AVCaptureSession

AVFoundation捕捉的核心是AVCaptureSession,官方文档的描述是You use an AVCaptureSession object to coordinate the flow of data from AV input devices to outputs.,可以有多个输入或输出。它是连接AVCaptureInput和AVCaptureOutput的桥梁，它协调input到output之间传输数据。
初始化方法

   AVCaptureSession *seesion = [[AVCaptureSession alloc] init];
    seesion.sessionPreset = AVCaptureSessionPresetHigh;

属性

/*!
 @property sessionPreset
 @abstract
    Indicates the session preset currently in use by the receiver.
 
 @discussion
    The value of this property is an NSString (one of AVCaptureSessionPreset*) indicating 
    the current session preset in use by the receiver.  The sessionPreset property may be set 
    while the receiver is running.
*/
@property(nonatomic, copy) NSString *sessionPreset;```

默认值是
AVCaptureSessionPresetHigh

当然还有其他值

AVCaptureSessionPreset1280x720
AVCaptureSessionPreset1920x1080
AVCaptureSessionPreset352x288
AVCaptureSessionPreset640x480
AVCaptureSessionPresetInputPriority
AVCaptureSessionPresetLow
AVCaptureSessionPresetMedium
AVCaptureSessionPresetPhoto
AVCaptureSessionPresetiFrame1280x720
AVCaptureSessionPresetiFrame960x540

![Paste_Image.png](http://upload-images.jianshu.io/upload_images/1643663-52024bd809520a33.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)


##一些方法
若想在一个已经使用上的session中（已经startRunning了）做更换新的device、删除旧的device等一系列操作，(比如可以通过调用下面两个方法切换手机的摄像头操作)那么就需要使用如下方法：

/*!
@method beginConfiguration
@abstract
When paired with commitConfiguration, allows a client to batch multiple configuration
operations on a running session into atomic updates.

@discussion
-beginConfiguration / -commitConfiguration are AVCaptureSession's mechanism
for batching multiple configuration operations on a running session into atomic
updates. After calling [session beginConfiguration], clients may add or remove
outputs, alter the sessionPreset, or configure individual AVCaptureInput or Output
properties. All changes will be pended until the client calls [session commitConfiguration],
at which time they will be applied together. -beginConfiguration / -commitConfiguration
pairs may be nested, and will only be applied when the outermost commit is invoked.
*/

• (void)beginConfiguration;

/*!
@method commitConfiguration
@abstract
When preceded by beginConfiguration, allows a client to batch multiple configuration
operations on a running session into atomic updates.
*/

• (void)commitConfiguration;

/*!
@method stopRunning
@abstract
Starts an AVCaptureSession instance running
*/

• (void)startRunning;

/*!
@method stopRunning
@abstract
Stops an AVCaptureSession instance that is currently running.
*/

• (void)stopRunning;