Apple 在 iOS Technology Overview 中的 Audio Technologies 和 Video Technologies 分别是这么介绍 AVFoundation 的：
AV Foundation is an Objective-C interface for managing the recording and playback of audio and video. Use this framework for recording audio and when you need fine-grained control over the audio playback process.
AV Foundation provides advanced video playback and recording capabilities. Use this framework in situations where you need more control over the presentation or recording of video. For example, augmented reality apps could use this framework to layer live video content with other app-provided content.
从这两个介绍中我们可以知道 AVFoundation 是用来播放和录制音频和视频的。
在 AVFoundation Programming Guide 中则是这么介绍的：
AVFoundation is one of several frameworks that you can use to play and create time-based audiovisual media. It provides an Objective-C interface you use to work on a detailed level with time-based audiovisual data. For example, you can use it to examine, create, edit, or reencode media files. You can also get input streams from devices and manipulate video during realtime capture and playback.
A:Currently the best documentation is the source. You can take a look at it here(arrts.xml).
You can define attributes in the top <resources> element or inside of a <declare-styleable> element. If I’m going to use an attr in more than on place I put it in the root element. Note , all attributes share the same global namespace. That means that even if you create a new attribute inside of a <declare-styleable> element it can be used outside of it and you cannot create another attribute with the same name of a different type.
An <attr> element has two xml attributes name and format. name lets you call it something and this how you end up refering to it in code, e.g., R.attr.my_attribute. The format attribute can have different values depending on the type of attribute you want.
reference - if it references another resource id(e.g, “@color/my_color”, “@layout/my_layout”)
enum - normally implicitly defined
flag - normally implicitly defined
You can set the format to multiple types by using |, e.g., format="reference|color".