Using Assets - 使用Assets

Assets can come from a file or from media in the user’s iPod library or Photo library. When you create an asset object all the information that you might want to retrieve for that item is not immediately available. Once you have a movie asset, you can extract still images from it, transcode it to another format, or trim the contents.

Assets 可以来自文件或者媒体用户的iPod库、图片库。当你创建一个 asset 对象时，所有你可能想要检索该项目的信息不是立即可用的。一旦你有了一个电影 asset ，你可以从里面提取静态图像，转换到另一个格式，或者对内容就行修剪。

Creating an Asset Object - 创建一个Asset对象

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:

为了创建一个 asset ，去代表任何你能用一个 URL 识别的资源，你可以使用 AVURLAsset .最简单的情况是从一个文件创建一个 asset 。

NSURL *url = <#A URL that identifies an audiovisual asset such as a movie file#>;
AVURLAsset *anAsset = [[AVURLAsset alloc] initWithURL:url options:nil];

Options for Initializing an Asset - 初始化一个Asset的选择

The AVURLAsset initialization methods take as their second argument an options dictionary. The only key used in the dictionary is AVURLAssetPreferPreciseDurationAndTimingKey. The corresponding value is a Boolean (contained in an NSValue object) that indicates whether the asset should be prepared to indicate a precise duration and provide precise random access by time.

AVURLAsset 初始化方法作为它们的第二个参数选项字典。本字典中唯一被使用的 key 是 AVURLAssetPreferPreciseDurationAndTimingKey. 相应的值是一个布尔值（包含在一个 NSValue 对象中），这个布尔值指出是否该 asset 应该准备标出一个精确的时间和提供一个以时间为种子的随机存取。

Getting the exact duration of an asset may require significant processing overhead. Using an approximate duration is typically a cheaper operation and sufficient for playback. Thus:

• If you only intend to play the asset, either pass nil instead of a dictionary, or pass a dictionary that contains the AVURLAssetPreferPreciseDurationAndTimingKey key and a corresponding value of NO (contained in an NSValue object).
• If you want to add the asset to a composition (AVMutableComposition), you typically need precise random access. Pass a dictionary that contains theAVURLAssetPreferPreciseDurationAndTimingKey key and a corresponding value of YES (contained in an NSValue object—recall that NSNumberinherits from NSValue):

获得一个asset的确切持续时间可能需要大量的处理开销。使用一个近似的持续时间通常是一个更便宜的操作并且对于播放已经足够了。因此：

• 如果你只打算播放这个 asset， 要么传递一个 nil 代替 dictionary ，或者传递一个字典，这个字典包含 AVURLAssetPreferPreciseDurationAndTimingKey 的 key和相应 NO(包含在一个 NSValue 对象) 的值。
• 如果你想要把 asset 添加给一个 composition (AVMutableComposition), 通常你需要精确的随机存取。传递一个字典（这个字典包含 AVURLAssetPreferPreciseDurationAndTimingKey key） 和一个相应的 YES 的值（YES 包含在一个 NSValue 对象中，回忆一下继承自 NSValue的 NSNmuber）

NSURL *url = <#A URL that identifies an audiovisual asset such as a movie file#>;
NSDictionary *options = @{ AVURLAssetPreferPreciseDurationAndTimingKey : @YES };
AVURLAsset *anAssetToUseInAComposition = [[AVURLAsset alloc] initWithURL:url options:options];

Accessing the User’s Assets - 访问用户的Assets

To access the assets managed by the iPod library or by the Photos application, you need to get a URL of the asset you want.

• To access the iPod Library, you create an MPMediaQuery instance to find the item you want, then get its URL using MPMediaItemPropertyAssetURL.For more about the Media Library, see Multimedia Programming Guide.
• To access the assets managed by the Photos application, you use ALAssetsLibrary.

The following example shows how you can get an asset to represent the first video in the Saved Photos Album.

为了访问由 iPod 库或者照片应用程序管理的 assets ，你需要得到你想要 asset 的一个 URL。

• 为了访问 iPod 库，创建一个 MPMediaQuery 实例去找到你想要的项目，然后使用MPMediaItemPropertyAssetURL得到它的 URL ，
• 为了访问有照片应用程序管理的 assets ，可以使用 ALAssetsLibrary。

下面的例子展示了如何获得一个 asset 来保存照片相册中的第一个视频。

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 - 将 Asset 准备好使用

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.)

初始化一个 asset （或者轨道）并不意味着你可能想要检索该项的所有信息是立即可用的。这可能需要一些时间来计算一个项目的持续时间（例如一个 MP3 文件可能不包含摘要信息）。当一个值被计算的时候不应该阻塞当前线程，你应该使用AVAsynchronousKeyValueLoading 协议去请求值，通过完成处理你定义使用的一个 block 后得到答复。(AVAsset and AVAssetTrack 遵循 AVAsynchronousKeyValueLoading 协议.)

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.

测试一个值是否是使用 statusOfValueForKey:error: 加载为一个属性。当 asset 被首次加载时，大部分的或全部属性值是 AVKeyValueStatusUnknown。为一个或多个属性加载一个值，调用loadValuesAsynchronouslyForKeys:completionHandler:。在完成处理程序中，你采取的行动是否恰当，取决于属性的状态。你应该总是准备加载不会完全成功，它可能有一些原因，比如基于网络的 URL是无法访问的，或者因为负载被取消。

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.

