# AVFoundation Programming Guide(官方文档翻译2)Using Assets - 使用Assets

– 第2章：Using Assets - 使用Assets
– 第3章：Playback - 播放
– 第4章：Editing - 编辑
– 第5章：Still and Video Media Capture - 静态视频媒体捕获
– 第6章：Export - 输出
– 第7章：Time and Media Representations 时间和媒体表现

CSDN博客：

– 第2章：Using Assets - 使用Assets
– 第3章：Playback - 播放
– 第4章：Editing - 编辑
– 第5章：Still and Video Media Capture - 静态视频媒体捕获
– 第6章：Export - 输出
– 第7章：Time and Media Representations 时间和媒体表现

# 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:

### 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 keyAVURLAssetPreferPreciseDurationAndTimingKey. 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):

• 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 AVURLAssetPreferPreciseDurationAndTimingKeykey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 NSValueNSNmuber

### 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.

## 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.)

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.

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 - 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:.

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.

### 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.

## 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.

• 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

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。

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

## 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

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

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:

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:

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.

• 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