailed:

            [self reportError:error forAsset:asset];

            break;

        case AVKeyValueStatusCancelled:

            // Do whatever is appropriate for cancelation.

            break;

   }

}];

If you want to prepare an asset for playback, you should load its tracks property. For more about playing assets, see Playback.

Getting Still Images From a Video

To get still images such as thumbnails from an asset for playback, you use an AVAssetImageGenerator object. You initialize an image generator with your asset. Initialization may succeed, though, even if the asset possesses no visual tracks at the time of initialization, so if necessary you should test whether the asset has any tracks with the visual characteristic using tracksWithMediaCharacteristic:.

AVAsset anAsset = <#Get an asset#>;

if ([[anAsset tracksWithMediaType:AVMediaTypeVideo] count] > 0) {

    AVAssetImageGenerator *imageGenerator =

        [AVAssetImageGenerator assetImageGeneratorWithAsset:anAsset];

    // Implementation continues...

}

You can configure several aspects of the image generator, for example, you can specify the maximum dimensions for the images it generates and the aperture mode using maximumSize and apertureMode respectively.You can then generate a single image at a given time, or a series of images. You must ensure that you keep a strong reference to the image generator until it has generated all the images.

AVAsset *myAsset = <#An asset#>];

AVAssetImageGenerator *imageGenerator = [[AVAssetImageGenerator alloc] initWithAsset:myAsset];

Float64 durationSeconds = CMTimeGetSeconds([myAsset duration]);

CMTime midpoint = CMTimeMakeWithSeconds(durationSeconds/2.0, 600);

NSError *error;

CMTime actualTime;

CGImageRef halfWayImage = [imageGenerator copyCGImageAtTime:midpoint actualTime:&actualTime error:&error];

if (halfWayImage != NULL) {

    NSString *actualTimeString = (NSString *)CMTimeCopyDescription(NULL, actualTime);

    NSString *requestedTimeString = (NSString *)CMTimeCopyDescription(NULL, midpoint);

    NSLog(@"Got halfWayImage: Asked for %@, got %@", requestedTimeString, actualTimeString);

    // Do something interesting with the image.

    CGImageRelease(halfWayImage);

}

AVAsset *myAsset = <#An asset#>];

// Assume: @property (strong) AVAssetImageGenerator *imageGenerator;

self.imageGenerator = [AVAssetImageGenerator assetImageGeneratorWithAsset:myAsset];

Float64 durationSeconds = CMTimeGetSeconds([myAsset duration]);

CMTime firstThird = CMTimeMakeWithSeconds(durationSeconds/3.0, 600);

CMTime secondThird = CMTimeMakeWithSeconds(durationSeconds*2.0/3.0, 600);

CMTime end = CMTimeMakeWithSeconds(durationSeconds, 600);

NSArray *times = @[NSValue valueWithCMTime:kCMTimeZero],

                  [NSValue valueWithCMTime:firstThird], [NSValue valueWithCMTime:secondThird],

                  [NSValue valueWithCMTime:end]];

[imageGenerator generateCGImagesAsynchronouslyForTimes:times

                completionHandler:^(CMTime requestedTime, CGImageRef image, CMTime actualTime,

                                    AVAssetImageGeneratorResult result, NSError *error) {

                NSString *requestedTimeString = (NSString *)

                    CFBridgingRelease(CMTimeCopyDescription(NULL, requestedTime));

                NSString *actualTimeString = (NSString *)

                    CFBridgingRelease(CMTimeCopyDescription(NULL, actualTime));

                NSLog(@"Requested: %@; actual %@", requestedTimeString, actualTimeString);

                if (result == AVAssetImageGeneratorSucceeded) {

                    // Do something interesting with the image.

                }

                if (result == AVAssetImageGeneratorFailed) {

                    NSLog(@"Failed with error: %@", [error localizedDescription]);

                }

                if (result == AVAssetImageGeneratorCancelled) {

                    NSLog(@"Canceled");

                }

  }];

Trimming and Transcoding a Movie

You can transcode a movie from one format to another, and trim a movie, using an AVAssetExportSession object. The workflow is shown in Figure 1-1. An export session is a controller object that manages asynchronous export of an asset. You initialize the session using the asset you want to export and the name of a export preset that indicates the export options you want to apply (see allExportPresets). You then configure the export session to specify the output URL and file type, and optionally other settings such as the metadata and whether the output should be optimized for network use.

You can check whether you can export a given asset using a given preset using exportPresetsCompatibleWithAsset: as illustrated in this example:

AVAsset *anAsset = <#Get an asset#>;

NSArray *compatiblePresets = [AVAssetExportSession exportPresetsCompatibleWithAsset:anAsset];

if ([compatiblePresets containsObject:AVAssetExportPresetLowQuality]) {

    AVAssetExportSession *exportSession = [[AVAssetExportSession alloc]

        initWithAsset:anAsset presetName:AVAssetExportPresetLowQuality];

    // Implementation continues.

}

You complete the configuration of the session by providing the output URL (The URL must be a file URL.) AVAssetExportSession can infer the output file type from the URL’s path extension; typically, however, you set it directly using outputFileType. You can also specify additional properties such as the time range, a limit for the output file length, whether the exported file should be optimized for network use, and a video composition. The following example illustrates how to use the timeRange property to trim the movie:

    exportSession.outputURL = <#A file URL#>;

    exportSession.outputFileType = AVFileTypeQuickTimeMovie;

    CMTime start = CMTimeMakeWithSeconds(1.0, 600);

    CMTime duration = CMTimeMakeWithSeconds(3.0, 600);

    CMTimeRange range = CMTimeRangeMake(start, duration);

    exportSession.timeRange = range;

To create the new file, you invoke exportAsynchronouslyWithCompletionHandler:. The completion handler block is called when the export operation finishes; in your implementation of the handler, you should check the session’s status value to determine whether the export was successful, failed, or was canceled:

    [exportSession exportAsynchronouslyWithCompletionHandler:^{

        switch ([exportSession status]) {

            case AVAssetExportSessionStatusFailed:

                NSLog(@"Export failed: %@", [[exportSession error] localizedDescription]);

                break;

            case AVAssetExportSessionStatusCancelled:

                NSLog(@"Export canceled");

                break;

            default:

                break;

        }

    }];

You can cancel the export by sending the session a cancelExport message.

The export will fail if you try to overwrite an existing file, or write a file outside of the application’s sandbox. It may also fail if:

• There is an incoming phone call

• Your application is in the background and another application starts playback

In these situations, you should typically inform the user that the export failed, then allow the user to restart the export.