如果你想准备一个 asset 去播放，你应该加载它的轨道属性。更多有关播放 assets，请看 Playback

Getting Still Images From a Video - 从视频中获取静态图像

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:.

为了从一个准备播放的 asset 中得到静态图像，比如缩略图，可以使用 AVAssetImageGenerator 对象。用你的 asset 初始化一个图像发生器。不过即使 asset 进程在初始化的时候没有视觉跟踪，也可以成功，所以如果有必要，你应该测试一下， asset 是否有轨道有使用 tracksWithMediaCharacteristic 的视觉特征。

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.

你可以配置几个图像发生器的部分，例如，可以指定生成的图像采用最大值，并且光圈的模式分别使用 maximumSize 和 apertureMode 。然后可以在给定的时间生成一个单独的图像，或者一系列图像。你必须确定，在生成所有图像之前，必须对图像生成器保持一个强引用。

Generating a Single Image - 生成一个单独的图像

You use copyCGImageAtTime:actualTime:error: to generate a single image at a specific time. AVFoundation may not be able to produce an image at exactly the time you request, so you can pass as the second argument a pointer to a CMTime that upon return contains the time at which the image was actually generated.

使用 copyCGImageAtTime:actualTime:error: 方法在指定时间生成一个图像。AVFoundation 在你要求的确切时间可能无法产生一个图像，所以你可以将一个指向 CMTime 的指针当做第二个参数穿过去，这个指针返回的时候包含图像被实际生成的时间。

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);
}

Generating a Sequence of Images - 生成一系列图像

To generate a series of images, you send the image generator a generateCGImagesAsynchronouslyForTimes:completionHandler: message. The first argument is an array of NSValue objects, each containing a CMTime structure, specifying the asset times for which you want images to be generated. The second argument is a block that serves as a callback invoked for each image that is generated. The block arguments provide a result constant that tells you whether the image was created successfully or if the operation was canceled, and, as appropriate:

• The image
• The time for which you requested the image and the actual time for which the image was generated
• An error object that describes the reason generation failed

In your implementation of the block, check the result constant to determine whether the image was created. In addition, ensure that you keep a strong reference to the image generator until it has finished creating the images.

生成一系列图像，可以给图像生成器发送 generateCGImagesAsynchronouslyForTimes:completionHandler: 消息。第一个参数是一个 NSValue 对象的数组，每个都包含一个 CMTime 结构体，指定了图像想要被生成的 asset 时间。block 参数提供了一个结果，这个结果包含了告诉你是否图像被成功生成，或者操作某些情况下被取消。结果：

• 图像
• 你要求的图像和图像生成的实际时间
• 一个 error 对象，描述了生成失败的原因

在 block 的实现中，检查结果常数，来确定图像是否被创建。此外，在完成创建图像之前，确保保持一个强引用给图像生成器。

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");
                }
  }];

You can cancel the generation of the image sequence by sending the image generator a cancelAllCGImageGeneration message.

你发送给图像生成器一个 cancelAllCGImageGeneration 消息，可以取消队列中的图像生成。

Trimming and Transcoding a Movie - 微调和转化为一个电影

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.

asset一律使用“资产”代码，切换还要加“略麻烦

你可以使用 AVAssetExportSession 对象，将一个电影的编码进行转换，并且对电影进行微调。工作流程如图1-1所示。一个 export session 是一个控制器对象，管理一个资产的异步导出。使用想要导出的资产初始化一个 session 和输出设定的名称，这个输出设定表明你想申请的导出选项（allExportPresets）。然后配置导出会话去指定输出的 URL 和文件类型，以及其他可选的设定，比如元数据，是否将输出优化用于网络使用。

<center style="box-sizing: border-box; color: rgb(63, 63, 63); font-family: "microsoft yahei"; font-size: 16px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);">

Figure 1-1 The export session workflow

</center>

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

你可以检查你能否用给定的预设导出一个给定的资产，使用 exportPresetsCompatibleWithAsset: 作为示例。

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:

完成会话的配置，是由输出的 URL （URL 必须是文件的 URL）控制的。AVAssetExportSession可以从 URL 的路径延伸推断输出文件的类型。然而通常情况下，直接使用 outputFileType 设定。还可以指定附加属性，如时间范围、输出文件长度的限制、导出的文件是否应该为了网络使用而优化、还有一个视频的构成。下面的示例展示了如果使用 timeRange 属性修剪电影。

    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:

调用 exportAsynchronouslyWithCompletionHandler: 创建新的文件。当导出操作完成的时候完成处理的 block 被调用，你应该检查会话的 status 值，去判断导出是否成功、失败或者被取消。

    [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.

你可以通过给会话发送一个 cancelExport 消息来取消导出。

如果你尝试覆盖一个现有的文件或者在应用程序的沙盒外部写一个文件，都将会是导出失败。如果发生下面情况也可能失败：

• 有一个来电
• 你的应用程序在后台并且另一个程序开始播放

在这种情况下，你通常应该通知用户导出失败，然后允许用户重新启动导出

原文链接：http://blog.csdn.net/zyq522376829/article/details/52144326