// 采集
- (void)setupCaputureVideo {
 // 1.创建捕获对话，必须要强引用，否则会释放
 _captureSession = [[AVCaptureSession alloc] init];
 // 2.捕获摄像头设备，默认前置摄像头
 AVCaptureDevice *videoDevice = [self getVideoDevice:AVCaptureDevicePositionFront];
 // 3.获取声音设备
 AVCaptureDevice *audioDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeAudio];
 // 4.创建对应视频设备输入对象
 _currentVideoDeviceInput = [AVCaptureDeviceInput deviceInputWithDevice:videoDevice error:nil];
 // 5.创建对应音频设备输入对象
 AVCaptureDeviceInput *audioDeviceInput = [AVCaptureDeviceInput deviceInputWithDevice:audioDevice error:nil];
 // 6.添加到会话中 注意：最好要判断是否能添加输入，会话不能添加空的
 // 6.1 添加视频
 if ([_captureSession canAddInput:_currentVideoDeviceInput]) {
 [_captureSession addInput:_currentVideoDeviceInput];
 }
 // 6.2 添加音频
 if ([_captureSession canAddInput:audioDeviceInput]) {
 [_captureSession addInput:audioDeviceInput];
 }
 // 7.捕获视频数据输出设备
 AVCaptureVideoDataOutput *videoOutput = [[AVCaptureVideoDataOutput alloc] init];
 //7.1 设置代理， 捕获视频样品数据
 dispatch_queue_t videoQueue = dispatch_queue_create("Video Capure Queue", DISPATCH_QUEUE_SERIAL);
 [videoOutput setSampleBufferDelegate: self queue:videoQueue];
 // 8.设置音频数据输出设备
 AVCaptureAudioDataOutput *audioOutput = [[AVCaptureAudioDataOutput alloc] init];
 // 8.1 设置代理，捕获音频样品数据 注意：必须是串行队列才能捕获到数据,而且不能为空
 dispatch_queue_t audioQueue = dispatch_queue_create("Audio Capure Queue", DISPATCH_QUEUE_SERIAL);
 [audioOutput setSampleBufferDelegate:self queue:audioQueue];
 // 9.添加到会话中 注意：最好要判断是否能添加输入，会话不能添加空的
 if ([_captureSession canAddOutput:videoOutput]) {
 [_captureSession addOutput:videoOutput];
 }
 if ([_captureSession canAddOutput:audioOutput]) {
 [_captureSession addOutput:audioOutput];
 }
 // 10.获取视屏输入与输出连接，用于分辨音视频数据
 _videoConnection = [videoOutput connectionWithMediaType:AVMediaTypeVideo];
 // 11.添加视屏预览图层
 _previewdLayer = [AVCaptureVideoPreviewLayer layerWithSession:_captureSession];
 _previewdLayer.frame = [UIScreen mainScreen].bounds;
 [self.view.layer insertSublayer:_previewdLayer atIndex:0];
 // 12.开启会话
 [_captureSession startRunning];
}
// 根据摄像头方向获取摄像头
- (AVCaptureDevice *)getVideoDevice: (AVCaptureDevicePosition)position {
 NSArray *devices = [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
 for (AVCaptureDevice *device in devices) {
 if (device.position == position) {
 return device;
 }
 }
 return nil;
}

AVCaptureDevicePositionUnspecified，那么是不能调用摄像头的。
typedef NS_ENUM(NSInteger, AVCaptureDevicePosition) {
 AVCaptureDevicePositionUnspecified = 0, // 不指定
 AVCaptureDevicePositionBack = 1, //后置
 AVCaptureDevicePositionFront = 2 // 前置
}

// 获取输出设备数据，有可能是音频，有可能是视频
- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection {
 if (_videoConnection == connection) {
 NSLog(@"采集到视屏数据");
 }else {
 NSLog(@"采集到音频数据");
 }
}

#pragma mark - 切换摄像头
- (IBAction)toggleCapture:(id)sender {
 // 1.获取当前设备方向
 AVCaptureDevicePosition cureentPosition = _currentVideoDeviceInput.device.position;
 // 2.获取需要改变的方向
 AVCaptureDevicePosition togglePosition = (cureentPosition == AVCaptureDevicePositionFront ? AVCaptureDevicePositionBack : AVCaptureDevicePositionFront);
 // 3.获取需要改变的摄像头设备
 AVCaptureDevice *toggleDevice = [self getVideoDevice:togglePosition];
 // 4.获取需要改变的摄像头输入设备
 AVCaptureDeviceInput *toggleDeviceInput = [[AVCaptureDeviceInput alloc] initWithDevice:toggleDevice error:nil];
 // 5.停止会话，否则会有一瞬间的白屏
 [_captureSession stopRunning];
 // 6.移除之前的摄像头输入设备,否则会崩溃，因为会话里只能有一个摄像头设备
 [_captureSession removeInput:_currentVideoDeviceInput];
 // 7.添加新的摄像头输入设备
 [_captureSession addInput:toggleDeviceInput];
 // 8.重新开始会话
 [_captureSession startRunning];
 // 记录当前摄像头输入设备
 //9.重新开始
 _currentVideoDeviceInput = toggleDeviceInput;
}